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3 Stars and Stellar Evolution Stars and stellar systems are the building blocks of the universe. Stars create most of the chemical elements, and they allow us to explore the laws of physics under conditions impossible to reproduce on Earth. They provide most of the observed light of galaxies and hence are bright signposts that allow the age, scale, shape, and content of the universe to be determined. An understanding of the workings of stars in all their variety is essential to comprehending the universe we see around us. Astronomers now know that all the objects they observe, from the other planets in the solar system to the most distant quasars, are composed of the same chemical elements found on Earth. All the heavy elements, moreover, were generated by the nuclear furnaces in stellar cores and released into space by the expulsions of matter that end the lives of the more massive stars. Understanding the chemical enrichment of a wide range of environments remains a central goal of astronomy, and one intimately tied to the study of stars. One of the most exciting recent discoveries in stellar astrophysics was the identification of a new class of binary star systems containing black hole candidates. Rather than orbiting massive stars, as in the case of Cygnus X-1, these black holes generally have low-mass companions. They are revealed by strong, transient x-ray emissions, often accompanied by optical and radio outbursts, and they provide a new wealth of observational detail and a challenge to theories of the ongin, evolution, and astrophysical manifestation of black holes. These black hole transients have many properties in common with the more massive variety of black holes thought to reside in galactic nuclei, including their nonthermal spectra and the capability of producing superluminal jets. Therefore, they give the opportunity to deepen our understanding of black holes on all scales. Explonng stars in all their manifestations allows astronomers to investigate physical conditions that will never be reproduced in terrestrial laboratories. This exploration is critical for the advancement of astronomy and fundamental physics because it pushes models of physical reality to their limits. As the sources of virtually all the visible light of galaxies, stars also provide direct means to make fundamental measurements of distance, age, and mass. An important problem is to resolve the apparent discrepancy in the age of the oldest stars and many measures of the age of the universe. Stellar research can also provide insight into some of the universe's deepest mysteries: the nature of dark matter and the origin of gamma-ray bursts.* *Dark matter is also discussed in Chapters 4 and 5. Gamma-ray bursts are discussed in Chapter 5. 21

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22 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS KEY THEMES The major scientific goals for the study of stars and stellar evolution can be organized according to the following four themes: The life cycles of stars; The origin of the elements; The behavior of matter under extreme conditions; and The use of stars as probes. The discussion below of each of these themes emphasizes issues for which space-based astronomy is crucial. LIFE CYCLES OF STARS Understanding the life cycles of stars, from turbulent clouds of gas to bright beacons of light to catastrophic explosions as supernovae, and hence to rebirth as neutron stars or black holes, remains a central challenge of the natural sciences. The theory of stellar evolution is still largely dependent on the simplifying assumption of spherical symmetry. Accretion disks, bound by stellar gravity but supported almost purely by rotation and forced to evolve by magnetic fields, remain a key topic in stellar astrophysics. To a great extent, the frontiers of stellar astronomy are concerned with the causes and effects of departures from symmetry. Key Questions About Stellar Evolution Important questions remaining to be answered about stellar evolution include the following: What physical processes determine the mass functions of single and binary stars, from their formation to their demise as compact stellar remnants such as white dwarfs, neutron stars, and black holes? How do rotation and magnetic fields influence stellar evolution? How are stellar magnetic fields generated? What are the progenitors of supernovae and the mechanisms of their explosion? Which stars leave neutron star remnants, and which leave black holes? How does transfer of mass take place in binary stars, and what regulates accretion onto compact objects? Recent Progress in Understanding Stellar Evolution Pre-Main Sequence Stars During their formation, stars are strongly influenced by rotation and magnetic fields that break spherical symmetry and channel angular momentum. The ubiquitous disks and bipolar flows that regulate stellar rotation through magnetic interaction were resolved for the first time earlier in this decade by means of interferometric observations at millimeter wavelengths. More recently, spectacular Hubble Space Telescope (MST) images of the young stars in the Orion nebula revealed protostellar disks (see Figure 2.1~. It is now clear that most young stars are members of binary systems. Thus even as astronomers have begun to discover candidate disks, they have also been forced to consider the impact of a second star embedded within the disk. Theory predicts that stellar- or jovian-mass companions will tidally clear large gaps in disks. Such tidal effects will fundamentally alter the distribution and flow of mass within the disk. The interaction of rotation and magnetic fields in young stars represents a qualitatively new area of study, one that may deeply change current understanding of stellar evolution.

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STARS AND STELLAR EVOLUTION Stellar Dynamos and Chromospheres 23 The Sun is still the gateway to a deeper understanding of all stars because it is typical in so many ways. Solar seismology has used oscillations to image phenomena such as the rotational profile of the core and the subsurface convective structure of sunspots. The lessons learned there will apply to stellar convection and magnetic struc- tures in other stars. The properties of sunspots and solar flares constrain theories of magnetic structure and reconnection that aid understanding of these processes in the broader contexts of both stars and accretion disks. Solar seismology techniques have been extended to other stars. The study of nonracial oscillations in white dwarfs has provided the ability to directly measure masses, radial variations in composition, differential rotation, and magnetic structure. Such studies also hold the promise of determining temperature evolution and hence age. More recently, Delta Scuti stars have been shown to display not only nonracial surface modes, but also radial modes in their deep interior. Space-based observations are crucial to extending this work when the noise from atmospheric turbulence dominates the signal from the stellar oscillations. Stellar chromospheres and coronae provide clues to the asymmetries that rotation and magnetic fields impose on stellar structure. Substantial features that vary with rotation rate and spectral type exist on the surfaces of both single and binary stars. The Extreme Ultraviolet Explorer (EUVE) and Advanced Satellite for Cosmology and Astrophysics (ASCA) have revealed dense coronal structures and the reconnection and flaring of magnetic fields in young stars. The first image of the surface of a star, the supergiant Betelgeuse, was recently obtained with HST. In rapidly rotating cool stars like Betelgeuse it has been shown that, unlike those in the Sun, magnetic spots frequently occur at the poles. As compared to single stars of the same temperature and luminosity, binary systems show greatly enhanced activity in the ultraviolet and may well show evidence of interaction between components. White Dwarfs Space missions over the past decade have produced important breakthroughs in the study of white dwarfs, especially those in close binaries. A combination of MST's high spatial resolution and the Roentgensatellit's (ROSAT's) ability to obtain deep x-ray images has enabled the identification of the long-sought population of white dwarf binaries in globular clusters. These stars are intrinsically interesting as test beds for binary evolution, and they can have a profound effect on the dynamical evolution of the clusters themselves. The physical condi- tions in white dwarf binaries were clarified when high-resolution ultraviolet spectroscopic observations with HST and the International Ultraviolet Explorer (IUE) identified "iron curtains" formed by a huge number of narrow iron absorption lines that have a major effect on flux distribution. These broad absorption features are seen in the early days of nova outbursts and when a dwarf nova's accretion disk is viewed edge on. A long-standing problem has been the nature of the boundary layer where the disk impacts the surface of an accreting white dwarf. EUVE spectra of white dwarfs accreting at high rates during dwarf novae outbursts confirmed the theoretically predicted lOO,000- to 500,000-K boundary layer. EUVE also provided critical con- straints on magnetic white dwarf accretion flow by establishing the vertical extent of the accretion spot and the spectrum of the heated white dwarf. New insight into the ultimate evolution of white dwarf binaries came from ROSAT observations of supersoft x-ray sources, many of which are accreting white dwarfs undergoing steady nuclear burning on their surfaces so that the white dwarfs are increasing in mass. Supernovae The origin of Type Ia supernovae is still a mystery, and fundamental issues of the nature of the explosion process continue to be debated. Recent observations have confirmed that Type Ia supernovae are not standard candles, but instead have a spread in maximum brightness that correlates with the rate of decline of the light. The peak brightness may also correlate with the nature of the host galaxy, and hence the stellar population that produced the supernova. Astronomers still do not know if Type Ia supernovae arise in a binary system containing a white dwarf and a

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24 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS normal star, two white dwarfs, or some other configuration of stars. The accreting white dwarf supersoft x-ray sources may be related to the progenitors of Type Ia supernovae. As yet, however, there is no clear evidence that these supernovae arise in binary systems at all. Despite these uncertainties, understanding of the physics of the thermonuclear explosions underlying Type Ia supernovae has advanced in a major way with the finding that departures from spherical symmetry are very important. Massive stars, in excess of 8 solar masses, end their lives by gravitational collapse that triggers supernovae, forms neutron stars and black holes, and ejects a significant fraction of the synthesized elements into space. Supernova (SN) 1987A continues to give insight into that dramatic event. HST images and spectra have given deeper understanding of the multiple rings (Figure 3.1) that may represent a link to the general process of planetary nebula formation. The debris of SN 1987A is expected to collide with the inner ring in about the year 2005, yielding a dramatic intensification of both x-ray and ultraviolet lines. Other work has shown that Type Ib and Ic supernovae and intermediate spectral class objects like SN 1993J have much to teach about the physics of core collapse and the dynamics of the explosion. Both SN 1987A and SN 1993J show evidence for asymmetric ejection of radioactive 56Ni. The enriched knots of Cas A and "bullets" of ejecta from the Vela supernova remnant also indicate departures from a spherically symmetric shell-like structure. Great progress has been made in understanding the physics of core collapse based on the recognition that three-dimensional convection is probably critical to the process. The current calculations still cannot-give suffi- cient asymmetry to account for pulsar runaway velocities whose rates are often 500 kmJs or more. Neutron Stars Neutron stars continue to be an exciting research frontier. ROSAT's observation of soft x-ray emission from single pulsars has supported the idea that the cooling of a neutron star is controlled by ordinary matter in its core, rather than by some more exotic form of matter. One of the most dramatic recent developments in the study of binary neutron stars is the Rossi X-Ray Timing Explorer's (RXTE's) discovery of the long-sought millisecond spin periods in low-mass x-ray binaries. This discovery opens the way to detailed study of the spin history of accreting x-ray binaries and their evolution into millisecond pulsars. The Compton Gamma-Ray Observatory's (CGRO's) Burst and Transient Source Experiment (BATSE) has verified that torque and spin acceleration are correlated, but has also observed long-term spin down and spin up of accreting pulsars with comparable time scales, in contradiction to popular theories. RXTE has also detected the first millisecond, quasi-periodic oscillations in x-ray binaries. These oscillations give direct evidence concerning physical processes in the inner accretion flow. Ground-based observations of one pulsar have shown one or more planetary-mass objects in orbit around it. It is not known whether these "planets" existed before the explosion, were somehow formed in the explosion, or instead condensed out of the accretion disk from an evaporated companion. Black Holes One of the most surprising recent findings is that many of the soft x-ray transients are very likely black holes with low-mass stellar companions. The all-sky monitoring capability of the BATSE instrument on CGRO has been especially useful in enabling the discovery of these transients and in providing the data required to understand them. There are now six such systems with well-defined mass functions in excess of 3 solar masses. The exponential decline of the light curve observed in many of these systems puts stringent constraints on the nature of the viscosity in the accretion disk and points strongly to an origin in a dynamo driven by internal waves. Ginga, EXOSAT, and now RXTE have found quasi-periodic oscillations and their first harmonics at various frequencies in black hole transients. Many of these frequencies are similar to those of neutron star sources, suggesting a common origin in the accretion process. Observations of some of these black hole candidates reveal tantalizing evidence for red-shifted annihilation radiation. The companions of some of these systems (and some neutron star systems) show lithium enhancements. The lithium could foe in the collision of high-energy particles with carbon or oxygen nuclei in the accretion disk.

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STARS AND STELLAR EVOLUTION 25 The black hole x-ray binary systems have a hard x-ray and gamma-ray spectral component that is very reminiscent of quasi-stellar objects (QSOs) and active galactic nuclei (AGN), and two of them have been observed to have superluminal radio jets. These systems may exhibit the fundamental nhv.sical nrocess~s at work in Arc In a better-constrained and more directly observable setting. Future Directions for Understanding Stellar Evolution -- r --A ~ ~ - ~~ r ~^ ~__#V~ V Future progress in the study of stellar evolution can be achieved in a number of different ways. These are, in approximately the order encountered in stellar evolution, the following: 1. Studying nonspherical effects in young stars and protostellar disks. Young binary stars serve as laboratories to help researchers understand the influence of massive companions on protoste~ar disks. Similarly, mass functions of young stars, both single and binary, are still very uncertain. With expanded astrometric capabilities and consequent determinations of mass, the nature of binary companions can be definitively estab- lished. Infrared spectroscopy will also be useful in helping to determine the radial variation in the opacity of protostellar disks. 2. Understanding the dynamics, heating, and energy balance of stellar chromospheres. The size and orientation of surface features on main sequence and giant stars enable differentiation between global pulsations, supergranulation cells, or spots. The discovery of spots on evolved luminous stars would be strong evidence for the presence of magnetic fields that are a possible source of heating and momentum deposition in the outer atmospheres, perhaps providing the energy for a stellar wind. A combination of extreme ultraviolet, ultraviolet, and infrared imaging and spectroscopy is the relevant set of techniques to advance such studies. 3. Determining the temperature, density, and velocity of the emitting areas on white dwarfs. Advances in the understanding of white dwarf binary systems in the next decade will come from increasing spectral resolution in the extreme ultraviolet and x-ray bands. Increased angular resolution at high energies will enable the identification of optical counterparts for both isolated white dwarfs and those accreting from binary companions. The ultimate goal of observations at longer wavelengths is to directly resolve the accretion areas in disk and in magnetic systems. The most useful observations will involve large-collecting-area, high-resolution x-ray and extreme-ultraviolet spectrographs. Also important is the capability for long-duration observations (high Earth orbit), a rapid response to transient events, better multiwavelength coordination by optical monitors on board satellites or dedicated ground-based telescopes, and an all-sky ultraviolet survey in the 90- to 300-nm band to locate hot objects, and to correlate findings with those obtained in existing radio, optical, and x-ray surveys. 4. Verifying the thermonuclear models of Type Ia supernovae. Gamma-ray spectroscopy should reveal whether the optical output from Type Ia events derives entirely from radioactive decay as surmised, and should be able to provide constraints on the physical models. Gamma-ray monitoring of radioactive species, 56Ni and 56Co, would determine when the explosion occurred. The data on kinematics and timing would give a clear test of models. Detection of the 511-keV emission from Type Ia supernovae would give a direct measurement of the manner in which positrons are emitted from these events. X-ray observations of the supersoft sources and related systems will yield greater understanding of their ultimate evolution and of the state of circumstellar matter that can be ionized by the x rays to give an extended HII region. Better knowledge of the nature of the circum-stellar medium may give clues to its prior evolution and the possible connection to Type Ia supernovae. 5. Understanding the mechanism of core-collapse supernovae. This task requires gamma-ray observa- tions to determine the quantities of radioactive Ni and its decay products, and whether or not they are ejected axially or otherwise asymmetrically. The element 44Ti is also of significant interest since it is produced near the critical region that divides the mass expelled during the explosion from that which falls onto the neutron star. Gamma-ray observations with a line resolution of several hundred kilometers per second of 56Ni, 56Co, 44Ti, and positronium would provide critical information on the dynamics of the ejecta of radioactive species. 6. Clarifying the evolutionary histories of neutron stars. Many issues, ranging from the origin of millisec- ond pulsars to the fate of old neutron stars and the question of whether or not any of them have high magnetic fields, remain to be clarified. The most useful observations will require large-collecting-area, high-resolution

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26 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS imagers and spectrographs throughout the x- and gamma-ray bands. Significantly increased angular resolution would aid the identification of optical, infrared, or radio counterparts. There is also a need to determine the spin and magnetic field of neutron stars in low-mass x-ray binaries. This can be done by high-temporal-resolution measurements at soft and hard x-ray wavelengths. 7. Measuring the density and spatial distribution of old, inactive neutron stars and various types of pulsars. Only a small fraction of neutron stars are active as pulsars or accreting binary systems. Neutron stars are likely to emit an observable fraction of their luminosity as cyclotron emission, thus allowing a measurement of the magnetic field on old pulsars. A large number of pulsars in our galaxy may be strong gamma-ray emitters rather than strong radio emitters; an important task is to determine the number of active isolated pulsars, the population of gamma-ray pulsars, and the fraction of these that are not powerful radio emitters. This requires high-sensitivity, wide-area surveys of the galactic plane for pulsars at MeV to GeV energies. 8. Understanding the stellar evolution that gives rise to black hole binaries. The new class of black hole transients raises fundamental questions about how stellar-mass black holes are formed. Several systems appear to have black holes of only 5 solar masses, which is both too large for plausible collapse of neutron stars and yet too small for formation from the massive helium cores of stars too massive to form neutron stars or supernovae. To better understand these issues, the sample of candidate black holes should be significantly extended by conducting an all-sky, hard x-ray, imaging survey. Follow-up optical and infrared spectroscopy will be needed to measure the system mass functions and black hole masses and thereby determine the mass function for stellar-mass black holes. 9. Exploring the astrophysical nature of black hole transients. The physics of accretion disks is reflected n their light curves. Obtaining multiwavelength light curves of black hole transients requires optical all-sky monitors coordinated with both all-sky and detailed imaging and spectroscopic observations at hard x-ray and gamma-ray wavelengths. All current soft x-ray, transient black hole candidates are in the Milky Way. This sample should be extended by a search for black hole x-ray transients in nearby galaxies (e.g., Large and Small Magellanic Cloud and Mat) and measurement of their time-dependent, multiwavelength spectra. This task requires large-collecting-area, x- and gamma-ray imaging and spectroscopy. To determine whether the "annihila- tion" line in some black hole transients is due to positrons or to a blend of spallation gamma rays requires high- resolution, time-dependent, hard x-ray and MeV spectroscopy. i] ORIGIN OF THE ELEMENTS Elements heavier than lithium are produced in stars by complex processes, and quantitative understanding of the chemical enrichment of the Milky Way and external galaxies is still rather primitive. Variations in metallicity as a function of position within the Milky Way and as a function of the redshift of more distant galaxies mav provide important clues in this area. In addition, further progress is required in the mixing processes within stars and of the quiescent and violent ejection mechanisms material back into the interstellar medium. Key Questions About the Origin of the Elements theoretical understanding of that release processed stellar How do mass loss and mixing influence stellar evolution and nucleosynthesis? What are the physical mechanisms that cause loss of mass from stars, and why is loss of mass so often bipolar? What elements are ejected in novae and supernovae? How does the relationship between age and metallicity vary with location in the Milky Way and else- where? How did quasars manage to produce a solar distribution of heavy elements so quickly? What are the mechanisms by which globular clusters and the lowest-metallicity stars are enriched?

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STARS AND STELLAR EVOLUTION Mixing 27 Recent Progress in Understanding the Origin of the Elements Mixing is a major factor in current uncertainties in models of stellar evolution and nucleosynthesis. Convec- tion is the primary mixing process in stellar astrophysics, and it is not well understood beyond the heuristic level of the one-parameter, mixing-length theory. At some level, convection is the issue preventing further understand- ing of stellar evolution at every stage. This is especially true both when dynamical or rapid evolutionary circum- stances demand a proper, time-dependent theory of convection, and in the face of compositional inhomogeneities that affect even the criteria for convective stability in nontrivial ways. These problems are of fundamental importance for the understanding of nucleosynthesis, the evolution of single and binary stars, and the use of stars as chronometers. The last decade has provided considerable evidence that convective mixing occurs at stages where standard, nonrotating, nonmagnetic theories predict no mixing. In many cases, the mixing cannot be attributed to convec- tion. It is now clear that the ratio of i2C to ]3C already is anomalous in stars barely evolved off the main sequence or up the subgiant branch. This observation implies a mixing to the surface by a mechanism that remains unknown of material that has been subject to nuclear processing in deeper layers. Mixing could be induced by some form of meridional circulation and turbulence driven by rotation. Turbulent diffusion might be especially important for massive stars that are radiation-pressure dominated and closer to neutral dynamical stability. In evolved stars, mixing between the interior and the surface is controlled by the depth of penetration of the deep, outer, convective envelope. Observed surface compositions again have revealed abundances created in the deep interior that are not predicted by the standard mixing theory for low-mass stars. In evolved, red-giant stars, substantial nucleosynthesis occurs specifically in thermal pulses in the helium- burning shell. Observations of red giants show that mixing from the shell occurs over a wider range of mass and metallicity than current theory predicts. An important factor may be overshoot driven by turbulence and, again, involving rotation. In massive stars the mixing, and hence the star's evolution, are thought to be strongly influenced by gradients in the composition and the accelerating pace of evolution that can become comparable to the mixing circulation time. Mass Loss Winds and other forms of mass loss from stars remove mass and angular momentum and hence may pro- foundly affect stellar evolution and nucleosynthesis. The mechanisms of mass loss may in turn be influenced by rotation and magnetic fields. While all main sequence stars lose mass in winds, this process is especially severe in massive stars whose mass can be substantially depleted on the hydrogen-burning time scale. While great progress has been made toward understanding spherically symmetric, radiation-driven winds emanating from massive stars, there are still difficulties in understanding time-dependent, nonspherical mass loss in massive stars such as luminous blue variables and Wolf-Rayet stars. , .. .. . . . . .. . Discovering the mechanisms responsible for mass loss from massive stars In all their variety is key to understanding their later evolution and nucleosynthesis. Ultraviolet spectroscopy has provided deeper understanding of the physics of radiatively driven winds from massive main sequence stars and Wolf-Rayet stars. The subject of planetary nebulae still contains a variety of astronomical puzzles. The ejection process carries matter, some of it newly synthesized, into the interstellar medium and leaves behind a white dwarf, the most common form of compact remnant. Recent work has made abundantly clear that planetary nebula ejection is not spherically symmetric (Figure 3.21. Bipolar flow is the norm, and more complex flow patterns have been revealed by HST images, most recently those of spectacular "globules" or "fliers" of ejecta. Does bipolar or more complex flow demand a binary companion, or can a single star manifest such complexity? Mass loss continues even after envelope ejection. As with main sequence and Wolf-Rayet stars, ultraviolet spectroscopy is the key technique for

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28 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS investigating the physical mechanisms responsible for radiatively driven winds coming from the nuclei of plan- etary nebulae. Supernovae Supernovae play a major role in nucleosynthesis. They create all the chemical elements more massive than carbon and much of the carbon as well. In the last decade, multiwavelength studies of supernovae and associated theory, especially of SN 1987A, have brought us closer to a quantitative understanding of how the major elements are made in supernovae. The role of neutrinos from gravitational collapse in forming nuclei via the reprocess has been clarified, although much about the process remains unknown. There has been progress in understanding how age-metallicity relationships and correlations of key elements depend on stellar kinematics and position in the galaxy, but the full role of the contribution of thermonuclear and core-collapse supernovae to determining the chemical enrichment of stars of all ages and in all environments is still elusive. As a supernova explosion expands into interstellar space, it heats the ambient material to x-ray temperatures and seeds it with ejecta. Atomic transitions in the x-ray regime provide signatures of the elements present in supernovae remnants. Theoretical plasma models yield abundances and hence a quantitative determination of nucleosynthesis at the sites of specific events. ASCA, the first true imaging spectrometer in the x-ray band, directly imaged supernova remnants in individual atomic transitions of recently expelled elements. These data, together with higher-resolution spectral imaging now being conducted by the Satellite per Astronomia in Raggi X (SAX), are providing, for the first time, the opportunity to determine the composition of the contents ejected from the cores of massive stars. Future Directions for Understanding the Origin of the Elements Future progress in the study of the origin of the elements can be achieved in a number of different ways. Among these are the following: 1. Understanding variations in the relative abundances of the elements throughout various phases of stellar evolution. Observational investigations pertaining to stellar mixing are hampered by the lack of reliable measurements of distance and hence estimates of luminosity. Accurate measurements of the parallaxes of a large sample of red giants would greatly ease this problem. A marked characteristic of most galactic globular clusters is that all stars within a cluster have essentially the same metallicity. However, detailed studies of the chemical compositions of giant stars in globular clusters have revealed star-to-star anomalies that may indicate nucleosyn- thesis and mixing within individual stars at levels not predicted by standard theories of stellar evolution. Studies of faint main sequence stars will indicate the extent to which abundance anomalies arise because of mixing or because the cloud from which the cluster formed was inhomogeneous. Many of the key resonance lines of heavy elements, especially those created by the reprocess, fall in the ultraviolet and hence require space-based observa- tions. In general, progress in understanding the physical mechanisms of convection and mixing will depend on careful observations of a range of elements that are synthesized under different conditions of density and tempera- ture, e.g., 7Li, 12C, 13C, i60, ~70 and RIO 2. Discovering the fundamental mechanisms of mass loss. To discover such mechanisms will require high-spatial-resolution ultraviolet, optical, and infrared observations close to the surface of stars losing mass. Observations are also needed of circumstellar shells at a large distance from the star with millimeter and far- infrared interferometers and molecular emission lines. High-resolution optical and infrared imaging is necessary to determine the mass-loss history and the distribution of dust and molecules in planetary nebulae. Ultraviolet and x-ray spectroscopy is necessary to determine nebular abundances and correlated properties of the central star and winds. 3. Understanding the process of planetary nebula ejection. This process, linking pulsating red-giant stars to white dwarfs, is poorly understood. Moreover, it has parallels with other mass-ejection processes in objects ranging from protostars to supernova progenitors. Recent HST images have revealed compact knots in the ejecta

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STARS AND STELLAR EVOLUTION 29 of both planetary nebulae and supernova remnants. These observations suggest underlying similarities in the physics of stellar mass ejection. An intense wind is suspected, but the physics of this process is not well understood, nor is the role of stellar pulsations. Direct determination of distances via measurement of the parallaxes of a large number of planetary nebulae would lead to greater understanding of the formation and evolution of the ejected shells and the evolution of the central stars. 4. Improving quantitative understanding of the physics of supernovae and associated nucleosynthesis. Of particular importance to the multiwavelength spectroscopic studies required to address this task are observa- tions in the infrared, where lines of important species are unblended and easier to analyze. Also important are gamma-ray observations of freshly produced radioactive species such as 56Ni, 56Co, 44Ti, and positrons. The pioneering work of ASCA should be followed in the coming decade with studies of spectra of supernova remnants of sufficiently high resolution that temperature, density, ionization state, and abundance effects can be sorted out and abundances can be deduced. Because 44Ti has a relatively long half-life, its 68-, 78-, and 1,100-keV lines might be measured by future missions with imaging, hard x-ray spectrometers of high sensitivity and high resolution. Such measurements would clarify the physics of the explosion, the abundances ejected, and the nature of the remnant. BEHAVIOR OF MATTER UNDER EXTREME CONDITIONS Some of the most challenging problems in astrophysics concern the behavior of systems under extreme conditions. Compact stars are among the most important and interesting in this regard. Because of the large gravity they generate, the natural form of radiation is in the x- or gamma-ray bands. Study of these bands is done uniquely or most efficiently from space. Key Questions About the Behavior of Matter Under Extreme Conditions Leading questions to be addressed include the following: What is the equation of state of neutron stars, and do so-called strange-matter stars exist? What is the mechanism bv which pulsars emit their rn~lintion' . r __ _ _ ~ ~ A ~ - Is Einstein's general theory of relativity correct in the strong-field limit characteristic of the innermost orbits around black holes? How do black holes create relativistic jets of matter, and what is the composition of the jets? How does matter behave near the event horizons of black holes? Recent Progress in Understanding the Behavior of Matter Under Extreme Conditions Equation of State of Neutron Stars Determining the equation of state of neutron star matter promises deeper understanding of nuclear physics and perhaps of more-exotic particle physics. One of the keys to determining the equation of state of a collapsed star is measurement of its mass. Binary radio pulsars provide accurate indications of neutron star masses that seem to fall within 10% of 1.4 solar masses. The masses of neutron stars accreting in binaries are less accurately known, but some are near 1.4 solar masses. If this is the upper limit to a neutron star's mass, as has been suggested, it has important implications for the equation of state. Different equations of state yield different masses and radii for neutron stars, so that the measurement of these can distinguish between different equations of state. The radii of neutron stars can be determined from their spectra and luminosities. Thermonuclear flash bursts and temperature variations on old, cooling neutron stars have provided relevant estimates of the radius, but in both cases other processes can distort the results, thus limiting their accuracy.

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30 Pulsars A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS Recent measurements of the light curves of gamma-ray pulsars by CGRO's Energetic Gamma-Ray Experi- ment Telescope (EGRET) show encouraging confirmation of the predictions of "outer-gap" models of the pulsar emission mechanism. Fits to these light curves enable determination of a pulsar's magnetic field and the orienta- tion of its dipole-field axis relative to its spin axis. ROSAT and ASCA observations have provided evidence for thermal x-ray emission from isolated pulsars. Such measurements place constraints on neutron star cooling and on the conductivity of the stellar surface. Strong Gravity The binary black hole candidates may give the best opportunity to observe astrophysical processes subject to strong gravity. Fluxes and temperatures obtained from x-ray observations of some binary black hole candidates show an inner accretion-disk radius that seems not to change with mass flow rate and is consistent with the size of the last stable circular orbit about a black hole. Recent observations with RXTE have discovered long-lived oscillations in the x-ray emission from at least one black hole candidate that may be a signature of the Keplerian frequency of the last stable orbit around the black hole. Measurement of this frequency yields an estimate of the black hole's mass as a function of its angular momentum, thereby indicating whether the black hole has Schwarzschild or Kerr geometry. Jets Jets are phenomena common to protostellar disks, planetary nebula, accreting stellar black holes, and galactic black holes but are not, interestingly, an obvious product of accreting neutron stars. Some bipolar flows are slow and some are relativistic, but all require some form of collimation. Jets associated with stellar black holes are especially interesting since they may be related to similar phenomena in active galactic nuclei. The outflows from black hole sources may involve large fluxes of positrons. Future Directions for Understanding the Behavior of Matter Under Extreme Conditions Progress in the study of matter under extreme conditions can be achieved in several ways. The leading ones in priority order are as follows: 1. Searching for spectral and temporal characteristics unique to black holes. How can astronomers discriminate between a solar-mass black hole and a solar-mass neutron star, and how can they determine if a black hole has Kerr or Schwarzschild geometry? Black holes will have no hard surfaces as would strange-matter or quark stars. The absence of surfaces can be demonstrated by detecting emission from very close to the black hole's event horizon, including matter orbiting in the last stable orbit. A large collecting area is required to obtain a good signal-to-noise ratio, and hence accurate x-ray power spectra down to the microsecond level for a larger sample. The time-dependent, x- to gamma-ray spectra of black hole candidates must also be determined with x- and gamma-ray observations of high temporal and spectral resolution, and hard x-ray imaging may help to detect and locate a much larger sample of black hole candidates. 2. Determining the geometry of the jets and characterizing the environments in the vicinity of the black holes creating them. Performing this task requires measurement of the emission spectrum of the black hole on time scales ranging from minutes to weeks in radio, infrared, x-ray, and gamma-ray bands. The x- and gamma-ray observations provide information on the most energetic and shortest-time-scale processes powering the jet. X-ray lines can give the charge state in the flow, which can be related to the continuum flux and which gives specific diagnostics of the density and flow instabilities. Gamma-ray spectroscopy with relatively high energy resolution will allow measurement of the stability of pair plasmas, their compactness parameters, and the conditions that give rise to them. Radio and infrared observations are needed to characterize the temporal and spatial history of the ejection of matter.

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STARS AND STELLAR EVOLUTION 31 3. Constraining the equation of state of neutron stars. To determine the mass and radius of a neutron star and thereby constrain its equation of state requires simultaneous measurement of the Stark broadening of its lines and the gravitational redshift from its surface. This can be done using high-resolution x-ray spectroscopy of appropriate lines of oxygen and iron, or perhaps using pair annihilation lines with high-resolution gamma-ray spectroscopy. 4. Determining the sites and mechanisms of particle acceleration in young pulsars. The high-energy emission from isolated pulsars is a long-standing and poorly understood problem. In particular, does outer-gap or polar-cap emission occur? Understanding this requires high-sensitivity MeV to GeV observations of isolated gamma-ray pulsars to determine the pulse profiles, spectra, spin periods, and spin-down rates. Broad wavelength coverage from radio to gamma rays is critical to exploring diagnostics of relativistic motion. ~, STARS AS PROBES: MEASURING THE UNIVERSE Stars are the beacons by which we measure the universe the basic yardsticks for distance measurement. They are also the absolute chronometers of galaxies and thus of the universe, and they provide a sample of the conditions in the cores of dense clusters and galactic nuclei where massive black holes may result from stellar collisions. Key Questions About Stars as Probes Answers to a number of important questions remain to be found. These include the following: meet? What are the absolute luminosities of stars, and how do they depend on metallicity and galactic environ What are the ages of globular clusters? How do dense star clusters and galactic nuclei form and evolve? What fraction of the dark matter is composed of brown dwarfs and compact objects (white dwarfs, neutron stars, and black holes)? Distance Scales Recent Progress in Understanding Stars as Probes Measuring distances has always been one of the most fundamental and challenging tasks of astronomy. Astronomers have developed an array of clever indirect methods, but these tend to be rife with systematic errors and are often built on chains of argument that are only as good as their weakest link. In addition, many of these methods are purely empirical and hence not grounded in fundamental understanding of the physical processes involved. Examples are those based on observations of Cepheids, planetary nebula luminosity functions, surface- brightness fluctuations induced by the brightest giant stars in a galaxy, and supernovae. Work on the HST has extended the Cepheid distance scale to the Virgo cluster, a previously inaccessible distance that allows comparison to several competitive methods. Stellar Evolution and Age Scales Extensive efforts have been made in the last decade to check the age of globular clusters based on standard techniques of isochrone fitting with attention to the details of metallicity-dependent opacities. White dwarf sequences have recently been measured in globular clusters with HST and the upper end used to calibrate the cluster distance and hence the age from the main sequence turnoff. Cooling of white dwarfs has given a new technique to determine the age of the Milky Way's disk. This technique tends to give a substantially smaller age than do measurements involving globular clusters. Yet another means to measure the ages of stars has been by radioactive dating through the use of the spectral lines of thorium or other radioisotopes.

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32 Dynamics A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS Ultraviolet imaging with HST has provided important new information on the contents of globular clusters. Star densities near the cluster cores may be as much as a million times larger than those in the solar neighborhood. The stars inhabiting the central core reflect both the stellar evolution and the dynamical evolution of the cluster. MST's images and spectra have provided direct evidence in the cluster cores of significant populations of both merged and stripped stars. Understanding the evolution of these objects presents new challenges for stellar astrophysics. Dark Matter Astronomers cannot be confident that their current ideas on the evolution of galactic structure, the chemical elements, or the global structure of the universe are correct until they have determined the nature of the dominant dark matter. If a significant fraction of dark matter is baryonic, ideas about stellar evolution and nucleosynthesis will require revision. If exotic particles constitute the dark matter, the required new physics may force adjustments in both adopted primordial abundances and subsequent stellar evolution and element synthesis. Searches for massive compact halo objects (MACHOs) have turned up microlensing events that indicate small stellar objects that may or may not be white dwarfs. The years 1995 through 1997 have brought definitive evidence for the existence of "brown dwarfs," stars too small ever to burn hydrogen, but large enough to shine from their own energy of contraction (and perhaps from deuterium burning) and therefore not planets. These brown dwarfs have been imaged as companions to other stars and identified spectroscopically by atmospheric methane features that indicate a low temperature. As yet nothing is known about the density of isolated brown dwarfs. Aside from their potential role as dark matter in the galaxy, brown dwarfs represent an important clue to low-mass star formation, star-formation efficiency, and binary mass ratios. Future Directions for Understanding Stars as Probes Additional progress in the use of stars as probes can be achieved with a number of different approaches. These are, in priority order, as follows: 1. Improving the accuracy of stellar distance measurements by a factor of order 1,000. Such a capability would expand the volume of space within which accurate distances are available by a factor of a billion and would have an impact on virtually every aspect of astrophysics. Space-based optical interferometers now under consid- eration may yield high-dynamic-range imaging with milliarc-second resolution. They should also be capable of measuring absolute parallaxes with microarc-second accuracy and proper motions to a precision of a microarc second per year. Such an interferometer could provide direct measurements of distances for stars of virtually all spectral types throughout the Milky Way. These data would allow qualitatively new studies of galactic structure, today plagued by uncertainties of 10 to 20% for such basic quantities as, for example, the distance to the galactic center. With the galactic structure and total-luminosity measures so derived, a wide variety of secondary distance calibrators can be utilized that would aid extension of the Cepheid distance scale to distant galaxies. 2. Improving the accuracy of measurements of stellar masses from interferometric studies of the dy- namics of binary systems. Measurements of stellar masses are critical to all of stellar astrophysics and especially to calibration of relationships between mass and age. For normal binaries as well as those with compact compo- nents, substantial progress can be made by accurate interferometric measurement of the orbital motion of the system's center of light. Data on this motion, coupled with observations of Doppler shifts, will give a binary star's orbital inclination, and thus stellar masses, with unprecedented accuracy. 3. Understanding the physics and evolution of Type Ia supernovae to enable their use as cosmological probes. Type Ia supernovae hold great promise for determining the cosmic distance scale and the value of the deceleration parameter, and with this information it may be possible to put constraints on the cosmological constant. Current searches are routinely discovering Type Ia events at a redshift of about 0.5, with the current

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STARS AND STELLAR EVOLUTION 33 record in excess of 0.8. To incorporate Type Ia and core-collapse supernovae as useful cosmological probes, astronomers must understand their origin and explosion mechanisms, and how these correlate with the stellar population in which they reside. A key objective is to compile a large sample of supernovae as a function of galaxy type, location in the galaxy, and age of the galaxy (redshift). Such a survey requires a capability to detect supernovae at large redshifts where key spectral features are displaced out of the optical band. This topic is discussed further in Chapter 4. 4. Performing absolute checks on models of stellar evolution and age. A key step to determining accurate stellar ages is the precise measurement of distances and hence luminosities. Accurate distances will serve to indicate the turnoff and to normalize the isochrones of globular clusters, the principal tool for determining cluster ages. In general, precise absolute distances and hence luminosities would give an absolute calibration of the Hertzsprung-Russell diagram in a variety of contexts. 5. Using globular clusters as laboratories for tests of stellar dynamics and evolution. The fate of blue stragglers and stripped giants remains to be determined with high-resolution images and spectra and with theoreti- cal studies. The origin and evolution of white dwarfs and neutron stars in globular clusters, and thus both initial mass functions and the dynamical evolution of clusters, can be studied with high-resolution optical, ultraviolet, and x-ray imaging. Precise positions of stars in globular clusters can be derived from interferometric astrometry, and binary stars in globular clusters can be studied by very high resolution imaging from optical to x-ray wave- lengths. When combined with new absolute measures of distances for stars, these data will allow the use of stars in globular clusters as probes of both internal motions within the clusters and the clusters' motion in the galactic gravitational field. This capability would give greater insight into equipartition and tidal-disruption time scales for the clusters in the Milky Way's gravitational field, and would allow much deeper study of the effects of cluster binaries and their influence on cluster evolution, the approach to equipartition, and their role in the gravothermal catastrophe. 6. Discriminating between baryonic and nonbaryonic dark matter in the Milky Way. Accurate mea- surements of the proper motions of stars far from the galactic plane and the galactic center can place important constraints on the distribution of dark matter in our galaxy. The discovery that a substantial amount of dark matter has accumulated in the galactic disk would imply that this material is baryonic. Such measurements have the potential to rule out most nonbaryonic candidates and many baryonic ones, and hence would be a critical advance. CONCLUSIONS Stellar astrophysics remains at the heart of modern astrophysics. A vigorous program of space-based stellar research will benefit not only stellar astronomy, but nearly every other field of astrophysics as well. From the wealth of research possibilities, TGSAA identified several issues of exceptionally high priority. They are, in priority order, as follows: 1. Understand the origin and astrophysical manifestations of black holes. The discovery of a new class of stellar-mass black hole candidates the soft x-ray transients promises to revolutionize current understanding of these exotic end points of stellar evolution. The fact that these candidates have known binary systematics, accretion disks, and jets makes them especially fruitful laboratories to understand black hole astrophysics, and, perhaps, to probe the nature of strong gravity. Extending the frontiers of stellar black hole research calls for all- sky monitors and hard x-ray imaging surveys to discover new candidates and also calls for higher spatial and spectral resolution throughout the high-energy band. Also needed is temporal coverage on time scales from submilliseconds to years. 2. Study the behavior of matter at extremes of gravity, rotation, magnetic field, and energy density. These conditions are best investigated by studying compact objects, especially those accreting in binary systems, using high-spatial- and high-spectral-resolution observations conducted at multiple wavelengths, with particular emphasis on the high-energy bands. 3. Investigate fundamental issues concerning the origin of the elements. High-spatial- and high-spectral- resolution observations in the infrared, ultraviolet, x-ray, and gamma-ray bands and fundamental calibrations of

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34 A NEW SCIENCE STRATEGY FOR SPACE ASTRONOMY AND ASTROPHYSICS stellar distances are needed to better understand the origin of the elements from the youngest galaxies and quasars to the most recent supernovae and supernova remnants. Key related topics are the roles of rotation and angular momentum in the stars, mass loss, mixing, and the mechanisms of explosion. 4. Improve understanding of stars as markers of the size and age of the universe. The use of stars to measure the cosmological distance scale, deceleration parameter, and age of the universe is central to modern cosmology. Important tasks include accurate determinations of the parallax distances and hence ages of globular clusters. Also important are the detection and physical understanding of a large sample of distant supernovae, where most of the flux will be in the infrared. 5. Understand the effects of rotation and magnetic fields, and the effects of binary companions. Young stars already provide special insight into these key unknown areas of stellar evolution. Significant progress in understanding the evolution of young stars and their disks, jets, and possible planets can be made with very high resolution imaging.