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2 Science Objectives INTRODUCTION Astronomy encompasses an enormous range of interesting and fundamental questions. The universe exhibits regions far more va- cant than the best vacuum that we could hope to generate in the laboratory, and compressed matter so dense that a thimbleful would weigh a billion tons. Temperatures, pressures, magnetic field strengths, and radiation densities range similarly across ex- tremes. A practical consequence of this great variety of conditions is a huge range of cosmic sizes, of velocities, and of time scales on which dynamic processes take place. Three decades ago, it was still possible to think of a universe in which changes occurred ponderously, if at all, over the aeons. Now we detect compact neu- tron stars whirling about their axes 1000 times a second, emitting streams of pulses at millisecond intervals and there are processes that take place on every other conceivable time scale between mil- liseconds and billions of years. These same neutron stars are no more than a few kilometers across, and there is accumulating evi- dence for the existence of stellar black holes that would be another 10 times smaller. At the other extreme we see radio jets that span an entire cluster of galaxies. The relativistic energies of such jets are the handiwork of electrons and protons traveling along thin 8
9 filaments at close to the speed of light. These particles, which stem from an unknown source, eventually are spent in gigantic billowing clouds known through their powerful radio emission. Closer to home, and of special interest, are planetary pro- cesses. Here the history of a system can be read in its chemical composition, and we may hope to trace the origins of life and determine the prevalence (or absence) of biological systems else- where in the solar system, in our galaxy, and potentially in the universe. To study all these different conditions and construct an un- derstanciable picture, we require an ability to detect the very small as well as the very large. To probe the great variety of dynamical processes, we need to be able to measure both extremely high and very low velocities; to recognize different chemical species, we need to be able to unravel their spectral signatures. These require- ments dictate that the instruments that we wit! need must cover the widest range of photon energies from the extreme gamma- ray portion of the spectrum to the longest radio waves. In each of these ranges we will require large, sensitive instruments that can follow rapid time variations. Higher angular resolution, as well, is needed to focus on extremely compact sources. Such compact sources will also need to be studied both at high spectral resolution and across the widest possible dynamic range to reveal internal motions and chemical composition. BASIC ASTROPHYSICAL QUESTIONS In identifying scientific objectives for astronomical research in the period 1995 to 2015, the Task Group on Astronomy and Astrophysics accepts that there will be major developments in as- tronomy and related sciences between now and then. Priorities will necessarily have to change, and entirely new projects may evolve. However, the study Astronomy and Astrophysics for the 1980s, conducted by the Astronomy Survey Committee of the National Research Council, enunciated a number of basic questions that are as relevant today as when they were published in 1982. The task group believes that these will remain compelling questions for the foreseeable future as well. The fundamental astrophysical issues fall into three major groups (a fourth topic-solar and stel- lar activity is discussed by the Task Group on Solar and Space
10 Physics in the companion volume Space Science in the Twenty- First Century: Solar and; Space Physics): . galaxies. The early universe, unidentified matter, and the origin of The physics of collapse and the physics of strong fields. The formation of stars, planets, and life. In what follows, the task group has not attempted to make predictions about the discoveries possible in each area before 1995. The task group points out, however, the relevance of major NASA missions to be initiated before that time. The Early Universe, Unidentified Matter, and the Origin of Galaxies "Big bang" cosmology the theory espousing the explosive origin of the universe met a crucial experimental test with the 1965 discovery of the cosmic blackbody background radiation. That measurement yielded a ratio of photons to protons in the universe and permitted calculation of the abundances of light elements produced in the first few minutes after the big bang. Studies of these abundances have since led to the conclusion that the density of normal nuclear matter in the universe is about 10 percent of that expected of a closed universe in fair agreement with dynamical constraints on the total amount of its mass. Grand unified theories of elementary particles postulate that the universe underwent a transition from a symmetric to an asym- metric phase 10-35 S after the big bang, when the temperature was 1028K. A rapid inflation of the universe then resulted in a present total density of matter almost exactly that of a closec3 universe according to thorn The Nits era Rile ~1~,~ for matter, however, tolls 10 times short of that predicted den- sity. Such theories thus imply that 90 percent of the mass of the universe is in unidentified form; if this mass is nonbaryonic, it could take the form of massive neutrinos or even more exotic new particles like photinos or anions. This would have important im- plications for the origin of galaxies, which must have formed in the gravitational collapse of density perturbations in the cosmically expanding matter. As the universe evolved, most of the "normal" matter has, theoretically, been compressed by gravitation to form the stars and galaxies that we observe; these processes would be ~ · ~flus ~- ~UL4~11] ~1 VO
11 profoundly influenced by the presence of the greater bulk of matter in unidentified form. Astronomers began to realize some years ago that there is some unidentified form of mass present in the universe. Galaxies have since been shown to be embedded in giant halos composed of material whose presence can be inferred from its gravitational effects, but which has not yet been detected directly at radio, infrared, optical, or x-ray wavelengths. The matter in such ha- los appears to make up 5 or 10 percent of the critical cosmic density, but the data are not accurate enough to decide whether this unidentified mass could be in the form of very faint stars or whether additional matter of novel form is required. Density estimates based on the dynamics of clusters of galaxies do not exceed 20 percent of the closure density that is calculated for a closed universe. Thus, these estimates are apparently not consis- tent with an inflationary universe based on grand unified theories. On the other hand, almost all visible galaxies are observed to lie within clusters or filamentary structures that occupy a small frac- tion of the volume of the universe. The rest of space seems to be largely void; but these voids in the distribution of galaxies may yet contain large amounts of hidden matter not counted in the analyses of clusters and superclusters enough, in fact, to make up the 100 percent of closure density predicted by grand unified theories. Current observations have not resolved this point. The 1982 report of the Astronomy Survey Committee recom- mended a number of facilities that promise to contribute to our understanding of the unaccounted mass. HST will determine the distribution of hidden mass in galactic halos by determining the motions of globular clusters. AXAF would permit a search for the x rays emitted by the hot envelopes of faint stars in galactic halos. SIRTF will search for cold matter, such as brown dwarfs- substelIar bodies emitting energy by virtue of a slow contraction that liberates gravitational potential energy. This search for cold matter by SIRTF will set limits on the baryonic composition of the dark matter component in our galaxy. AXAF can also exploit the recent discovery of hot gas in the halos of some elliptical galaxies to determine the halo mass through its gravitational ejects. The NASA instruments that will be operational or under con- struction by 1995 have extraordinary sensitivity at wavelengths ranging from the x-ray to the infrared region. They will thus per- mit us to Took across large distances, far back in time to the epoch
12 of galaxy formation, when the universe was probably only 10 or 20 percent of its current size. Theorists are currently developing computer simulations of galaxy formation in a universe dominated by various types of unidentified matter. Such models have al- ready demonstrated that if the unidentified matter is composed of massive neutrinos, then the large-scale structures such as clusters and sup erclusters should be more highly developed than galaxies themselves are observed to be. Hence, current attention is focused upon other particles that remain consistent with grand unified theories but are more exotic than neutrinos. · . . The evidence for exotic particles may come from direct measurements of their anni- hilation products such as proton/antiproton pairs and gamma-ray photons. By 1995, it is expected that theoretical models of galaxy formation will be well-developed, and it will be possible to com- pare observations of distant matter in the remote past with the predictions of particle physics. S~RTF is essential for t.hi~ work ~ _~ 1:_L~ 31 _ ~ 1 · ~e ~ ~ ;5111~: 1l'~ll' embed nv n~lmnr~l~ I an I=Y1~O ;C! h^~r;l~r m1~;~ the infrared. - ~-A^ ~^ ,br~- 15= 4~at~ fly OllllUG~ lllbU A puzzling feature of the universe is the asymmetry of the matter/antimatter balance within the galaxy in view of the sym- metry of the laws of nature. We have no clear-cut understanding of mechanisms that would have led an initially hot, explosive uni- verse to favor matter over antimatter, though several symmetry- breaking mechanisms can be postulated. Whether the universe on its largest scales preserves the matter/antimatter symmetry is, therefore, a question of great interest. If matter and antimatter exist in proximity, we mav expect annihilat.inn t.n t.~k" r~lar.= ot the interface where gases from these regions mix. The annihila- tion radiation has its own spectral signature, readily identified, and we may expect to find this annihilation radiation, if it exists. Also, antimatter of extragalactic origin might be detected with magnetic spectrometers that could analyze the cosmic particle radiation with high sensitivity. - ~^ van v~ lows- mu ~. , . . _ The Physics of Collapse and Strong Fields The discovery of radio pulsars, neutron stars, and candidate black holes in x-ray binary stellar systems demonstrated that some stars collapse beyond the density of atomic nuclei. Theoretical models indicate that the collapse occurs within a fraction of a second when the core of a star becomes unstable; the bounce of an
13 imploding stellar envelope off the just-formed neutron-star core is sufficient to eject the stellar envelope explosively in a super- nova eruption. Such events are important in the evolution of our galaxy because they are the principal source of turbulence and heat in the interstellar medium. They account for the acceleration of cosmic rays, and create the heavy elements that later become incorporated into stars and planets like those of our solar system. These elements, which are believed to originate by such explo- sive nucleosynthesis, can be detected through their characteristic gamma-like decay. Neutron stars represent a unique state of matter of excep- tionally high density. In spite of their high temperatures, their interiors are superfluid and are capable of sustaining magnetic fields up to 10~2 Gauss. Observations of their rotational proper- ties yield information about their interior structure. Further, the radiation accompanying accretion of matter from companion stars is teaching us much about the behavior of matter in strong gravi- tational and magnetic fields. X-ray burst sources, for instance, are believed to be neutron stars on which a sufficient layer of matter has accumulated from a companion star to initiate a thermonu- clear explosion. Recent observations of the x-ray spectrum during the decay phase of such a burst in the source 4U1636-53 indicate the presence of an absorption line. If attributed to iron (as seems probable), this spectral line is redshifted by 40 percent from the spectrum of iron observed in a laboratory. If this represents the gravitational red shift at the surface of a neutron star, then such stars are both smaller and denser than indicated by models based on general relativity and the equation of state of nuclear matter. If confirmed, this interpretation may teach us something quite un- expected about particle physics or, conceivably, about gravitation. Some collapsing cores do not stop at the neutron star stage but plunge on toward a black hole singularity. It is of great interest to learn about the black hole state. Astronomers have identified several objects believed to be stellar black holes in orbit around a normal stellar companion. With the aid of computer simulations, theorists are now studying the accretion process near rotating black holes. They are finding that these relativistic flows exhibit novel features that could help us to understand temporal variabil- ity in the emission from black hole candidates. We anticipate that more sophisticated theoretical models will yield new insight that may tell us whether individual sources are, in fact, black holes.
14 Before 1995, relativistic jets will be observed in greater detail by the Very Long Baseline Array (V[BA), now under construc- tion. Current observations have demonstrated structure on the microarcsecond scale. In several jets there are condensations that appear to be moving apart faster than the speed of light. The -VCOA, worn z~u-,uarcsec resolution, will pursue this finding with greater sensitivity and angular resolution, and the QUASAT mis- sion will extend resolution still further, probing more deeply into the jet formation site. Much more will remain to be done, since an- gular resolution of a few microarcseconds will resolve details about 3 light-days across in the quasar 3C 273, and only 2 light-hours at the nucleus of the active galaxy M87. Because the Schwarzschild radius of a black hole whose mass is 109 times that of th`~ chin in ~ 1- L ~L _ 1 1 . ~ ~ rT ~ A · I 1 rams ~ . ~ou~ ~ ~gr~-nours, sucn observations will permit study of events less than 20 Schwarzschild radii from the black hole (the r~r~'mP~ ~:~ r ~ 1 . ~ ~ ~ ~ lit OI one accretion DISKS, provided that the opacity is not too great. Such observations will be of extraordinary interest in testing the black hole mode} of active galactic nuclei and in studying the novel relativistic effects expected in the vicinity of black holes. Both the Long-Baseline Optical Space Interferometer (LBOSI) and the large space radio array (ASTROARRAY see Chapter 4) aim at achieving 1-,uarcsec resolution at optical wave- lengths. We should be able to probe phenomena in active galactic nuclei at distances even closer to the black hole (liaht-hours) than the size of the light-emitting region ~light-days) indicated by the time scales involved in light variations. Already, x-ray observa- tions have demonstrated strong variability on a time scale of an hour or less in active galactic nuclei (AGN) of a wide range of luminosities. SIRTF will also play a vital role in the study of these , . . . superenergetic phenomena. Gamma radiation also is a tracer for highly relativistic par- ticles cosmic rays both in the galaxy and beyond. In our galaxy the gamma rays emanate from a number of discrete sources, such as the Crab and Vela pulsars, but they also emanate from dense molecular clouds. This diffuse gamma flux indicates the presence of highly energetic particles that interact with matter in these clouds. In ways we do not yet understand, this gamma flux could reflect mechanisms that play a significant role in the formation of stars. It could, for example, indicate destruction of magnetic fields that have to be eliminated in order for protostellar matter to collapse to form stars.
15 The mystery of the origin of cosmic rays is only partially solved at present. Acceleration of particles to relativistic energies occurs at a variety of scales: in the solar system in solar flares and planetary magnetospheres, and in the galaxy probably in stel- lar winds and supernova-driven shock fronts. Recent observations have indicated that binary x-ray sources containing compact ob- jects such as Cyg X-3 appear to be powerful particle accelerators at extremely high energies. To sustain the average luminosity in cosmic rays of our galaxy (about 104 ergs/s), a substantial energy source is required. If supernovae are responsible, almost 10 per- cent of their total energy output would be required to account for the cosmic-ray luminosity. We also know through observations of secondary photons in the radio, and sometimes in the gamma-ray region, that powerful particle acceleration must occur in almost all classes of extragalactic objects. In fact, the nonthermal radio spectra of most bright spiral galaxies seem to have almost identical slope, indicating that some common, but as yet poorly understood, mechanism governs the acceleration and propagation of the parent electrons in these galaxies. Cosmic rays from our galaxy can be directly observed with detectors in space. At their very highest energies, they can be ob- served indirectly in ground-based air-shower installations. Most spaceborne gamma-ray detectors have been relatively small in size and therefore restricted to studies at low energies (below about 1 GeV) where interactions with the solar wind ("solar modula- tion") change the composition and energy spectra of the radia- tions reaching us from the galaxy. Radioactive dating, using the i°Be isotope, has shown that cosmic rays are a relatively young sample of galactic matter, with an age of about 107 years. Yet the elemental abundance distribution of cosmic rays seems to be quite sirn~lar to that of the much older solar system. However, subtle but characteristic differences in the isotopic abundances reflect a different nucleosynthesis history of the matter from which cosmic rays are accelerated. This may constitute evidence for the contin- uous chemical evolution of our galaxy, but much more sensitive measurements are required before this interesting question can be resolved. First information on the composition of the extremely rare ultraheavy cosmic rays, up to uranium, came from detectors on the HEAD-3 and Ariel-6 spacecraft. Measurements on the Space Shuttle are providing the energy spectra of individual nuclear
16 species to much higher energies (approximately 1013 eV) than previously accessible and thus are expected to help reveal the character of the galactic acceleration mechanism. Cosrn~c-ray measurements could potentially also lead to major changes in our understanding of fundamental physics and the formation of the universe if they detected exotic particles, such as magnetic monopoles, superheavy nuclei, or primordial antimatter. The Formation of Stare, Planets, and Life Our own solar system was formed 4.5 billion years ago, prob- ably through the gravitational contraction of a fragment of an interstellar molecular cloud. With radio and infrared telescopes (IRAS is a notable recent example) we have the opportunity to observe similar processes at work today. Large numbers of active regions in molecular clouds appear to be candidates for star for- mation. Clouds of small particles have been discovered around several bright stars. These could conceivably represent a stage in the process of planet formation. Astronomers are already study- ing such events, but need higher spectral and angular resolution to sort out the complex gas dynamics. STRTF will be able to infer the chemical composition, temperature, and density of gas flows involved in star formation regions. However, the angular resolu- tion will be relatively limited (6 arcsec at 30-,um wavelength for SIRTF), so the images of such regions will be fuzzy. The Large Deployable Reflector (I.DR), described in Chapter 5, will attain angular and spectral resolution that will permit study of details as small as 50 AU, or somewhat smaller than the diameter of the solar system, in the nearest star-forming region. We have not yet been able to demonstrate the existence of a single planet beyond our solar system. Our instruments have so far been unable to detect any of the manifestations of such bodies: reflected light, infrared reradiation of stellar light, wobble on the plane of the sky, or periodic radial velocity variation. It should be possible in the 1990s to undertake a meaningful search for ex- trasolar planets by using special high-resolution spectrographs to look for the radial velocity variations in a parent star produced by planetary gravitational pulls. Space-based astrometric instru- ments may also be used to Took for the corresponding periodic wobble of the star position. Planetary systems may well be a common phenomenon around
17 late-type dwarf stars like the Sun roughly 50 of which lie within 10 parsecs distance. Whether they are organized like our solar system into large outer planets, rich in light elements, and smaller inner planets that have high concentrations of the heavy elements is more conjectural, but plausible. Both theoretical analysis and the example of our own solar system support this view. A more speculative, but intensely interesting subject is the prevalence of life in the universe. We know of only one example-our own Earth. Whether the phenomenon of life is unique, rare, or a common oc- currence appears now to be a problem that can be approached in a preliminary way. Instruments specifically designed for this purpose, with spatial resolution of the order of 10-3 to 10-4 arc- sec, could obtain images of other solar systems. Furthermore, they could carry out spectrophotometric analyses of the chemical composition of the planetary atmospheres. For our own Earth, the oxygen concentration is far higher than one would expect if it were near chemical equilibrium. Nearly two aeons ago, the oxygen concentration was enriched dramatically by the action of living organisms. A similarly high oxygen concentra- tion, if found in the atmosphere of another planet, would be highly suggestive of the presence of biological activity there as well. Our knowledge of how planets form and how life arises is slight, and surprises would surely result from such a program of exploration. A further, more specialized search involves intelligent life. Such a search has a different character than the broader-based search for life and is not addressed in this study. Obviously, if the NASA Search for Extra-terrestrial Intelligence (SETI) program were to find radio signals from another planetary system, it would be a tremendously exciting and significant event. In the case of the more restricted study described here, even the hint of life in another planetary system would trigger a new era in planetary research. RELATIONSHIP TO OTHER DISCIPLINES OF SPACE SCIENCE Astronomical instrumentation is becoming increasingly so- phisticated and powerful in the high-precision measurement of objects far beyond our immediate solar system. This remote- sensing capability of astronomy can clearly be applied to other · ,- · ~ space science alSClp lneS.
18 Sensitive cameras and spectrographs designed for high spatial and spectral resolution of cosrn~c phenomena allow imaging and spectroscopy of solar system objects that contribute to studies of the chern~stry and physics of planetary atmospheres and magneto- spheres. Images and spectroscopy of planets, circumsteliar disks, and bipolar outflows from young objects can help to unravel the sequence of events leading to formation of stars and planetary systems. Studies of the magnetic activity of stars, and even di- rect imaging of stellar surfaces, enable us to extrapolate physical theories of solar activity and magnetic dynamos to objects like our Earth. Eventually, these studies may contribute to a deeper understanding of climatic variations on Earth, of the terrestrial dynamo action (including periodic reversals of the Earth's mag- netic field), and of other phenomena of long-term importance to life on Earth.
