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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics (2011)

Chapter: 5 Report of the Panel on Stars and Stellar Evolution

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Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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5
Report of the Panel on Stars and Stellar Evolution

SUMMARY

The science frontier for stars and stellar evolution is as close as the Sun and as distant as exploding stars at redshift 8.3. It includes understanding processes of exquisite complexity that connect the rotation of stars with their magnetic fields and areas of nearly total ignorance about phenomena that have been imagined but not yet observed, such as accretion-induced collapse. Because astronomers understand stars well, they have the confidence to use them as cosmic probes to trace the history of cosmic expansion; but because this understanding is not complete, there is much to learn about the subtle interplay of convection, rotation, and magnetism or the not-so-subtle violent events that destroy stars or transform them into neutron stars or black holes. Although the topics of stars and their changes over time comprise great chunks of introductory astronomy textbooks, and although the tools for these investigations are tested and sharp, many of the simplest assertions about the formation of white dwarfs, mass loss from giant stars, and the evolution of binary stars are based on conjecture and a slender foundation of facts.

The future is promising. X-ray and radio observations allow astronomers to probe stars where strong gravity is at work. These settings stretch the understanding of fundamental physics beyond the range of laboratory investigations into unknown areas of particle interactions at higher densities than those produced in any nucleus or terrestrial accelerator. By testing three-dimensional predictions against the evidence, more-powerful computers and programming advances put astronomers on the brink of understanding the violent events that make stars explode

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

and collapse. Advances in laboratory astrophysics lead to a better understanding of the underlying nuclear, atomic, and magnetohydrodynamic (MHD) processes. New technology for optical and infrared (IR) spectropolarimetry and for interferometry open up the possibility of seeing magnetic fields and resolving the disks of stars. When well-sampled imaging is coupled to powerful systems for processing vast quantities of data sampled over time, subtle features of stellar interiors can be inferred. Similarly, rare and rapid transients that have eluded surveys to date will surely be found, and may be connected with gravitational waves.

These advances are certain to open up a new and unexplored world of investigation on timescales from seconds to decades. In this report, the Astro2010 Science Frontiers Panel on Stars and Stellar Evolution sketches the most fertile opportunities for the coming decade in the field of stars and stellar evolution. The panel is confident that it will prove a fruitful decade for this field of astronomy, with the resolution of today’s questions producing many new problems and possibilities.

As requested by the Astro2010 committee, the panel formulated its report around four science questions and one outstanding discovery opportunity. The panel is under no illusion that this short list is complete: the field is so rich that there will surely be advances in areas not emphasized here. The panel does, however, have every reason to believe that these questions capture some of the most promising areas for advances in the coming decade. The four questions and discovery opportunity are as follows:

  1. How do rotation and magnetic fields affect stars?

  2. What are the progenitors of Type Ia supernovae and how do they explode?

  3. How do the lives of massive stars end?

  4. What controls the mass, radius, and spin of compact stellar remnants?

  5. Unusual discovery potential: time-domain astronomy—in which the technology on the horizon is well matched to the many timescales of stellar phenomena.

The subsections below summarize the main points.

How Do Rotation and Magnetic Fields Affect Stars?

There’s an old chestnut about a dozing theorist at the weekly colloquium who opens his eyes at the end of every talk and rouses himself to ask, to great approbation for his subliminal understanding, “Yes, all very interesting, but what about rotation and magnetic fields?”

Astronomers are now in a position to address this question in a serious way. In the Sun, the effects are visible; in many other stars they are likely to be much more important. It is not sufficient to think of rotation and magnetism as perturbations on a one-dimensional star. These are fundamental physical phenomena that demand a three-dimensional representation in stars.

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

Astronomers are poised to learn how stars rotate at the surface and within and how that rotation affects mass loss and stellar evolution. They seek a better understanding of how magnetic fields are generated in stars across the mass spectrum and of how these fields power the chromospheres and coronae that produce observed magnetic activity. Finally, the origin of highly magnetized main sequence stars, in which surface fields approach 104 gauss, remains mysterious, and the investigation of these stars promises to shed light on the star-formation process that produced them as well as on the origin of even more highly magnetized compact objects.

The prospects for progress on this question in the next decade stem from the emergence of greatly improved tools for measuring magnetic fields from polarization, for resolving the atmospheres of some stars with interferometry, for probing the interiors of stars through their vibration spectra, and for extending observations into X-rays and gamma rays. When combined with more thorough understanding of the static and dynamic properties of magnetic atmospheres, astronomers will learn how stellar atmospheres really work and how rotation and magnetism affect the evolution of stars.

What Are the Progenitors of Type Ia Supernovae and How Do They Explode?

Many lines of evidence converge on the idea that Type Ia supernovae are thermonuclear explosions of white dwarfs in binary systems. Because of their high luminosity, and with effective empirical methods for determining their distances from light-curve shapes, Type Ia supernovae have acquired a central role not just in stellar astrophysics but also in tracing the history of cosmic expansion and in revealing the astonishing fact of cosmic acceleration. Because this result points to a profound lack in the understanding of gravitation, a problem right at the heart of modern physics, completing the astronomical story of Type Ia supernovae is a pressing priority for the coming decade.

First of all, the provenance of the exploding white dwarfs seen in other galaxies is not known with certainty. The prevailing picture is that Type Ia explosions arise in binary systems in which a white dwarf accretes matter until it approaches the Chandrasekhar limit, simmers, and then erupts in a thermonuclear flame. But it is not known how this picture is affected by chemical composition or age, two essential ingredients in making a precise comparison of distant events with those nearby. Events that are precipitated by the merger of two white dwarfs are not excluded. The Type Ia supernovae in star-forming galaxies and in ellipticals are at present treated in the same way, but this is the result of small samples, not of evidence that they should be analyzed together. More broadly, these gaps in knowledge illuminate the need for a better understanding of the evolution of interacting binary stars, which are responsible for a variety of crucial, yet poorly understood, phenomena.

It can be expected that both theory and improved samples will place this work on a firmer foundation. The complex, turbulent, unstable nuclear flame that

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

rips through the star and incinerates its core is at the present time impossible to compute fully in three dimensions. But the prospects for achieving that goal in the coming decade are intriguing. Samples today amount to a few hundred objects at low redshift and similar numbers beyond redshift 0.5. Much larger and significantly more uniformly discovered samples are coming soon through targeted aspects of time-domain surveys. They will create a much tighter connection between chemistry, binary stellar populations, and supernova properties. Inferences on dark-energy properties are at present limited by inadequate understanding of the intrinsic properties of Type Ia supernovae as distorted by interstellar dust. Observing in the rest-frame infrared will expand the basis for comparing observations with computations and provide more accurate measurements of dark energy.

How Do the Lives of Massive Stars End?

Ninety-five percent of stars will end their lives as white dwarfs. For the rest, stellar death is spectacular and dramatic: these massive stars can explode as supernovae, emit gamma-ray bursts (GRBs), and collapse to form neutron stars or black holes. The elements that they synthesize and eject become the stuff of other stars, planets, and life. The energy and matter that they produce are crucial for the evolution of galaxies and clusters of galaxies.

Despite a basic understanding that gravity is the energy source for these events, a clear connection between the mass and metallicity of the star that collapses, the nature of the collapse and explosion, and the properties of the compact remnant remain mysterious. The rotation of the progenitor and its mass loss, areas of uncertainty highlighted in the panel’s first question, seem to be essential aspects of the link between core collapse supernovae and GRBs. Exploring these frontiers will require continued thoughtful analysis and full-blown first-principles calculations.

The full range of outcomes for stellar deaths may not be well represented in current observational samples. Deeper, faster, wider surveys will surely detect rare or faint outcomes of stellar evolution that have not yet been seen. These could include pair-instability supernovae and other types of explosions that have only been imagined.

The role of massive stars in the evolution of the universe is coming into view. The fossil evidence of massive stars is embedded in the atmospheric abundance patterns of our galaxy’s most metal-poor stars. The most distant object measured so far is a gamma-ray burst, presumably from a massive star, at redshift 8.3. In the coming decade, the direct observation of the first generation of stars, which are predicted to be exceptionally massive, will be within reach with the James Webb Space Telescope (JWST).

Massive stars could be the source of gravitational wave signals, a neutrino flash, or nuclear gamma-ray lines. All of these novel messages from stars are within reach

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

for very nearby cases, and could be exceptionally important in shaping the future understanding of the deaths of massive stars.

What Controls the Mass, Radius, and Spin of Compact Stellar Remnants?

Unanswered questions about the magnetic fields and rotation of stars carry through to similar questions about the exotic remnants that they leave behind as neutron stars and black holes. These are exceptional places in the universe where understanding of physics is extended beyond the reach of any laboratory.

For example, the equation of state for nuclear matter sets the relation between mass and radius for neutron stars. Theoretical understanding of the forces at work is uncertain where the density exceeds that of the densest nuclei. Prospects for measuring masses for radio pulsars and neutron star radii from X-ray techniques promise a glimpse into the strange world of quantum chromodynamics and the possibility of hyperons, deconfined quark matter, or Bose condensates.

The spins of neutrons stars and black holes are rich areas for future work. It is known that millisecond pulsars are spinning much faster than when the neutron stars were formed, and it is understood how accretion in a binary system can accelerate their rotation, but the mechanism that limits how fast these neutron stars can whirl is not known. The answer is expected to come from new pulsar surveys that are less biased against detecting the fastest pulsars. Similarly, there are now plausible measurements that imply that black holes are spinning rapidly. It seems very likely that these black holes are telling us about the conditions in which they formed, during the collapse of a massive star—one of the key points in the panel’s third question. In the coming decade, X-ray spectroscopy should be a powerful technique for expanding the slender sample of spinning black holes, all identified in binaries.

Most stars surely become white dwarfs, but present understanding of the white dwarf mass for a main sequence star of a given initial mass is seriously incomplete. How stars lose mass is not understood well enough to predict which stars will become white dwarfs. Important details of the white-dwarf population remain unsolved and could lead to types of supernovae that have not yet been recognized. Large surveys will be powerful tools for finding these objects, making it possible to fill in these embarrassing gaps in understanding.

Discovery Area: Time-Domain Astronomy

For poets, stars are symbols of permanence. But astronomers know that this is not the whole story. Stars reveal important clues about their true nature by their rotation, pulsation, eclipses and distortions, mass loss, eruptions, and death. Across a wide range of timescales from seconds to years, stars are changing, and scientific

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

knowledge has been obtained through narrow windows of time set by practical matters of telescope time, detector size, and the ability to sift the data. The panel foresees a rich flood of data from specialized survey instruments capable of exploring this new frontier in astronomy, across the electromagnetic spectrum. These instruments will provide new time-domain data, with the potential for major impact on stellar astronomy, ranging from the precise understanding of stars through seismological data and the periodicities that rotation produces, to the detection of rare transient events that have not yet been revealed in extant surveys. An example provides a glimpse of the excitement: wide, deep, and frequent surveys will be the way to find the electromagnetic counterparts of gravitational wave events. The broad problems of binary star evolution, about which so much is assumed and so little is known, can be sampled by such an undertaking, perhaps advancing the knowledge of the progenitors of thermonuclear supernovae from being a plausible story to becoming an established fact. The range of stellar phenomena that will be addressed with large, accessible, time-domain databases goes far beyond the four questions of the panel.

Summary of Panel’s Conclusions

The conclusions of this panel report are summarized in Table 5.1.

INTRODUCTION

The advent of quantum mechanics and the study of nuclear fusion in the 1930s led to the first successful models for energy generation in stars and to a basic understanding of the distribution of stars in the Hertzsprung-Russell diagram. This work, along with the study of stellar atmospheres, laid the foundation for modern astrophysics. In just the past decade, the resolution of the solar neutrino problem demonstrated conclusively the presence of new neutrino physics and the accuracy of solar models originally developed in the 1960s. The success of this work relied in part on the approximate spherical symmetry of stars, which enabled accurate one-dimensional models of their structure.

The opening of new parts of the electromagnetic spectrum dramatically broadened astronomers’ views of stellar phenomena, leading to a number of breakthrough discoveries. Newly discovered radio pulsars in binaries, including the unique double-pulsar system, provided some of the most stringent tests of general relativity. Advances in X-ray astronomy have led to new discoveries related to accreting neutron stars and black holes, compact remnants of massive stars, and energetic phenomena, such as coronae and flares, on normal stars. The characterization of brown dwarfs, cool low-mass objects at the border between stars and planets, was due to the development of new infrared surveys. And the

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

TABLE 5.1 Summary of Conclusions of the Panel on Stars and Stellar Evolution

 

Question 1: How Do Rotation and Magnetic Fields Affect Stars?

Question 2: What Are the Progenitors of Type Ia Supernovae, and How Do They Explode?

Question 3: How Do the Lives of Massive Stars End?

Question 4: What Controls the Mass, Radius, and Spin of Compact Stellar Remnants?

Current and expected facilities

ATST, SDO, HST, 4-m and 8-m telescopes, Kepler, CoRoT, Gaia

PTF, PanStarrs-1, KAIT, PAIRITEL, JWST, Swift, HST, Chandra, 8- to 10-m telescopes

PTF, PanStarrs-1, Swift, NuSTAR, EVLA, 8- to 10-m telescopes, HST, GALEX, Chandra

EVLA, ALMA, LOFAR, Gaia FAST, LIGO, FRIB, Chandra, XMM, Suzaku, RXTE, Fermi

New facilities needed

High spatial and synoptic solar magnetometry; helio-and asteroseismology; OIR interferometry; OIR time-domain, large-FOV observations; high-resolution multiobject OIR spectroscopy; plasma physics experiments

OIR time-domain, large-FOV, high-cadence observations; precise IR follow-up; X-ray spectroscopy; 20- to 30-m telescope

Multiwavelength (radio to X-ray) time-domain, large-FOV, high-cadence observations; post-Swift GRB studies; X-ray spectroscopy; neutrino and gravitational wave observatories; 20- to 30-m telescope

Large-area decimeter-wavelength telescope; large-effective-area X-ray timing and spectroscopy; gravitational wave observatory

Crucial capabilities

Detailed solar and stellar studies of internal rotation and magnetism; surveys of stellar surface rotation, activity, magnetism, and mixing diagnostics; three-dimensional MHD simulations; pulsation theory; UV and X-ray spectroscopy

Panchromatic spectroscopy of a representative sample; large sample for finding diverse objects and correlations with environment; advanced three-dimensional simulation capability; progenitor surveys; nuclear cross sections

Large-scale three-dimensional simulations; discovery of broad range of transients and multiwavelength follow-up; nuclear data and oscillator strengths; abundance studies of extremely low metallicity stars

High-sensitivity X-ray timing and spectral observations of known neutron stars and black holes; laboratory measurements of nuclear equation of state; deep radio pulsar searches; star-cluster white-dwarf searches; gravitational wave detection of compact binary inspirals

Other priorities

Dedicated follow-up for inferring fundamental stellar properties; progress on abundance determinations; laboratory measurements of opacities; support for basic theory and computational astrophysics

NOTE: Acronyms are defined in Appendix C.

conclusive determination that long-duration gamma-ray bursts are associated with the deaths of massive stars showed that the central collapse of some massive stars can produce relativistic jets.

In parallel with the continued exploration of the electromagnetic spectrum, high-energy neutrino and gravitational-wave views of the universe will likely be unveiled in the next decade. Observational techniques such as astrometry, interfer-

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

ometry, and time-domain surveys, all of which are well suited to studies of stellar phenomena, will reach maturity. Given this influx of new data, there are tremendous opportunities in stellar astrophysics, from the lowest-mass stars to compact objects. In “traditional” topics such as stellar structure and evolution and stellar seismology, it is crucial that the United States support the intellectual infrastructure—observational, experimental, and theoretical—to take full advantage of these new opportunities.

Stellar astrophysics has informed many other areas of physics, including nuclear physics, particle physics, and general relativity. Moreover, an understanding of stellar astrophysics is needed for many other problems in astronomy. The study of galaxies at high redshift relies critically on an understanding of the stellar populations that make up those galaxies. The formation of stars, galaxies, and the intracluster medium in galaxy clusters is strongly influenced by the heavy elements, ionizing photons, and explosions produced by massive stars. The Sun continues to be a working template for understanding magnetohydrodynamics and plasma physics “in practice”—physics that is crucial in many other arenas, including that of compact objects. The most distant known object in the universe is now a GRB. These GRBs are (temporarily) much brighter than quasars in the optical-ultraviolet (UV), allowing unique studies of the intergalactic medium at high redshift. A distinct and complementary probe of star formation at these early times is provided by the discovery of nearby extremely metal-poor stars, which constrain the nucleosynthetic products of the first generations of stars.

Just as stellar astrophysics has a significant impact on other branches of physics and astrophysics, it also requires input from other disciplines, notably laboratory experiments. For example, the stellar interior models now in use rely on purely theoretical opacity calculations, but these will be tested by laboratory data in the next decade. Next-generation solar-neutrino experiments can measure the central solar temperature and potentially constrain solar abundances. Key nuclear cross sections are needed for a quantitative understanding of nucleosynthesis and energy generation in stars and stellar explosions. More recently, experiments focused on studying complex hydrodynamic processes have provided key insights into aspects of stellar physics. For example, laboratory experiments and space-physics measurements contributed to the recognition that fast magnetic reconnection occurs only in plasmas with low collision rates, with potential applications to stellar and accretion disk coronae. Continued laboratory studies of basic physical processes important in stellar astrophysics, such as reconnection, angular-momentum transport, and combustion, would complement more traditional observational and theoretical work. More generally, a transition to models grounded in experimental data has the capability to open up new realms of precision stellar astrophysics that could have a major impact on astronomy and physics.

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

One-dimensional stellar models have yielded considerable insight and are quantitatively sufficient for many applications. However, there are also crucial aspects of stellar structure and evolution that require a three-dimensional approach. In these areas, a combined effort involving numerical simulations informing observations, and vice versa, is needed for progress. Studying these inherently three-dimensional problems computationally has become feasible in the past 10 to 15 years, with the tremendous advances in computational resources. Similarly, observational progress on many of these problems is just now feasible. This includes the realization of interferometric techniques in the past decade to illuminate the rotationally distorted shapes of massive stars, the enlarged radii of active M-dwarfs, and the details of how stars lose mass. In addition, observations of the Sun show that convection (inherently multidimensional) plays a critical role in shaping both its rotation profile and magnetic structure. The observational and theoretical study of the solar case provides a point of departure for the study of rotation and magnetism in other stars (science question SSE 1).

Other problems in which a full three-dimensional understanding is crucial are the thermonuclear explosions of white dwarfs in Type Ia supernovae, and the core collapse and explosion of massive stars (science questions SSE 2 and SSE 3). These topics will benefit from the development of new time-domain surveys (SSE discovery area). Already, surveys are finding supernovae with extreme properties in terms of luminosity and energy (both high and low). The evolution leading up to stellar explosions—in particular, mass loss and the dynamics of binaries—is crucial for an understanding of the variety of observed explosions. The study of compact stellar remnants has become mature, but fundamental questions remain about the basic properties of compact objects, such as what determines their masses, spins, and radii (science question SSE 4).

The entire subject of stellar astrophysics will benefit tremendously from large time-domain surveys, given that the variability of the timescales to which large surveys are sensitive matches well with those of many stellar phenomena (see the section “SSE Discovery Area: Time-Domain Surveys”). In addition to explosive events often related to compact objects, a variety of variable and binary stars can be studied. Many time-variable events may be found that have been predicted but not yet discovered (e.g., orphan afterglows of gamma-ray bursts).

THE SCIENCE FRONTIERS

SSE 1. How Do Rotation and Magnetic Fields Affect Stars?

The standard models of stellar structure are traditionally one-dimensional. Observational and theoretical results in the past decade have demonstrated that

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

the study of rotation and magnetic fields, coupled with convection, demands a dynamic, three-dimensional approach that is now achievable. This complex reality is most vividly illustrated by the Sun’s bewildering array of multiscale inhomogeneity, which is now seen with remarkable magnetic sensitivity, and with temporal and spatial resolution, and understood with the help of detailed simulations. It is sobering that this rich structure and its few-gauss average dynamo fields result from the interaction of only a few-kilometers-per-second surface rotation with comparable subsurface convective velocities. The much broader range of stellar rotation, convection, and magnetism poses both a challenge and an opportunity to complement the detailed views of the Sun.

Beyond the Standard Picture

Rotation is now recognized as fundamental to the understanding of stellar evolution. Photometric monitoring programs have mapped the mass-dependent range of pre-main sequence rotation rates and have shown that star-disk interactions (planet and star formation processes) are key to the subsequent evolution of stellar rotation. It is known that Sun-like stars spin down from magnetized winds. Yet recent large samples of active stars now show that the stellar-mass dependence of these winds is uncorrelated with the boundary at which stars become fully convective, contrary to theoretical expectations. Helioseismology has revealed the internal solar rotation, invigorating debate over dynamo theories and confirming the strong coupling between the radiative core and convective envelope. However, mechanisms for angular-momentum transport in stellar interiors, the related mixing, and relevant timescales are still poorly understood. Nascent asteroseismic constraints on internal stellar rotation exist, but crucial knowledge, such as the core rotation rate in pre-supernova stars, is lacking. Understanding rotation-induced mixing in stellar interiors and its effect on stellar and chemical evolution demands that these uncertainties be resolved.

It is known that magnetic fields in cool stars are generated by a dynamo mechanism, but its precise nature, and the heating mechanism for chromospheres and coronae, remain subjects of vigorous debate. Computational advances now allow three-dimensional, radiative MHD models of the outer solar atmosphere that are consistent with observations showing no meaningful “average” or homogeneous chromosphere. Such models are finally making progress on the important question of how magnetic energy from the convective regions makes its way into the magnetically dominated layers. Strong magnetic fields are also detected in hot, higher-mass stars without strong surface convection, and in their stellar remnants. Their origin and evolution, possibly from relic fields, is a mystery.

The prospect of exciting breakthroughs in the understanding of these issues arises from great leaps in stellar observational capabilities, such as interferometry,

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

spectropolarimetry, magnetometry, asteroseismology, and extended waveband surveys from radio and infrared to X-rays and gamma rays. Stars can now be imaged and modeled as three-dimensional objects. Realistic three-dimensional atmosphere simulations are a fruitful new frontier in the understanding of the envelopes of stars. Powerful diagnostics of internal properties are available, and time-domain surveys will allow the study of stellar rotation in samples of unprecedented size, precision, and duration. The complementary approaches of large surveys and detailed studies of smaller samples promise new observational constraints on the origin, nature, and consequences of stellar rotation and magnetic fields. Gaia will provide precise astrometric data and spectroscopic information for an unprecedented sample, which will be invaluable for characterizing stellar properties. These will be combined with unprecedented new capabilities for high-resolution solar observations, particularly with the Advanced Technology Solar Telescope (ATST) and the Solar Dynamics Observatory (SDO). On the theoretical side, fully three-dimensional, time-dependent MHD solar simulations will provide the template for more realistic two-dimensional and three-dimensional stellar models. Against this backdrop, the following subsections describe five important problems involving stellar rotation and magnetic fields that seem particularly ripe for progress.

How Are Magnetic Fields Generated in Stars?

Helioseismology, spectropolarimetry, and radio gyrosynchrotron observations offer new tools for measuring the complex global magnetic structures in the Sun and stars. Local helioseismology with the SDO (a satellite to be launched in early 2010) will revolutionize astronomers’ vision of subphotospheric magnetic fields, while sensitive polarimetry obtained with ATST (now under construction) will unveil the dynamic magnetized atmosphere from photosphere to corona (Figure 5.1). Lower-spatial-resolution coronal magnetometry over the full disk using microwave gyrosynchrotron or IR Zeeman measurements could effectively sample intermittent solar atmospheric explosive events. This would also be a powerful trigger for more sensitive ATST observations. This new generation of high temporaland spatial-resolution magnetometry will help disentangle the influence of flares and mass ejections on chromospheric and coronal structure. For stars, accurate spectro polarimetry with high Stokes Q/U/V sensitivity is the demonstrated key to measuring surface magnetism. A significant increase in the stellar sample size and duration of these observations could be made with just a single, dedicated 4-m-class telescope and suitable instrumentation equipped with a spectropolarimeter.

A new generation of sophisticated simulations that include realistic treatments of radiation and non-equilibrium ionization can be harnessed to interpret these data. The panel urges the provision of support for such theoretical investigations and the extension of detailed solar models to the broader stellar regime. This re-

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
FIGURE 5.1 The solar chromosphere has a fundamentally dynamic structure. This spectacular high-resolution CaII image from the Hinode satellite shows how fine-scale magnetic structures (spicules) dynamically set local physical boundary conditions with the photosphere below. A new generation of high-resolution ground-based measurements (ATST) will directly measure this so-far-invisible driving magnetic field from the photosphere far into the solar atmosphere. SOURCE: Y. Suematsu, K. Ichimoto, Y. Katsukawa, T. Shimizu, T. Okamoto, S. Tsuneta, T. Tarbell, and R.A. Shine, High resolution observations of spicules with Hinode/SOT, p. 27 in First Results from Hinode, Astronomical Society of the Pacific Conference Series (S.A. Matthews, J.M. Davis, and L.K. Harra, eds.), Vol. 397, Astronomical Society of the Pacific, San Francisco, Calif., 2008. Courtesy of Dr. Y. Suematsu, National Astronomical Observatory of Japan.

FIGURE 5.1 The solar chromosphere has a fundamentally dynamic structure. This spectacular high-resolution CaII image from the Hinode satellite shows how fine-scale magnetic structures (spicules) dynamically set local physical boundary conditions with the photosphere below. A new generation of high-resolution ground-based measurements (ATST) will directly measure this so-far-invisible driving magnetic field from the photosphere far into the solar atmosphere. SOURCE: Y. Suematsu, K. Ichimoto, Y. Katsukawa, T. Shimizu, T. Okamoto, S. Tsuneta, T. Tarbell, and R.A. Shine, High resolution observations of spicules with Hinode/SOT, p. 27 in First Results from Hinode, Astronomical Society of the Pacific Conference Series (S.A. Matthews, J.M. Davis, and L.K. Harra, eds.), Vol. 397, Astronomical Society of the Pacific, San Francisco, Calif., 2008. Courtesy of Dr. Y. Suematsu, National Astronomical Observatory of Japan.

search includes exploring dynamos in the cores of massive stars and examining a range of rotation rates, masses, and evolutionary states, including fully convective stars and giants and supergiants. Precise radius measurements in stars with differing rotation and starspot properties, along with detailed seismic studies, will allow the measurement of how interior stellar structure is affected by magnetic activity. Understanding magnetized winds in close-binary systems and how they differ from those of single stars may prove important in understanding phenomena such as blue stragglers and cataclysmic variable stars. It is essential to future progress to train and support theorists who work across the boundaries of solar and stellar studies.

What Are the Stellar Surface and Internal Rotation Distributions?

Rotation periods from spot modulation can be now be inferred for large samples of cool stars and brown dwarfs, including slow rotators (Figure 5.2). From stars with a broad range of masses and ages, astronomers can understand angularmomentum evolution and learn about the star-formation process. Obtaining rotation rates and activity measurements in samples of known age will characterize rotation-age relationships, whereas measuring rotation in numerous old-field stars is a new capability with real discovery potential. Large samples including rotation are especially important for understanding binary and interacting-binary evolu-

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 5.2 The light curve of a solar-like star fitted with a multiple-spot model—with two sets of spots at different angular velocities—shows the potential for studying the dependence of magnetic activity patterns on stellar age, metallicity, and rotation using time-series data of relatively high precision and high cadence from planet search programs. In the top panel, CoRoT data are fitted by a model; in the lower panel, the residuals are shown. SOURCE: A.F. Lanza, I. Pagano, G. Leto, S. Messina, S. Aigrain, R. Alonso, M. Auvergne, A. Baglin, P. Barge, A.S. Bonomo, P. Boumier, A. Collier Cameron, M. Comparato, G. Cutispoto, J.R. De Medeiros, B. Foing, A. Kaiser, C. Moutou, P.S. Parihar, A. Silva-Valio, and W.W. Weiss, Magnetic activity in the photosphere of CoRoT-Exo-2a—Active longitudes and short-term spot cycle in a young Sun-like star, Astronomy & Astrophysics 493:193-200, 2009, reproduced with permission © ESO. Courtesy of A.F. Lanza, INAF-Osservatorio Astrofisico di Catania, based on data obtained with CoRoT, a space project operated by the French Space Agency, CNES, with participation of the Science Programme of ESA, ESTEC/RSSD, Austria, Belgium, Brazil, Germany, and Spain.

FIGURE 5.2 The light curve of a solar-like star fitted with a multiple-spot model—with two sets of spots at different angular velocities—shows the potential for studying the dependence of magnetic activity patterns on stellar age, metallicity, and rotation using time-series data of relatively high precision and high cadence from planet search programs. In the top panel, CoRoT data are fitted by a model; in the lower panel, the residuals are shown. SOURCE: A.F. Lanza, I. Pagano, G. Leto, S. Messina, S. Aigrain, R. Alonso, M. Auvergne, A. Baglin, P. Barge, A.S. Bonomo, P. Boumier, A. Collier Cameron, M. Comparato, G. Cutispoto, J.R. De Medeiros, B. Foing, A. Kaiser, C. Moutou, P.S. Parihar, A. Silva-Valio, and W.W. Weiss, Magnetic activity in the photosphere of CoRoT-Exo-2a—Active longitudes and short-term spot cycle in a young Sun-like star, Astronomy & Astrophysics 493:193-200, 2009, reproduced with permission © ESO. Courtesy of A.F. Lanza, INAF-Osservatorio Astrofisico di Catania, based on data obtained with CoRoT, a space project operated by the French Space Agency, CNES, with participation of the Science Programme of ESA, ESTEC/RSSD, Austria, Belgium, Brazil, Germany, and Spain.

tion. Rotation samples of early-type stars will require moderate-resolution, multi-object spectroscopy.

Direct tests of internal stellar rotation through interferometry (resolved imaging of distorted massive stars) and asteroseismology (in a variety of stellar masses and evolutionary regimes) are now possible. Kepler and the Convection, Rotation and Planetary Transits (CoRoT) spacecraft will provide extensive asteroseismic data in the first few years of the 2010-2020 decade. These should be complemented with next-generation, midsize space missions or networks of modest-aperture, ground-based telescopes using long time-baseline and high-cadence campaigns for modest-size and carefully selected samples. Support for theoretical pulsation

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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studies, especially of massive and evolved stars, will also be required so that the full benefits of the new seismic data can be reaped. Extending sophisticated solar simulations of convection and angular momentum transport to the stellar regime (including rotationally induced mixing) should be a priority, as should the training and supporting of young investigators to take advantage of the computational advances and new data.

What Is the Impact of Rotation on Stellar Evolution and Mass Loss?

Rotation can impact stellar structure directly and induce mixing in radiative regions. The effects will be most dramatic in massive, rapidly rotating stars. Evolved massive supergiants often show enhanced N/O and N/C in their atmospheres and winds, implying interior mixing and the dredging up of CNO-processed material from the core. Strong rotation-induced interior mixing can also alter evolution, weakening redward loops. Rotation in massive stars is even observed to approach the critical rate, generating distortion and the associated equatorial gravity darkening that can now be directly measured interferometrically (Figure 5.3). The relatively bright poles drive a strong wind, leading to the bipolar, prolate mass-loss nebulas seen in luminous blue variable (LBV) stars such as Eta Carina. In classical Be and supergiant B[e] stars, near-critical rotation can induce the centrifugal ejection of material into equatorial “excretion disks.” A key issue for future studies is the relative importance of these competing mass-loss effects on the associated angular-momentum loss and its role in spin-down, or in limiting spin-up, during massive star evolution. The insensitivity of centrifugal excretion to metallicity means that it could be particularly important in the first stars.

In intermediate-mass stars ( [solar masses]), asteroseismic studies of convective-core sizes will establish the degree of overshooting and whether it depends on rotation. Progress in theoretical pulsation studies may permit the measurement of internal stellar rotation rates in intermediate-mass stars. Such data will be the most direct diagnostic of the impact of rotation on stellar lifetimes. Multi-object, high-resolution spectroscopy can map out the mixing of nuclear-processed material to the surface and can detect subsurface mixing from changes in the degree of dredge-up. In addition to serving as tests of stellar physics, such abundance data will be a powerful asset in assigning stellar ages and in investigations of chemical evolution. Both photometric and spectroscopic abundance diagnostics will also need to be refined to elucidate mixing patterns as a function of metallicity.

How Are Chromospheres and Coronae Formed?

Much of the knowledge of solar and stellar magnetism comes from observing the activity that it induces in their atmospheres. A key goal is to identify the heating

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 5.3 Temperature variation across the surface of α Cep (Alderamin) imaged with the CHARA interferometry array, interpreted through modeling. The apparent surface brightness is the result of both rotation (hot poles and cool equator) and limb darkening (seeing into higher, cooler layers away from the disk center) and thus does not peak inside the latitude oval surrounding the rotational pole. Yellow lines are constant latitude lines from the standard model used by the authors to interpret the data. The white circle indicates the convolving beam used to produce the image, 0.68 mas. SOURCE: M. Zhao, J.D. Monnier, E. Pedretti, N. Thureau, A. Mérand, T. Ten Brummelaar, H. McAlister, S.T. Ridgway, N. Turner, J. Sturmann, L. Sturmann, P.J. Goldfinger, and C. Farrington. Imaging and modeling rapidly rotating stars: α cephei and α ophiuchi, Astrophysical Journal 701:209, 2009, reproduced by permission of the AAS.

FIGURE 5.3 Temperature variation across the surface of α Cep (Alderamin) imaged with the CHARA interferometry array, interpreted through modeling. The apparent surface brightness is the result of both rotation (hot poles and cool equator) and limb darkening (seeing into higher, cooler layers away from the disk center) and thus does not peak inside the latitude oval surrounding the rotational pole. Yellow lines are constant latitude lines from the standard model used by the authors to interpret the data. The white circle indicates the convolving beam used to produce the image, 0.68 mas. SOURCE: M. Zhao, J.D. Monnier, E. Pedretti, N. Thureau, A. Mérand, T. Ten Brummelaar, H. McAlister, S.T. Ridgway, N. Turner, J. Sturmann, L. Sturmann, P.J. Goldfinger, and C. Farrington. Imaging and modeling rapidly rotating stars: α cephei and α ophiuchi, Astrophysical Journal 701:209, 2009, reproduced by permission of the AAS.

mechanism that transfers energy from the magnetic field to heat the chromosphere and corona, providing observable diagnostics of magnetic activity. Comprehensive, multiwavelength observations of stellar activity from radio to X-ray wavelengths are being carried out at present with such facilities as the Expanded Very Large Array (EVLA), 4-m-class and 8-m-class ground-based telescopes, the Galaxy Evolution Explorer (GALEX), Chandra, and the Hubble Space Telescope (HST), with the newly refurbished Space Telescope Imaging Spectrograph (STIS) instrument. New facilities, especially those focused on ultraviolet and X-ray spectroscopy, will allow such studies to expand to cover much larger samples of stars. The Sun can be observed at very high spatial and temporal resolution (particularly with the expected advent of ATST in the near future), and—along with its own intrinsic interest—also acts as a template for the stellar observations. Solar observations from radio to X-ray wavelengths are needed. Laboratory experiments and plasma simulations should also contribute to this effort.

Both steady and episodic components of magnetic activity and photometric variability are of interest. Steady components illustrate how activity changes on long timescales; flares and coronal mass ejections trace very short-lived energetic events; and spots and activity cycles address changes on timescales from weeks to years. The study of changes in magnetic activity as a function of stellar mass and age will benefit from large time-domain surveys, especially in clusters. Phenomena to be understood include the following: the spindown time and its connection to magnetic activity; the origin of a seeming upper limit to activity (often termed saturation); the still-inexplicable presence of strong, long-lived magnetic fields and magnetic activity in fully convective stars; and the appearance of chromospheres in objects as disparate as red giants and brown dwarfs. All of these diagnostics will help constrain the dynamo mechanism(s) that produce the fields. Additionally, late

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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M-dwarfs are promising targets for finding Earth-mass planets, but the impact of their persistent activity on habitability needs to be understood.

What Is the Origin of Highly Magnetized Stars?

Spectropolarimetry reveals a subset (~5 to 10 percent) of massive, hot stars that possess large-scale magnetic fields with global strengths up to 104 G, dwarfing what is seen on the Sun or other later-type stars. In marked contrast to the dynamo activity cycles of lower-mass stars, the global properties of massive-star fields seem stable for years and even decades. This, and an apparent decline of occurrence fraction with age, hint at a fossil origin during star formation. These strong fields can profoundly affect the mass and angular-momentum loss from winds. The rotational modulation associated with magnetic clouds and surface abundance patterns allows the determination of very accurate rotation periods, with potential for the direct measurement of stellar spindown. Improved spectropolarimeters on larger (10-m-class) telescopes should make possible field detection at the 1-10 gauss level in statistical samples of massive stars. Comprehensive surveys are needed to characterize accurately the underlying stellar populations and thus to provide clearer constraints on the origin of both strong and weak fields.

Not only are main-sequence stars highly magnetized; about 10 percent of field white dwarfs have strong fields (B ~ 106-109 G). The fraction is even larger, approximately 25 percent, for white dwarfs in close binaries. There is also a significant population of neutron stars with magnetic fields ~1014-1015 G. The birthrate of these “magnetars” is uncertain, but they probably represent at least 10 percent, and perhaps up to 50 percent, of all neutron stars. The connection between highly magnetized compact objects and their progenitors remains uncertain. Binarity is clearly implicated for some magnetized white dwarfs, but its potential role in neutron-star magnetism is unknown. Numerical simulations of dynamos during different stages of stellar evolution would clarify the likely origin of compact-object magnetism. Improved statistics on the numbers of magnetized hot stars and observational constraints on internal stellar rotation may also distinguish between fossil-field and dynamo models.

SSE 2. What Are the Progenitors of Type Ia Supernovae and How Do They Explode?

The evidence is strong that Type Ia supernovae (SNe Ia) are thermonuclear explosions of white dwarfs. Yet there is much that is not known. A physical understanding for the mechanism of the explosions and an astronomical understanding of which stars become SNe Ia are sought. The next decade presents opportunities for major advances on these questions.

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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The Standard Picture

A Type Ia supernova is probably an accreting carbon-oxygen white dwarf in a binary system. Theoretical models of those explosions generally match the data, although direct evidence for this mechanism has proved elusive. The nature of the white dwarf’s companion is not established. Although single-degenerate binary progenitors are favored, merging white dwarfs (double-degenerate binaries) are not ruled out. Merging white dwarfs are disfavored because there is not direct evidence for sufficiently many appropriate binary progenitor systems, and the mergers may not lead to explosions with the uniform properties of SN Ia. SN Ia explosions triggered by merging white dwarfs are not ruled out. Type Ia supernovae are an important area of stellar astrophysics and cosmic evolution: they are violent end points of stellar evolution, and they create much of the iron in the universe. Using SN Ia light curves provides strong evidence for the accelerating expansion of the universe. Subtle effects of age and composition on stellar evolution are likely to modify the properties of SNe Ia over cosmic time. Understanding these will strengthen inferences about dark energy. Because these challenging questions in stellar physics have such broad significance, elevating the understanding of SNe Ia to a new level of precision will be an important task for the next decade.

The basic picture of an exploding white dwarf seems sound: only SNe Ia occur in elliptical galaxies, indicating long-lived progenitors for these SNe Ia. The spectrum at maximum light shows no hydrogen, as expected for a highly evolved star, whereas the spectrum seen months after the explosion is a mass of blended lines from iron, as expected for a star that burns its interior to the iron peak. Theory indicates that fusion ignites near the center and that a thermonuclear fusion flame then rips through the star. But theory also shows that the flame is unstable and creates turbulence. This complex, unstable, turbulent flame burns much of the star to radioactive 56Ni. The radioactive decay of that nickel is responsible for the 4 × 109 solar luminosities that allow SNe Ia to be seen halfway across the universe. Although there is a range of outcomes from nuclear burning, and SNe Ia show a factor-of-three range in peak luminosity, there is an effective way to determine the intrinsic brightness of an SN Ia from its light curve in many colors. This is thought to be the result of diffusion of energy from various amounts of 56Ni through the expanding envelope, but a theoretical model that can account quantitatively for the observed relationship is still lacking. The precision of corrections based on light-curve shape makes SNe Ia the most powerful extragalactic distance indicators. But the path of evolution to an accreting white dwarf, the sites of ignition, the turbulent nuclear burning, and a possible transition from subsonic burning to detonation all remain uncertain in this model. Empirical work may be lumping together explosions that have different chemical composition and progenitor paths. We can do better.

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Opportunities for the Coming Decade

Major advances in understanding SNe Ia lie just ahead: supernova samples will improve dramatically, enabling progenitor evolution to be traced, and it can be expected that computations will cope more effectively with the underlying physics of the explosions. Improved observational capabilities across a wide range of wavelengths will be applied effectively to the unsolved puzzles of SNe Ia.

  • Samples. The world’s sample of well-observed SNe Ia is only a few hundred objects. Ground-based surveys underway—such as the Katzman Automatic Imaging Telescope (KAIT), Palomar Transient Factory (PTF), and PanStarrs-1—and planned will discover orders-of-magnitude more events. Light curves in several colors will be determined for most. A large number need firmly measured redshifts and types; a smaller number will have detailed spectroscopic histories. With large samples, the diversity of this class of explosion will be more fully explored to determine the connection between supernova properties and the chemistry and star-forming history of the host galaxies. At present, uncertainties in dust properties pose the limit to the precision of distance measurements with Type Ia supernovae. During the coming decade, measurements in the near IR with the Peters Automated Infrared Imaging Telescope (PAIRITEL) and JWST should provide much better understanding of this problem and provide the most precise and accurate distance measures. Studies may be extended to higher redshift with wide-field IR surveys from space.

  • Tracing evolution. Progress is also expected in tracing the long path of stellar evolution to an SN Ia explosion. This path includes mass transfer and common-envelope phases: matching the numbers and lifetimes of progenitors with the rates and types of stellar explosions is an incomplete task that future synoptic surveys will probe. The close binaries that precede mergers will be sources of gravitational radiation in the galaxy and have broad interest beyond the question of which stars become SNe Ia (or possibly other types of stellar explosions not yet categorized). Some of these binaries will be detected in synoptic surveys that search the time-scales associated with their orbits, which range from hours to years. Matching the observed populations with the evolutionary paths and the observed rates of stellar death requires large, intensive, and carefully characterized surveys to find and count the stars at each stage. In the next decade, surveys in the galaxy should aim to discover binaries that contain white dwarfs approaching the Chandrasekhar mass, on their way to becoming SNe Ia. Progress in binary evolution models, especially poorly understood phases such as common envelope evolution, is needed along with the observational developments.

  • Computation. Major advances in modeling will come from three-dimensional simulations of ignition and burning physics with spatial resolution that can fol-

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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low the turbulence (Figure 5.4). Light curves and time-dependent spectra will be calculated from three-dimensional models. Improvements in three-dimensional radiative-transfer calculations will be important for matching simulations to observations. The required improvement of roughly a factor of 10 in resolution is a challenging, yet reasonable goal. Although the factor-of-104 increase in central processing unit (CPU) capability to do this by brute-force computing alone is not feasible, significant improvements in codes may achieve this goal. As the large observational and model databases accumulate, finding the best matches will demand new ways to analyze the results—perhaps machine intelligence will help. Time series of spectra will map the distribution of ejected composition with velocity for many events. A fruitful confrontation of models with data will ensue. In a decade, today’s conceptual issues for SN Ia explosions, including the possible transition from a deflagration to a detonation, should be settled. This is not the same as saying that the accuracy of theoretical predictions by themselves will exceed the requirements for the most demanding applications of SN Ia. Since the goal in dark-energy studies is to determine the distance to an ensemble of SNe Ia with an accuracy better than 1 percent, a combination of theoretical insight and better empirical evidence will be required. Major advances are also expected in the understanding of important nuclear physics model input such as the 12C+12C fusion rate and electron capture rates. Progress depends on the development of experimental facilities as well as of nuclear theory, because not all relevant transitions can be measured in the laboratory.

FIGURE 5.4 This hydrodynamic simulation of the turbulent development of a Type Ia supernova explosion by means of deflagration, starting with a C+O white dwarf, shows the potential of three-dimensional codes now becoming available. SOURCE: F.K. Röpke, W. Hillebrandt, W. Schmidt, J.C. Niemeyer, S.I. Blinnikov, and P.A. Mazzali, A three-dimensional deflagration model for Type Ia supernovae compared with observations, Astrophysical Journal 668:1132, 2007. Reproduced by permission of the AAS. Courtesy of F. Röpke, Max Planck Institute for Astrophysics, Garching, Germany.

FIGURE 5.4 This hydrodynamic simulation of the turbulent development of a Type Ia supernova explosion by means of deflagration, starting with a C+O white dwarf, shows the potential of three-dimensional codes now becoming available. SOURCE: F.K. Röpke, W. Hillebrandt, W. Schmidt, J.C. Niemeyer, S.I. Blinnikov, and P.A. Mazzali, A three-dimensional deflagration model for Type Ia supernovae compared with observations, Astrophysical Journal 668:1132, 2007. Reproduced by permission of the AAS. Courtesy of F. Röpke, Max Planck Institute for Astrophysics, Garching, Germany.

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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  • Many wavelengths. Major advances in observing SNe Ia from improved searches and more effective follow-up over a wide range of wavelengths from the radio to gamma rays are expected. More-sensitive radio observations with the EVLA and future instruments can pose strict limits on circumstellar matter near the explosion. Recent near-IR observations demonstrate that SNe Ia are better standard candles at 1.6 microns than in the visible, and the infrared corrections for dust extinction are also significantly smaller. This leads to the prospect that SNe Ia, already the best standard candles, can be made more precise and accurate through infrared follow-up. Follow-up in the rest-frame IR can be carried out at low redshifts from the ground, but a space-based survey would be extremely valuable to reach the rest-frame infrared beyond z ~ 0.5. Rolling searches that have a cadence of a few days will provide excellent sampling of thousands of SNe Ia light curves. Spectroscopic follow-up remains a high scientific priority: queue observing on Gemini has been effective, and improved access would be useful. Samples that are drawn from well-defined searches with carefully prescribed selection criteria can be used to establish rates and can be compared across cosmic time to learn more about the gestation time for supernova progenitors.

The ultraviolet is directly observable from the ground for high-redshift samples, where observations hint that the UV emission from SNe Ia is more variable than the optical. There are some observations from HST and Swift, but overall the situation in the UV is not very satisfactory and offers a rich opportunity for future work.

Current 8- to 10-m telescopes have allowed the detailed study of SNe Ia. The next generation of large telescopes (20 to 30 m) will provide opportunities to study distant supernovae in much the same way that the nearby sample is studied. This will provide direct comparisons for observing any drifts in the properties of supernovae with cosmic epoch that may result from shifts in the mean age of the stellar population and chemical evolution in galaxies. Very large telescopes will be more capable for the high-resolution spectroscopy needed to search for residual gas in the neighborhood of the explosions and for the photon-starved work of spectropolarimetry. Polarization measurements teach astronomers about asymmetries in explosions that will be fruitful to compare with predictions of multidimensional explosion models. Near-IR spectroscopy is still poorly explored for SNe Ia, and the advantages of large telescopes with adaptive optics (AO) systems for this work are substantial.

In our galaxy, Chandra has produced beautiful X-ray observations of Tycho’s and Kepler’s SNe (Figure 5.5), giving both spatial and spectral information on the distribution of heavy elements. Explosion models do not yet match the observations well. Detection of faint X-ray lines with future missions is promising for providing diagnostics of the explosion mechanism and the metallicity of the progenitor. 56Ni

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 5.5 In this X-ray image of Tycho’s supernova remnant (known to be a normal Type Ia supernova from echo emission), the blue outer emission is from the interstellar shock wave and the flocculent emission is from ejecta, which are Si and Fe rich. Red, green, and blue colors are assigned to bands 0.95-1.26, 1.63-2.26, and 4.1-6.1 keV of the Chandra X-ray telescope. SOURCE: J.S. Warren, J.P. Hughes, C. Badenes, P. Ghavamian, C.F. McKee, D. Moffett, P.P. Plucinsky, C. Rakowski, E. Reynoso, and P. Slane, Cosmic-ray acceleration at the forward shock in Tycho’s supernova remnant: Evidence from Chandra X-ray observations, Astrophysical Journal 634(1):376, 2005, reproduced by permission of the AAS.

FIGURE 5.5 In this X-ray image of Tycho’s supernova remnant (known to be a normal Type Ia supernova from echo emission), the blue outer emission is from the interstellar shock wave and the flocculent emission is from ejecta, which are Si and Fe rich. Red, green, and blue colors are assigned to bands 0.95-1.26, 1.63-2.26, and 4.1-6.1 keV of the Chandra X-ray telescope. SOURCE: J.S. Warren, J.P. Hughes, C. Badenes, P. Ghavamian, C.F. McKee, D. Moffett, P.P. Plucinsky, C. Rakowski, E. Reynoso, and P. Slane, Cosmic-ray acceleration at the forward shock in Tycho’s supernova remnant: Evidence from Chandra X-ray observations, Astrophysical Journal 634(1):376, 2005, reproduced by permission of the AAS.

and 56Co are powerful gamma-line emitters, and SNe Ia are copious producers of these isotopes (more than half the mass of the explosion is converted to iron-peak elements). In principle, a detailed map of the speed and abundance of radioactive 56Ni and 56Co would sharply constrain the explosion physics. To get a reasonable rate of events, the sensitivity of a useful gamma-ray instrument needs to be much better than has been achieved so far. The detection of the integrated gamma-ray background from all the SNe Ia that have ever occurred over cosmic time would be a major accomplishment and warrants further study. Instrument development is needed in the coming decade for this form of astronomy to realize its potential.

SSE 3. How Do the Lives of Massive Stars End?

What they lack in numbers, massive (initial mass ) stars make up for in energy and extreme conditions. In their deaths, they produce spectacular fireworks—supernovae and gamma-ray bursts—that leave behind exotic objects: neutron stars and black holes. They create the elements necessary for life. The interior of a “core-collapse supernova” is a physical laboratory with conditions not seen elsewhere in the universe. The neutrino burst that announces the central core collapse is one of the most powerful events in the universe, and the jet that makes a GRB is one of the fastest flows of matter. Yet no one is really sure how it all works.

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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The Standard Picture

Most massive stars end their lives when their inner cores of heavy elements collapse to form neutron stars or black holes. This releases an enormous amount of energy, chiefly radiated away as neutrinos. As already noted in the late 1930s, if a small fraction—approximately 1 percent—of this binding energy could be tapped, this could account for the comparatively modest kinetic energy of common supernovae. For nearly 70 years, however, finding a robust mechanism to drive the explosion has proven elusive. Today, the picture is both complicated and made more tractable by the realization that probably not every star blows up (or collapses) in the same way. In particular, stars that die with rapidly rotating cores have an additional source of energy to tap, and the conduit may involve magnetic fields. This has probably been illustrated by observations showing that “long-soft” gamma-ray bursts are connected with massive-star death and are often accompanied by supernovae. In the next decade, driven especially by advances in computation and large observational surveys, it is likely that major advances will occur in the understanding of how massive stars of all masses, metallicities, and rotation rates end their lives.

Opportunities for the Coming Decade: The Progenitor Stars

The outcome of core collapse depends on the properties of the pre-supernova star. The greatest uncertainties lie at the extremes of mass, metallicity, and rotation rate. On the low-mass end , what is the main-sequence mass that separates white-dwarf remnants from supernovae? Might some progenitor stars even have a white dwarf (carbon and oxygen or oxygen and magnesium) core? At the high-mass end, does the well-established metallicity dependence of line-driven winds for lower-mass stars carry over to the episodic ejections that now appear to dominate for very massive stars? A deeper understanding of mass loss is essential to extend what is learned from relatively close Type II SNe (those with observed hydrogen) to those that exploded when the universe was young. It is also important for determining what main-sequence mass separates stars that form neutron stars from those that form black holes.

Recent models suggest that stars of only explode with neutrino heating alone but that more massive stars are more tightly bound and are harder to blow up this way. About a dozen Type II supernovae currently have reliably determined progenitors based on pre-supernova archival imaging, especially with HST. These progenitors generally have properties consistent with main-sequence masses around 10 to , as expected for average core-collapse SNe, although a Type IIn (narrow line) SN has been found to have a massive, luminous blue variable progenitor (Figure 5.6). The strong episodic mass loss in LBVs affects the appearance of the

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 5.6 Three views of the location of a supernova in galaxy NGC 266. (a) In 1997, five stars can be located in the field, one of them a luminous blue variable (LBV), inside the white circle. (b) The supernova, SN 2005gl, is observed in October 2005, at the location of the LBV. (c) When the supernova has faded, the location of the LBV is now empty, clear evidence that the star seen in 1997 was the progenitor. Using multicolor imaging of nearby galaxies with a space telescope or a large ground-based telescope, many more progenitor stars should be identified. SOURCE: Reprinted by permission from Macmillan Publishers Ltd.: Nature, A. Gal-Yam and D.C. Leonard, A massive hypergiant star as the progenitor of the supernova SN 2005gl, Nature 458:865, 2009, copyright 2009.

FIGURE 5.6 Three views of the location of a supernova in galaxy NGC 266. (a) In 1997, five stars can be located in the field, one of them a luminous blue variable (LBV), inside the white circle. (b) The supernova, SN 2005gl, is observed in October 2005, at the location of the LBV. (c) When the supernova has faded, the location of the LBV is now empty, clear evidence that the star seen in 1997 was the progenitor. Using multicolor imaging of nearby galaxies with a space telescope or a large ground-based telescope, many more progenitor stars should be identified. SOURCE: Reprinted by permission from Macmillan Publishers Ltd.: Nature, A. Gal-Yam and D.C. Leonard, A massive hypergiant star as the progenitor of the supernova SN 2005gl, Nature 458:865, 2009, copyright 2009.

supernova, and an understanding of the relation between stellar properties, episodic mass loss, and supernova type is needed. Observations of young massive star clusters show that highly magnetized neutron stars, magnetars, form from stars with main-sequence masses above 40 to . One possibility is that rotation and magnetic fields become a bigger factor in the explosion as the mass increases. Some ultrabright supernovae such as SN 2007bi suggest that the most massive stars may not even die by core collapse at all, but by an electron-positron “pair instability.” Binary evolution can also affect the appearance of a supernova through mass transfer to or mass loss from the progenitor star. Supernovae with stripped H envelopes (Types IIb, Ib, Ic) may have binary, as well as single-star, progenitors. Better understanding of binary-star populations is needed to discern these possibilities.

New surveys, such as PTF and the Catalina Real-Time Transient Survey, are revealing the surprising diversity of core-collapse supernovae (Figure 5.7). In the next decade, large-volume optical and near-IR time-domain surveys will increase the observational database of supernovae by orders of magnitude. In particular, surveys with approximately 5-day cadence and volumes 10 to 100 times larger than current surveys could double the known SNe population in about 2 years. These surveys will find the very rare events that are often keys to progress. They will also increase the number of known pre-supernova stars, detect large numbers of LBV-like outbursts, and determine whether there is a class of “orphan afterglows” (long-wavelength emission from GRBs for which the higher-energy emission is beamed out of our line of sight). These discoveries will help to constrain directly the white dwarf–neutron star transition mass of the progenitor star, the maximum stellar mass for main-sequence stars, the uncertain stellar evolution prior to core collapse (e.g., the relative importance of steady winds versus LBV eruptions), and the role of rotation and magnetic fields in core-collapse explosions.

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 5.7 The wide variety of light curves of likely core-collapse supernovae is shown, along with that of one Type Ia supernova. The light curves provide physical constraints on the explosion parameters. SOURCE: N. Smith, W. Li, R.J. Foley, J.C. Wheeler, D. Pooley, R. Chornock, A.V. Filippenko, J.M. Silverman, R. Quimby, J.S. Bloom, and C. Hansen, SN 2006gy: Discovery of the most luminous supernova ever recorded, powered by the death of an extremely massive star like η Carinae, Astrophysical Journal 666(2):1116, 2007, reproduced by permission of the AAS. Courtesy of Nathan Smith, Astronomy Department, University of California, Berkeley.

FIGURE 5.7 The wide variety of light curves of likely core-collapse supernovae is shown, along with that of one Type Ia supernova. The light curves provide physical constraints on the explosion parameters. SOURCE: N. Smith, W. Li, R.J. Foley, J.C. Wheeler, D. Pooley, R. Chornock, A.V. Filippenko, J.M. Silverman, R. Quimby, J.S. Bloom, and C. Hansen, SN 2006gy: Discovery of the most luminous supernova ever recorded, powered by the death of an extremely massive star like η Carinae, Astrophysical Journal 666(2):1116, 2007, reproduced by permission of the AAS. Courtesy of Nathan Smith, Astronomy Department, University of California, Berkeley.

Radio and X-ray observations will also probe shock interactions in the circumstellar medium and mass loss in the hundreds to thousands of years leading up to the explosion. The EVLA will allow detailed radio observations of all types of core-collapse supernovae. For young galactic supernova remnants, the Chandra X-ray observatory is a powerful tool and has yielded useful imaging and composition information on objects such as Cassiopeia A. Higher sensitivity (by a factor of approximately 10) is required to obtain spectra of extragalactic supernovae. Wide-field X-ray and UV surveys should yield observations of shock breakout from core-collapse supernovae, which can be used to determine the radii of the progenitor stars and to tie down the times of the explosions. To date, this has been observed only for the Type Ib supernova SN 2008D, which had a relatively compact progenitor and produced fainter, harder shock-breakout emission. The more common Type II supernovae emit longer, brighter transients, chiefly in the ultraviolet. Swift, which discovered SN 2008D, and GALEX have made a start on this problem.

How Do the Stars Explode?

On the theoretical front, it is not currently understood which stars leave behind neutron stars and which leave behind black holes, nor is it understood what determines the observed masses, spins, and magnetic fields of these compact objects (see discussion of science question SSE 4). In the next decade, the exponential increase in computer power, plus the wide availability of multidimensional codes that scale well on these big machines, will lead to major advances in the understanding of how

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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all kinds of stars explode. It is just now becoming feasible to do three-dimensional simulations of stellar core collapse with multigroup neutrino transport coupled to the hydrodynamics, an essential for a first-principles model. By the middle of the decade, such calculations will be commonplace and should, at a minimum, answer the long-standing problem of whether the simplest (non-rotating) neutrinotransport model works for any range of stellar masses. What about the rest? Driven in part by the interest in gamma-ray bursts, codes that realistically couple magnetic fields, rotation, and general relativity in the context of the core-collapse problem are also in an advanced stage of development. Neutrinos have not yet been included in these codes in a realistic way, but before the end of the decade they will. By the end of the decade there should be a quantitative theory of how massive stars of all masses die—given the pre-supernova characteristics. While computational modeling of core-collapse supernovae may produce the most spectacular results, analytic “pencil and paper” theory will also remain crucial for the interpretation of numerical results, for refining the microphysics that goes into the simulations, and for exploring uncertain aspects of stellar evolution and core collapse.

On the observational front, spectroscopic and spectropolarimetric observations of supernovae will ensure major advances in studies of the composition, kinematics, and asphericity of the explosion at a time when numerical models will become increasingly predictive. When combined with time-domain surveys that will discover new and rare events, these observations will significantly advance the understanding of core-collapse explosion physics. As multidimensional simulations become, it is hoped, increasingly successful at producing supernovae, comparison to the observed polarimetry is likely to become particularly productive. Large telescopes (10 to 30 m) are needed for spectropolarimetric observations, while smaller telescopes, which can spectroscopically study broad lines (1,000 km/s resolution), are important for providing input for modeling supernova parameters. Estimates of supernova energy are useful constraints on the supernova mechanism. The observation of neutron-star kicks received during a supernova provides another constraint on the symmetry of the explosion mechanism.

A gravitational wave and/or neutrino detection of a nearby SN in the next decade would provide a wealth of unique information about the dynamics in the central approximately 100 km during core collapse. Such a detection would provide stringent and unique tests of theoretical models as well as a laboratory for studying the properties of the neutrino itself. Unfortunately, a neutrino detection can only be made for a very nearby (e.g., in the galaxy or Large Magellanic Cloud [LMC]) supernovae, and the prospects for gravitational wave detections depend on uncertain core-collapse physics.

Only about 1 in 100 massive-star deaths end in a GRB. The distinguishing properties of the stars that end this way are not known, although a high central angular momentum is likely to play a role. A combination of theoretical studies

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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of stellar evolution and larger observational samples of GRB-SN associations and more “orphan afterglows” are needed for progress on this question.

Gamma-ray line signals from radioactive decays offer a way of sampling SN nucleosynthesis and dynamics in a most direct way, if the signals can be seen. The strongest gamma-ray line expected from galactic supernova remnants that are centuries old comes from the decay of 44Ti. The only remnant detected to date is Cas A, which is about 330 years old. The Nuclear Spectroscopic Telescope Array (NuSTAR) should provide more detections of 44Ti through the 68- and 78-keV hard X-ray lines. Determining the amount of 44Ti in an explosion is important for determining the mass cut between the central compact object and the ejected material. More generally, nuclear physics is a crucial ingredient in core-collapse SN calculations. This includes the nuclear equation of state, rates for electron capture on nuclei, properties of neutron-rich nuclei produced in the r-process, and, potentially, the physics of neutrino oscillations in the core-collapse environment. In the next decade, advances in observations, theory, and laboratory nuclear physics will likely determine whether some core-collapse SNe can produce r-process nuclei (approximately half of the nuclei above the iron peak); this would solve one of the major puzzles regarding the origin of heavy elements.

Blasts from the Past

The most distant point source in the universe is now a GRB at redshift 8.3. GRBs can be used as tools for exploring stellar evolution throughout cosmic time because of their brightness and the penetrating power of gamma rays. GRBs may be the deepest direct probe of first-generation stars. In addition, observations of the composition of nearby metal-poor stars provide information on the chemical traces of early supernovae and their evolution, and thus provide a critical piece of information for understanding supernovae. Currently, the number of very metal-poor stars known is extremely small, and larger samples are critical for progress. This requires large-scale surveys and high-resolution spectroscopic follow-up with optical telescopes up to 30 m in diameter.

SSE 4. What Controls the Mass, Radius, and Spin of Compact Stellar Remnants?

The deaths of stars give rise to compact stellar remnants—neutron stars, black holes, and white dwarfs—that produce the most exotic and energetic phenomena in the universe, from the brightest known sources of radiation to the steadiest astrophysical clocks. The properties of compact stellar remnants provide not only unique information about the late stages of stellar evolution, but also a testing

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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ground for studying physics that is otherwise inaccessible to astronomers and physicists alike.

Many fundamental questions about compact stellar remnants center on understanding, both empirically and theoretically, their basic physical properties, including mass, radius, spin, and magnetic field. Here several key questions in which major progress can be made in the coming decade are highlighted.

What Is the Equation of State of Ultradense Matter?

The basic properties of neutron stars are closely coupled to the physics of their interiors. Neutron star cores have densities up to several times that found in atomic nuclei (ρ ~ 3 × 1014 g cm3) and are therefore the densest objects, without event horizons, known. Because the equation of state for such ultradense matter is still poorly constrained, the basic compositions of neutron star cores are unknown, with exotic new states of matter (e.g., deconfined quark matter or Bose condensates) possible. This high-density regime is mostly inaccessible to terrestrial laboratories, but its properties determine the mass-radius relation of neutron stars, providing an astrophysical probe (Figure 5.8). In the past decade, the ability to measure masses of binary radio pulsars has dramatically improved, and several X-ray techniques for constraining neutron-star radii have been developed.

Radio pulsars are the most commonly observed neutron stars known, with almost 2,000 cataloged to date. Precise neutron-star-mass determinations can be obtained in “recycled” binary millisecond pulsars by the measurement of relativistic orbital effects, but only a handful of suitable systems are known. Pulsars with unusually high or low mass directly constrain the dense matter equation of state, and in fact the measurement of a single neutron star with mass would rule out most forms of exotic material in neutron star cores. Current and future galactic surveys for millisecond radio pulsars are crucial for detecting new systems for neutron-star-mass measurements (currently only approximately 1 percent of pulsars are suitable), as well as finding other exotic pulsars for gravitational-wave-detection experiments and strong-field gravity tests. The past 5 years have seen the discovery of several eccentric binary millisecond radio pulsars that contain neutron stars likely more massive than .

Radio surveys with current or soon-to-be-available facilities (such as the Green Bank Telescope, Arecibo Observatory, Parkes Observatory, Effelsberg Radio Telescope, and the EVLA) should double the number of known radio pulsars in the next decade and (because of computational and instrumentation improvements) should increase the number of millisecond radio pulsars by an even larger factor. Further in the future, the Chinese Five-hundred-meter Aperture Spherical Telescope (FAST) could find approximately 5,000 new radio pulsars by 2020, while a

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
FIGURE 5.8 The radius of a neutron star is sensitive to details of the equation of state for matter at near-nuclear densities. Measurement of masses and radii for neutron stars in the next decade should suffice to rule out some of the equations of state displayed here (as black and green curves) in a figure from Lattimer and Prakash, 2007. The green region at the lower right is excluded from observations of pulsar J1748-2446ad. The recent discovery of pulsars with rules out equation-of-state models that do not extend to such high masses. SOURCE: Reprinted from J.M. Lattimer and M. Prakash, Neutron star observations: Prognosis for equation of state constraints, Physics Reports 442(1-6):109-165, copyright 2007, with permission from Elsevier.

FIGURE 5.8 The radius of a neutron star is sensitive to details of the equation of state for matter at near-nuclear densities. Measurement of masses and radii for neutron stars in the next decade should suffice to rule out some of the equations of state displayed here (as black and green curves) in a figure from Lattimer and Prakash, 2007. The green region at the lower right is excluded from observations of pulsar J1748-2446ad. The recent discovery of pulsars with rules out equation-of-state models that do not extend to such high masses. SOURCE: Reprinted from J.M. Lattimer and M. Prakash, Neutron star observations: Prognosis for equation of state constraints, Physics Reports 442(1-6):109-165, copyright 2007, with permission from Elsevier.

large-area decimeter-wavelength radio telescope could find approximately 20,000 new radio pulsars, including thousands of new millisecond pulsars. Gamma-ray surveys with facilities such as Fermi Gamma-ray Space Telescope will likely also contribute, albeit with smaller numbers. Current-generation pulsar surveys and timing observations are completely sensitivity-limited, and larger telescope collecting areas are needed.

The most extreme outliers from such surveys may be of greatest interest. A recent example is the so-called Double Pulsar J0737-3039, an exceptionally relativistic pulsar binary discovered in 2003. That system has provided the most precise tests of general relativity in the strong-field regime to date, and long-term timing (which may require a span of 10 years or more) will eventually allow the measurement of spin-orbit effects on periastron advance, which in turn will determine the neutron-star moment of inertia, strongly constraining the neutron-star equation of state (since the neutron-star mass is already precisely known in that system).

X-ray observations hold great promise as well. Measurements of neutron-star

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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radii by means of X-ray timing are possible by observing certain bright low-mass X-ray binaries (LMXBs) as well as nearby faint isolated millisecond pulsars. For the LMXBs, observations with the Rossi X-ray Timing Explorer (RXTE) have identified both millisecond oscillations during thermonuclear X-ray bursts and longer-lasting accretion-powered millisecond pulsations that encode information about neutron star parameters. Pulse shape and spectral modeling of these phenomena with a larger-area X-ray timing instrument can strongly constrain both neutron-star radius and mass. Broadband X-ray spectroscopy of Eddington-limited radius-expansion bursts in many of these same systems can also independently constrain neutron-star parameters. Neutron-star radii and masses can also be constrained through pulse shape modeling of the faint thermal pulsations seen from some isolated millisecond pulsars. Recent observations with X-ray Multi-Mirror Mission (XMM)-Newton have demonstrated this technique, but a more-sensitive, focusing X-ray telescope is required to obtain strong constraints. In addition, soft X-ray spectroscopic observations of transiently accreting neutron stars in quiescence show thermal X-ray spectra from the cooling neutron star surfaces that yield the neutron star radius when the source distance is known and the atmospheric model is correct. The measurement of these faint targets requires a sensitive focusing X-ray telescope with moderate spectral resolution.

Finally, laboratory nuclear physics experiments are expected to provide complementary constraints on the nuclear-matter equation of state. These measurements will constrain some aspects of the neutron-star equation of state that can serve as input to neutron-star models, allowing one to interpret observations, probe models, and constrain the regimes of the neutron-star equation of state not accessible in the laboratory. Major advances will be possible in the next decade. Examples include constraints on the nuclear symmetry energy around nuclear density from precision measurements of the neutron skin thicknesses of heavy nuclei, using parity-violating electron scattering, and—for higher densities—from heavy-ion collisions at a range of energies and asymmetries at various advanced rare isotope facilities, including the Facility for Rare Isotope Beams (FRIB). Nuclear-theory work to identify the most useful signatures of equation-of-state properties in heavy-ion collisions and how to interpret them quantitatively is also needed.

What Is the Spin Distribution and Maximum Spin of Neutron Stars and Black Holes?

Millisecond pulsars (MSPs) are neutron stars that have undergone mass transfer from companion stars in LMXBs and have been “spun up” to rapid rotation rates in the process. The physical processes that stop the transfer of angular momentum and thereby establish the maximum spin rates of neutron stars are currently unknown. One of the primary candidates is the emission of gravitational

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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radiation which, if correct, would be of major importance to current and future gravitational-wave-detection facilities such as the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO). Identifying the correct spin frequency distribution of MSPs will help to determine both the maximum spin rates of neutron stars and the limiting physical processes. Current and future radio and X-ray timing surveys have many fewer selection effects toward rapidly rotating pulsars than those in the past. If such systems are detected, they would directly limit the neutron-star equation of state by determining the maximum radius of the neutron star as a function of its mass for which it does not shed material at its equator. New constraints on neutron-star physics would come from the detection of a neutron star spinning more rapidly than 1,000 Hz.

Astrophysical black holes are completely described by just two quantities, their mass and spin. Although the masses of stellar black holes in X-ray binaries have been measured dynamically for decades, it is only in the past few years that it has become possible to constrain the spins of black holes. The spin is constrained by determining the inner radius of the accretion disk, either by fitting the thermal disk component of the X-ray continuum spectrum, or through the relativistically broadened shape of the Fe K disk fluorescence line. The radius inferred by these methods is believed to be comparable to that of the last stable orbit in general relativity, but there are systematic uncertainties in this association that limit the precision of current constraints on black hole spin. Inferences about spin have now been made in 10 systems (using a variety of X-ray missions, most recently including Chandra, XMM-Newton, and Suzaku) and most are rotating significantly, with a wide variety of black hole spins measured, and several are believed to be spinning near the maximal amount allowed by general relativity. A slowly spinning, disk-accreting black hole must double its mass in order to spin rapidly, which is impossible for a black hole in an X-ray binary; thus, the measured spin distribution is essentially sampling the birth properties of these black holes. An alternative way of measuring black hole spin is through the spin-orbit coupling of a pulsar with a black hole. This method will require pulsar searches to discover pulsars with black hole companions. Gravitational-wave detection of stellar black-hole/black-hole or black-hole/neutron-star inspirals offers still another promising method for measuring black hole spins (and masses).

A larger sample of black hole spin measurements will provide very strong constraints on models of massive star evolution and core-collapse supernovae. More broadly, an improved understanding of black hole spin can be used to address a number of important issues, including the role of black hole spin in producing jets and in powering GRBs. Accurate knowledge of the black hole spin distribution is also crucial for designing theoretical search templates required for the direct detection of gravitational waves from black-hole/black-hole and black-hole/neutron-star mergers. In order to make continued progress, soft X-ray continuum spectros-

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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copy and medium- to high-resolution X-ray line spectroscopy of a larger sample of black holes in X-ray binaries is needed, requiring a more sensitive telescope than Chandra or XMM-Newton. In addition, further numerical and theoretical work on general-relativistic MHD models of black hole accretion disks is essential for interpreting X-ray continuum observations, and better theoretical models of the X-ray irradiation and fluorescent Fe line emission from the inner accretion disk are needed to interpret Fe line spectra.

What Determines the Initial-Final Mass Relation Connecting Progenitors to White Dwarfs?

Observational constraints on the initial-final mass relation come primarily from white dwarfs in open clusters and require accurate main-sequence turnoff ages plus white-dwarf cooling ages (to infer the initial mass) and precise final masses (Figure 5.9). The largest source of error is the variance in theoretical mass-lifetime relationships for stars with convective cores (and, by extension, open cluster ages). Gaia will provide precise distances and membership information for open clusters, and asteroseismology from missions such as CoRoT and Kepler may constrain the sizes of convective cores. Uncertainties in the white-dwarf cooling timescale also need to be addressed, in particular the properties of the atmospheric “blanket” that governs heat transport and is a poorly understood by-product of asymptotic giant branch (AGB) evolution. The mass of the blanket is a result of the processes that end the AGB evolution of the star. The recent discovery of carbon-atmosphere white dwarfs is as yet unexplained and points to interesting discovery areas in white-dwarf formation. Also crucial to understanding white-dwarf properties is the onset of crystallization, which alters the internal energy structure and causes an abrupt change in effective temperature and luminosity. Both have been constrained by asteroseismology on a small number of stars, giving a partial picture and great promise for future advances.

The mass of a white dwarf originating from a single star such as the Sun is related to the luminosity of the star as it leaves the AGB. This luminosity, and thus the white-dwarf mass, is determined by the mass-loss process. Theoretical and observational studies of the dependence of mass-loss rates on stellar parameters have not reached a consensus, and prescriptions advocated and used differ dramatically from one another. Empirical and theoretical formulas span a wide range of slopes. To match observed initial-final mass relations with evolutionary and population models, empirical laws have been “corrected” with a variety of tuning parameters. However, there is no widely accepted or demonstrably correct mass-loss formula for the mass loss that produces white-dwarf stars, and thus no predictive power for extrapolating to understudied populations (such as young, low-metallicity cases).

To measure mass-loss rates for large numbers of stars, infrared surveys and

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 5.9 The fates of stars—the relationship between their initial and their final mass—for stars that lose mass and thereby (if they start with ) avoid exploding as supernovae, are observationally constrained by the study of white dwarf stars in clusters of known age. Considerable uncertainty exists for intermediate-mass stars (2 to ), including how much initial composition affects the result. With the detection of many more white dwarfs in clusters, enough information to constrain mass-loss models and perhaps enough to extrapolate to unobservable populations (low metallicity, high mass—as in the early universe) may become available. SOURCE: J.S. Kalirai, B.M.S. Hansen, D.D. Kelson, D.B. Reitzel, R.M. Rich, and H.B. Richer, The initial-final mass relation: Direct constraints at the low-mass end, Astrophysical Journal 676(1):594, 2008, reproduced by permission of the AAS. Courtesy of Jason Kalirai, Space Telescope Science Institute.

FIGURE 5.9 The fates of stars—the relationship between their initial and their final mass—for stars that lose mass and thereby (if they start with ) avoid exploding as supernovae, are observationally constrained by the study of white dwarf stars in clusters of known age. Considerable uncertainty exists for intermediate-mass stars (2 to ), including how much initial composition affects the result. With the detection of many more white dwarfs in clusters, enough information to constrain mass-loss models and perhaps enough to extrapolate to unobservable populations (low metallicity, high mass—as in the early universe) may become available. SOURCE: J.S. Kalirai, B.M.S. Hansen, D.D. Kelson, D.B. Reitzel, R.M. Rich, and H.B. Richer, The initial-final mass relation: Direct constraints at the low-mass end, Astrophysical Journal 676(1):594, 2008, reproduced by permission of the AAS. Courtesy of Jason Kalirai, Space Telescope Science Institute.

molecular-line surveys have already proven useful, although the IR measures require reliable gas/dust ratios and the radio lines measure mass loss at a different time (farther out in the flow); many mass-losing stars have variable outflows. The modeling of mass-loss processes requires non-local thermodynamic equilibrium hydrodynamics with shocks, non-equilibrium chemistry, and grain nucleation and growth. Strong tests of models are coming from interferometric studies of the structures of the atmospheres of mass-losing stars, as, for example, the discovery of “molecular shells” at about twice the stellar radius and coincident with the region where dust is expected to form. Molecular lines are ideal for mass-loss studies because (1) they trace the gas, and (2) they carry velocity information. CO has been widely used in our galaxy. The Atacama Large Millimeter Array (ALMA) should be

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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able to detect CO and thus measure mass-loss rates for AGB and red giant branch (RGB) stars in the Magellanic Clouds, a vital laboratory for the study of these populations. Additionally, CO traces mass loss for both C-rich and O-rich giants, which is not the case for tracers currently used to study the highest-mass-loss-rate objects in the Small Magellanic Cloud and LMC.

Most white-dwarf stars are believed to be composed of carbon and oxygen. Observations of novae show that some O-Ne-Mg white dwarfs are formed, presumably from relatively high mass progenitors, with details of the formation channel(s) as yet unclear. There are also He white dwarfs, including a surprisingly large population of He white dwarfs in very metal-rich clusters and a population of single-field He white dwarfs. Explaining the origin and evolution of these different classes of objects should be illuminating. The formation of He white dwarfs is understood only in the context of binaries, so further understanding of other ways of forming them potentially by single stars or through disrupted binaries or ejections from dense star clusters is needed. Stars that ignite C in their degenerate cores before losing their envelopes to mass loss may also produce unusual thermonuclear supernovae. Very little is known about this potential channel, but large surveys should yield valuable information.

SSE DISCOVERY AREA: TIME-DOMAIN SURVEYS

Astronomical timescales evoke the long stretches of time, reckoned in gigayears, that characterize cosmic expansion and most phases of stellar evolution. For these phenomena, a single comprehensive survey can reveal the essential facts, as in a Hertzsprung-Russell diagram for a cluster. But there are phenomena of rotation and pulsation, of orbiting binaries, of explosions and mass loss, and most spectacularly, of stellar death, for which the physical timescales are measured in seconds, days, or months. For a wide range of stellar events, knowledge has been obtained by observing through narrow windows in time, often set by single-investigator observing strategies or by the technical capabilities of the detectors being used. Narrow windows produce limited views. The panel anticipates that in the coming decade the burgeoning technological change due to detector development, fast computers, automated pipelines, and the ability for the entire community to interact with large-volume public databases (from a distance, over the Internet) will lead to significant scientific progress in revealing and exploring a wide range of stellar phenomena. For these reasons, time-domain surveys represent a significant discovery potential for the study of stars and stellar evolution.

Discovery in the time domain in the next decade will be driven by detectors with large fields of view, which scan the sky with approximately daily-weekly cadence and provide all-sky data sets. In addition to unanticipated discoveries (Figure 5.10), there are expected events; Table 5.2 gives a sample of the wide range

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 5.10 An unusual optical transient with a nearly symmetric light curve was discovered during the Hubble Space Telescope Cluster Supernova Survey. Most explosive events have a rapid rise and a slower decline; this symmetry is anomalous. From its redshift and comparison with other recent supernova discoveries, the object was found to be an unusual, luminous supernova. Surprising, rare events continue to be discovered as time-domain surveys expand in reach and duration and improve in cadence and precision. SOURCE: K. Barbary, K.S. Dawson, K. Tokita, G. Aldering, R. Amanullah, N.V. Connolly, M. Doi, L. Faccioli, V. Fadeyev, A.S. Fruchter, G. Goldhaber, A. Goobar, A. Gude, X. Huang, Y. Ihara, K. Konishi, M. Kowalski, C. Lidman, J. Meyers, T. Morokuma, P. Nugent, S. Perlmutter, D. Rubin, D. Schlegel, A.L. Spadafora, N. Suzuki, H.K. Swift, N. Takanashi, R.C. Thomas, and N. Yasuda for the Supernova Cosmology Project, Discovery of an unusual optical transient with the Hubble Space Telescope, Astrophysical Journal 690(2):1358, 2009, reproduced by permission of the AAS. Courtesy of Kyle Barbary, University of California, Berkeley, Lawrence Berkeley National Laboratory.

FIGURE 5.10 An unusual optical transient with a nearly symmetric light curve was discovered during the Hubble Space Telescope Cluster Supernova Survey. Most explosive events have a rapid rise and a slower decline; this symmetry is anomalous. From its redshift and comparison with other recent supernova discoveries, the object was found to be an unusual, luminous supernova. Surprising, rare events continue to be discovered as time-domain surveys expand in reach and duration and improve in cadence and precision. SOURCE: K. Barbary, K.S. Dawson, K. Tokita, G. Aldering, R. Amanullah, N.V. Connolly, M. Doi, L. Faccioli, V. Fadeyev, A.S. Fruchter, G. Goldhaber, A. Goobar, A. Gude, X. Huang, Y. Ihara, K. Konishi, M. Kowalski, C. Lidman, J. Meyers, T. Morokuma, P. Nugent, S. Perlmutter, D. Rubin, D. Schlegel, A.L. Spadafora, N. Suzuki, H.K. Swift, N. Takanashi, R.C. Thomas, and N. Yasuda for the Supernova Cosmology Project, Discovery of an unusual optical transient with the Hubble Space Telescope, Astrophysical Journal 690(2):1358, 2009, reproduced by permission of the AAS. Courtesy of Kyle Barbary, University of California, Berkeley, Lawrence Berkeley National Laboratory.

of stellar science that can be addressed with this type of time-domain survey, at many wavelengths. Follow-up observations of various types are often essential to carry out the science goals. For example, evolved giants and brown dwarfs observed interferometrically show evidence for time-variable spatial structures, possibly associated with dust-cloud formation and weather-like phenomena; follow-up observations with new interferometric facilities will provide important constraints on the physical mechanisms behind the observed time variations. Other kinds of time-domain studies not mentioned in Table 5.2 will also be valuable—particularly, continuous monitoring observations with high cadence for extended duration on individual objects, as in the case of asteroseismology.

This panel’s four science questions all mention time-domain observations as

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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TABLE 5.2 Time-Domain Surveys, Large Field of View, All-Sky Coverage, Daily-Weekly Cadence

Science

Survey Capabilities

Follow-up

Variable Stars and the Sun

 

 

Starspots and rotation

Optical/multicolor

Spectroscopy, spectropolarimetry

Massive stars—LBVs

Optical/IR/UV

Spectroscopy, photometry, spectropolarimetry

Eclipsing binaries

Optical/multicolor, long duration

Photometry, spectroscopy, radial velocities

Clouds and weather on brown dwarfs

Optical/IR

Multiwavelength photometry, spectroscopy; improved models

Pulsating variables—classical, rare

Optical/IR, long duration, Milky Way and nearby galaxies, helio- and asteroseismology

Photometry, spectroscopy; interferometry; improved models

Rare stages of stellar evolution

Optical/IR, long duration, clusters and nearby galaxies

Photometry, spectroscopy; improved models; interferometry

Stellar mergers on dynamical or thermal timescale

Optical/IR, long duration, resolved stellar populations

Photometry, spectroscopy; improved models

Stellar flares

Optical (blue, U/u filter), UV, X-ray, radio

Time-resolved, multiwavelength photometry, spectroscopy; improved models

Solar corona

Optical/IR/radio magnetometry

ATST follow-up of energetic events

Pulsars—rare types

Radio

Radio

Magnetar flares

X-ray, γ-ray

Multiwavelength photometry, spectroscopy

Variable Accreting Systems

 

 

CVs, novae

Optical, UV

Spectroscopy—optical, UV, IR

Tidal disruption of stars

Optical, UV, X-ray, radio

Multiwavelength photometry, spectroscopy; host galaxy properties

LMXBs (black-hole/neutron-star novae, X-ray bursters, superbursters)

X-ray, wide-field

Time-resolved X-ray photometry, spectroscopy

Supernovae and GRBs

 

 

Milky Way supernova

Optical/IR, ν, gravitational waves, γ-ray, IR, radio

Multiwavelength photometry, spectroscopy; rapid response

Supernova searches including rare forms, optical transients

Optical/IR

Multiwavelength photometry, spectroscopy

Shock breakout in supernovae Types II and Ibc

UV, X-Ray

Multiwavelength photometry, spectroscopy

Electromagnetic counterparts to gravitational wave sources

Optical/IR, UV, X-ray

Photometry, spectroscopy—radio; rapid response

Gamma-ray bursts

γ-ray, X-ray

Multiwavelength photometry, spectroscopy; rapid response

Orphan afterglows of GRBs

Optical/IR, radio

Multiwavelength photometry

NOTE: Acronyms are defined in Appendix C.

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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essential to making progress in the next decade (e.g., supernova searches, rotation, episodic mass loss from massive stars, pulsar searches). A few illustrative examples of additional stellar topics from Table 5.2 that have particular discovery potential are described in more detail below.

Accretion-Induced Collapse (Rare Type of Supernovae)

How often does a white dwarf approaching the Chandrasekhar mass in a binary undergo an accretion-induced collapse (AIC) to form a neutron star, rather than blowing up as an SN Ia? An understanding of this question is essential for understanding the evolution of white dwarfs in binary systems and would dramatically constrain the allowed progenitors of SNe Ia. AIC has also been proposed as one of the most promising sites for third-peak r-process nucleosynthesis. AICs are predicted to be accompanied by the ejection of up to approximately 0.01 to of Ni, produced in an accretion disk around the newly formed neutron star. This outflowing Ni produces a short, approximately 1 day, optical/near-IR SN-like transient with a peak luminosity of 1041 to 1042 ergs/s, significantly fainter and of shorter duration than ordinary SNe Ia.

Electromagnetic Counterparts to Gravitational Wave Sources

In the coming decade, it is likely that transient gravitational wave sources will be discovered by experiments such as Advanced LIGO and VIRGO (a gravitational wave detector at the European Gravitational Observatory), with lower-frequency gravitational wave sources perhaps becoming detectable toward the end of the decade. To optimize the astrophysics that results from such detections, it is critical to have nearly simultaneous electromagnetic observations. Wide-field-of-view cameras are essential given the rather poor localizations provided by gravity-wave detectors (fractions of a square degree). A unique electromagnetic counterpart temporally and spatially coincident with a gravitational wave source would provide more confidence in the gravitational wave detection. Combined gravitational wave and electromagnetic observations could potentially provide detailed information about stellar sources, including neutron-star/neutron-star mergers, black-hole/black-hole mergers, short gamma-ray bursts, and (perhaps) core-collapse supernovae. This information is unique given the gravitational wave constraints on the masses and spins (magnitudes and direction) of the objects.

Rare Stages of Stellar Evolution (He Core Flash)

At the tip of the first-ascent RGB, for stars of , He ignition in the degenerate core leads to the He core flash. Very little is known observationally

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

about what happens during the He core flash: for example, does the star lose mass? The ultimate fate of Earth—into the Sun or backing away—depends on whether the Sun will lose 20 percent of its mass before or during this event. Yet one cannot point to a single object in the sky that is currently undergoing an He core flash. The probable duration of this phase is about a thousand years, assuming that the star’s response resembles its response to He shell flashes, evolving on a thermal timescale. One probable observational signature will be erratic pulsation with rapid period changes. Large samples, including stars in clusters of various ages, are needed to identify individual objects in this critical stage of stellar evolution. These data will provide essential input to theoretical models of late-stage stellar evolution.

Eclipsing Binaries and Binary Star Evolution

Eclipsing binary stars are powerful diagnostics of stellar structure and evolution, and they are relatively easy to find in time-domain surveys (Figure 5.11). They also provide a secure way to measure masses and radii for stars of all spectral types, metallicities, and ages. Longer-period eclipsing binaries, although geometrically less favorable, should be found in large surveys with long durations. Such stars will be valuable because their evolution is less likely to be impacted by the presence of the companion (synchronous rotation, enhanced activity), which may influence the stellar radius. Large samples will permit tests of the mass ratios and distributions of orbital separations of close binaries, which will in turn inform theories of binary-star formation and evolution. Studies of interacting binaries will also benefit, particularly for unusual and rare systems such as contact binaries and common-envelope systems, probable precursors to stellar mergers (leading, for example, to blue stragglers). Investigations of white-dwarf/white-dwarf and white-dwarf/massive-star systems are important for understanding the origin of SNe Ia and cataclysmic variables. For very wide eclipsing binaries and stars with large planets, detailed information on the resolved stellar surface can be obtained during the eclipse/transit.

Radio Transients

Stars of all kinds produce a surprisingly wide variety of nonthermal radio emission from timescales of nanoseconds (the giant pulses from the Crab pulsar), to months (the radio afterglows of supernovae). Recently, several relatively small-scale radio surveys have uncovered new forms of transients from known sources, such as extremely rare millisecond-duration pulses from rotating neutron stars (the so-called RRATs, or rotating radio transients), and bright coherent emission from brown dwarfs. Other surveys have found unidentified radio transients in extragalactic blank fields and toward the galactic center. Yet these surveys have covered

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
FIGURE 5.11 Multicolor light curves for an eclipsing binary, from Sloan Digital Sky Survey and 2 Micron All Sky Survey data. The two stars are low-mass, M0 + M1, dwarfs with an orbital period of 2.639 days. Time-domain surveys will provide large numbers of new eclipsing binary systems for stars across the Hertzsprung-Russell diagram, allowing for much-improved basic data for unusual as well as common types of stars. Follow-up for systems such as this one—for example to get radial velocities—will require large telescopes and/or substantial telescope time. SOURCE: Reprinted with permission from A.C. Becker, E. Agol, N.M. Silvestri, J.J. Bochanski, C. Laws, A.A. West, G. Basri, V. Belokurov, D.M. Bramich, J.M. Carpenter, P. Challis, et al., Two-Micron All-Sky Survey J01542930+0053266: A new eclipsing M dwarf binary system, Monthly Notices of the Royal Astronomical Society 386:416, 2008, copyright 2008 Royal Astronomical Society.

FIGURE 5.11 Multicolor light curves for an eclipsing binary, from Sloan Digital Sky Survey and 2 Micron All Sky Survey data. The two stars are low-mass, M0 + M1, dwarfs with an orbital period of 2.639 days. Time-domain surveys will provide large numbers of new eclipsing binary systems for stars across the Hertzsprung-Russell diagram, allowing for much-improved basic data for unusual as well as common types of stars. Follow-up for systems such as this one—for example to get radial velocities—will require large telescopes and/or substantial telescope time. SOURCE: Reprinted with permission from A.C. Becker, E. Agol, N.M. Silvestri, J.J. Bochanski, C. Laws, A.A. West, G. Basri, V. Belokurov, D.M. Bramich, J.M. Carpenter, P. Challis, et al., Two-Micron All-Sky Survey J01542930+0053266: A new eclipsing M dwarf binary system, Monthly Notices of the Royal Astronomical Society 386:416, 2008, copyright 2008 Royal Astronomical Society.

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

either only tiny fractions of the sky or a very small range of timescales. The situation is improving with the development of the Low Frequency Array (LOFAR), the Long Wavelength Array, the Murchison Widefield Array, and ATA-42 (the Allen Telescope Array, configured with 42 radio-telescope dishes). As radio fields of view continue to increase and computing capability grows to allow wide-field, rapid-cadence, radio imaging, new surveys will uncover many more transient events of both known and unknown origin. These events have the potential to tell about particle acceleration, stellar magnetic fields and rotation, strong-field gravity, the interstellar and intergalactic media, the violent deaths of stars, and possibly physics beyond the standard model.

Summary of SSE Discovery Area

In summary, the time domain represents great discovery potential well matched to the timescales that are relevant for stellar phenomena during their lifetimes and their death throes. Astronomers look forward to the next decade as a period of renaissance for stellar astronomy as time information is added to the new advances in three-dimensional spatial resolution and the idealization of a star as a static, spherical object is put to bed.

Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Suggested Citation:"5 Report of the Panel on Stars and Stellar Evolution." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics Get This Book
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Every 10 years the National Research Council releases a survey of astronomy and astrophysics outlining priorities for the coming decade. The most recent survey, titled New Worlds, New Horizons in Astronomy and Astrophysics, provides overall priorities and recommendations for the field as a whole based on a broad and comprehensive examination of scientific opportunities, infrastructure, and organization in a national and international context.

Panel Reports--New Worlds, New Horizons in Astronomy and Astrophysics is a collection of reports, each of which addresses a key sub-area of the field, prepared by specialists in that subarea, and each of which played an important role in setting overall priorities for the field. The collection, published in a single volume, includes the reports of the following panels:

  • Cosmology and Fundamental Physics
  • Galaxies Across Cosmic Time
  • The Galactic Neighborhood
  • Stars and Stellar Evolution
  • Planetary Systems and Star Formation
  • Electromagnetic Observations from Space
  • Optical and Infrared Astronomy from the Ground
  • Particle Astrophysics and Gravitation
  • Radio, Millimeter, and Submillimeter Astronomy from the Ground

The Committee for a Decadal Survey of Astronomy and Astrophysics synthesized these reports in the preparation of its prioritized recommendations for the field as a whole. These reports provide additional depth and detail in each of their respective areas. Taken together, they form an essential companion volume to New Worlds, New Horizons: A Decadal Survey of Astronomy and Astrophysics. The book of panel reports will be useful to managers of programs of research in the field of astronomy and astrophysics, the Congressional committees with jurisdiction over the agencies supporting this research, the scientific community, and the public.

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