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262 4. Subject to periodic review, long-term support should be provided to strengthen existing centers to ensure that laboratory astrophysics is a major influen- tial part of research. 5. Funding agencies should encourage the convening of workshops attended by astronomers and by laboratory astro- physicists to ensure that the astronomical needs are recognized. 6. A number of visiting fellowships should be created to encourage astronomers and laboratory astrophysicists to visit institutions where research relevant to astronomy can be pursued. 7. Support should be maintained for data centers to provide critical compilations and assessments of the basic physical data required in quantitative applications. I I . THEORETICAL ASTROPHYSICS A. The Nature and Role of Theory in Astrophysics The relationship between theory and observation in astron- omy is complex and symbiotic, and it must be examined with care if resources are to be distributed wisely. In a fun- damental sense, observation is the prime mover of the re- lationship. Without observational input, theory can say nothing about what actually is; it can only explore the multitudinous possibilities of what logically might be. No theoretical astronomer, however abstract in orienta- tion, would seriously dispute that the ultimate goal of astronomy is to understand the Universe. It should be equally apparent that observations without theoretical input can at best yield a superficial view of the cosmos. Real understanding can only occur when the observational information is incorporated into a concep- tual framework of unifying principles. A healthy science is characterized by the intertwined and parallel develop- ment of new concepts as new information is gathered. Attempts to place in order of priority the support of theory and the construction of new observational facil- ities create an artificial and misleading dichotomy. Theoretical astrophysics resists easy characterization. It spans a broad range of activities, from the interpreta- tion of observations to the development of new concepts. Theory unifies the science by revealing underlying connec- tions between seemingly diverse phenomena. For example, in the past decade we have learned that the gaps in the

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263 ring systems of planets are created by resonances similar to those responsible for spiral structure in galaxies. We have also seen the emergence of the concept of the accretion disk for the construction of models of cata- clysmic variables, binary x-ray sources, the nuclei of active galaxies and quasars, and the origin of planetary systems. Theory links astronomy to other branches of science. In the past decade we have witnessed the incorporation of a large body of chemistry into astronomy, revealing con- nections between planetary atmospheres, comets, and inter- stellar gas clouds and showing how laboratory measurements and theoretical calculations can be combined with radio- astronomy observations to infer the physical conditions m e relationship is often in regions of star formation. reciprocal: astronomical observations have provided information on atomic and molecular processes that has not been obtained experimentally and indeed have dis- covered molecules never seen in the laboratory. There are many such relationships: nuclear matter theory and low-temperature physics and the structure and formation of neutron stars, particle physics and cosmology, and controlled thermonuclear fusion research and spectral . . ~ - observations of cosmic x-ray sources, to name a few. Others may emerge in the next decade, for example, the link between solar-system plasma physics and macnetohYdro- dynamics and the study of similar phenomena in active stars and galaxies, which is at present tenuous, will probably be strengthened. As observational techniques become more diverse and sophisticated, the demands on theory increase. Thus advances in radio, optical, and x-ray astronomy have yielded a growing body of evidence for well-collimated beams of relativistic particles emerging from radio galaxies and the Galactic binary SS433. The cause of the jets and the relationships between the emissions at different wavelengths and between the different objects are yet to be recognized. Sometimes theory leads observations. For example, a well-developed theory of gravitational radiation was in place before the discovery of the binary pulsar. After its detection it was clear immediately that detailed observations of the pulse-arrival times could provide constraints on the nature of the system, as well as confirmation of the theory of gravitational radiation and information on possible alternatives to the general theory of relativity. Entire areas of astrophysics can

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264 come into existence without a specific observational reason, but because astronomers work out the consequences of what is already known. The concept of the black hole is an excellent example. It would not be worth taking seriously if it were only an ad hoc hypothesis, advanced to explain the peculiarities of Cygnus X-1. What gives compelling intellectual force to the concept is its under- lying basis of well-developed theories of general relativ- ity, steller evolution, and the equation of state of mat- ter at high densities, whose development long preceded the observational discovery of the first black hole candidate objects. Neutron stars were likewise preconceived. An important role of theory is the generation of such concepts or paradigms, which have great intrinsic value even in the absence of an observational basis. A marvel of recent astronomy is the frequency with which these are reflected in nature. Although evidence for their reality is usually found by chance, the theoretical concepts spur new observational programs and provide a focus for further development once a phenomenon is discovered. Other para- digms exist, such as stars powered by accretion onto a neutron star core and the radiative decay of low-mass black holes, which are not strongly indicated by present observations but which we believe to be important issues for the next decade. Often theory lags observations. Then the observations must await theoretical insight to realize their full value. For example, a wealth of data on the distribution of galaxies in the sky has existed for many years, but until recently its meaning remained obscure. The new discoveries only occurred with the development of a power- ful and different point of view. A triumph of the 1970's was the analysis of these data in terms of correlation functions' which revealed a power-law spectrum of fluc- tuations that may have deep cosmological significance. Such theoretical insights not only enhance the meaning of existing observations, they drive new observational programs--in the case of galaxy clustering, systematic red-shift surveys. Theoretical knowledge is "consumed" by observational advances, which often exhaust the power of a theoretical concept to accommodate them. For example, it became apparent in the 1970's that the enormously successful theory of the solar wind was inadequate to deal with the complex variations observed in the x-ray brightness of the corona and in the flow velocities and magnetic fields of the wind. Some new ingredient--the concept of coronal

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265 holes--needed to be incorporated into the theory. As another example, it appears today that the theory of the electrodynamics of neutron stars is inadequate to explain the inner structure of radio pulsations and that a vital ingredient is lacking. The discovery of an unexpected phenomenon is usually followed by a period of model building--attempts by observers and theorists to fit the phenomenon into the existing conceptual framework. Elaborations are usually carried out by theorists who have a responsibility to make predictions that can be tested by observations. Predictions may be most valuable when they fail to be confirmed by observation. m is has been the case with the 37C1 solar neutrino experiment, in which the dis- crepancy between the theoretical predictions and the initial observations stimulated many careful investiga- tions of the solar interior and is a powerful motivation for the development of new detectors of solar neutrinos. At the time of this writing we are not sure whether the discrepancy is a fundamental one or whether it is a mat- ter of details that will soon be resolved. However, we clearly know much more about the Sun today than we would have had the theoretical prediction been confirmed by the first observations. Another kind of model building involves attempts to synthesize our understanding of diverse phenomena into global models. m us we use our knowledge of stellar evolution, mass loss from stars, supernova explosions, and interstellar gas dynamics to model our Galaxy as an ecosystem, in which the observed distributions of stars and interstellar matter result from cyclic flows of mass and energy. Understanding our own Galaxy in such a global sense is a first step toward understanding the differences among galaxies and unifies galactic and extragalactic astronomy. As predicted by the Greenstein report, computers have played an increasing role in theoretical astrophysics during the 1970's. We have witnessed the development of powerful numerical techniques by which we can explore astronomical phenomena that have long resisted concise analytical descriptions. Tree outstanding examples come to mind: many-body codes to simulate the evolution and interactions of star clusters, galaxies, and clusters of galaxies; hydrodynamic codes to simulate the formation of protostars, stellar explosions, and interactions with interstellar gas; and relativity codes to simulate the behavior of systems with strong gravitational fields,

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266 such as collapsing stars. With these codes, theorists specify hypothetical astrophysical environments and use computers as laboratories for experiments in astrophysics. Another vital role of theory is the development of interpretive tools to translate data into meaningful information. An obvious example is radiative-transfer theory, by which a spectrum is transformed into physical parameters such as velocity, density, temperature, and chemical composition. Without the theory, a spectrum is little more than a jumble of photons. Radiative transfer was once regarded as a mature area of theory, but with the discovery of new phenomena and the advance of astronomy into unexplored spectral ranges, it has again become a rapidly growing and exciting field. For example, the advent of ultraviolet spectroscopy made it obvious that many hot stars were losing mass in powerful supersonic stellar winds. Not only the interpretation of the observed spectra but also the very understanding of the mass-loss phenomenon require the development of new tech- niques to describe radiative transfer in differentially expanding media. The new theoretical principles provide a common ground for such different phenomena as the sPe tra of hot stars, supernovae, ana one emission spectra or quasars. Extensions of radiative transfer theory are also needed to interpret the observations of maser emission from interstellar and circumstellar molecules. In the coming decade further demands will be made on radiative- transfer theory, particularly as a result of the employ- ment of powerful spectrometers in the far-infrared, ultra- violet, x-ray, and gamma-ray bands. The development of theoretical and computational tools to interpret observa- tions is an essential part of any observational program and must be supported even as the telescopes and detectors are designed. Another example of a mature but vital field is dynami- cal astronomy, the study of motions dominated by gravita- tional force. Although the basic concepts were developed by scientists such as Newton and Laplace, the field con- tinues to advance. Some of the most exciting investiga- tions of the last decade have been the elucidation of evidence for unseen matter in galaxies, the formation of planets from the solar nebula, and solar-system tests of general relativity. The study of the dynamics of self- gravitating stellar systems, from globular clusters to clusters of galaxies, provides an important link between astrophysics and plasma physics. While substantial progress has been made in understanding the consequences

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267 of particle interactions, the elucidation of collective or collisionless phenomena in these systems remains a major challenge. B. Accomplishments of the 1970's Important theoretical progress was made in a wide range of areas of astronomy in the 1970's. In this section we describe a few of the highlights of the past decade. m e topics are chosen not to provide a comprehensive summary of the field but rather to exhibit the ways in which theo- retical research strengthens and advances astronomical knowledge and the variety of systems on which major progress has been made. Galactic Evolution. Decades from now, the 1970's are likely to be remembered as a period when the complexity of galactic evolution was glimpsed for the first time. Galactic colors and luminosites change as their stellar populations evolve: galaxies collide, leading to violent transitory tidal distortions, and coalesce, because the collisions are inelastic; galaxies may lose all their gas through ram pressure stripping by the intergalactic med- ium, but they may also acquire gas from the accretion of intergalactic clouds. Even the spiral structure in ordi- nary spiral galaxies causes important secular changes in the galaxy's mass and angular momentum distribution over a Hubble time. m e theoretical work on each of these processes has in turn stimulated new observations and the reinterpretation of existing observations. we appear tn be in the midst of a major revolution in our thinking about galaxies. .. _ . ., _ ~ ~~` Em. _~ General Relativity. The advances in General Relativity can be roughly classified as belonging to the strong-field limit or the weak-field limit. Consider the strong-field limit first. Black holes began the decade as mathematical abstraction (an exact solution to the Einstein equations). By the end of the 1970's we knew with some confidence that, for example, black holes are dynamically stable and uniquely described by the Kerr metric, that their colli- sions produce new black holes and radiate gravitational waves, and that quantum processes cause very small black holes to explode into a shower of photons and other par- ticles. In the weak-field limit the main achievement has been the high-precision observational verification of the theory. A variety of new and old solar-system tests, including the retardation and deflection of radio signals

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268 plus lunar laser ranging and spacecraft tracking, support General Relativity as the correct post-Newtonian gravita- tional theory. Parenthetically, they also demonstrate our impressive theoretical understanding of nonrelativistic effects in the solar system: the range residuals in the tests are typically no more than one part per billion. The binary pulsar has provided the first test of General Relativity ever made on an object outside the solar sys- tem and has provided striking evidence for the existence of the quadrupole gravitational radiation predicted by the theory. Shocks in Interstellar Gas. In the 1970's, observa- tions in radio, infrared, optical, ultraviolet, and x-ray astronomy have shown that the interstellar medium is per- vaded by shocks. These observations have stimulated extensive development of the theory and consequences of such shocks; their origin in stellar explosions and stel- lar winds; the effects of radiative precursors, thermal conduction, and magnetic fields on their structure; the roles of shocks in the cycling of the gas between cool dense phases and a hot tenuous phase, in destroying interstellar dust grains, and in the acceleration of cosmic rays. We now appreciate that shocks play a major role in the cycle of interstellar matter between gas and stars, and we suspect that they may cause propagating waves of star formation. Recent studies show the impor- tance of shocks in dense interstellar clouds. It appears that they are responsible for the infrared emission from H2 molecules in the Orion nebula. We are beginning to understand how such shocks can initiate networks of chem- ical reactions in the clouds. Volcanism on Io. One of the most spectacular images of the 1970's is the Voyager picture of a volcanic plume rising 200 km above the surface of Jupiter's satellite Io. The prediction of volcanism on Io, shortly before the discovery of eight active volcanoes during the Voyager flyby, is also a spectacular theoretical accomplishment. The satellite Europa perturbs Io's orbit away from cir- cularity, so that the Jovian tides on Io are not constant. The consequent deposition of energy on Io due to the tidal working of the rock was predicted to be sufficient to heat all but the outer crust of the satellite to a molten state. X-Ray Astronomy. The flood of x-ray observations in the 1970's offered a difficult challenge and a remarkable opportunity to theoretical astronomers. As we enter the 1980's we have a broad understanding of the nature and

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269 origin of the x-ray emission from a wide variety of ob- jects. Many of the galactic sources were recognized as close binary systems, with mass flow onto an accretion disk around a compact object, either a neutron star or a black hole. The x-ray emitting regions of these sources are some of the most exotic environments known to physics; to understand them requires consideration of gas dynamics and new radiative processes in relativistic flows--for the case of neutron stars, in magnetic fields greater than 1012 gauss. The x-ray burst phenomenon was interpreted as thermonuclear explosions on neutron star surfaces. The need to understand the origin of the x-ray binaries sting ulated extensive preliminary development of the theory of the evolution of binary star systems with mass transfer. These new developments in the "mature" area of stellar evolution theory have led in turn to a better understand- ing of novae and cataclysmic variable stars. The efforts to understand why compact x-ray sources occur so fre- quently in globular clusters and in galactic nuclei have led to a better understanding of the dynamical evolution of dense stellar systems. Accurate x-ray position deter- minations were used to determine the masses of x-ray sources in globular clusters, indicating that the x-ray sources are low-mass binary systems. There is now little doubt that the x-ray emission from clusters of galaxies is due to hot intracluster gas enriched in heavy elements, and this realization has had a profound impact on our understanding of galactic evolution in these clusters. Galaxy Clustering. Detailed statistical analysis of galaxy catalogs has shown a remarkably simple and regular clustering structure. It is now widely recognized that galaxy correlation functions offer us important clues to conditions in the early universe, and attempts have begun to exploit these clues. The progress of this field exhib- its the synergistic relationship of theory and observa- tion: galaxy catalogs compiled in the 1960's were ex- ploited by intensive theoretical analysis in the 1970's, and the results from this analysis stimulated the compila- tion of extensive new catalogs (including red shifts), which will be ready early in the 1980's. Ephemerides. The ephemerides of the Moon and inner- most planets have improved in accuracy by several orders of magnitude since the late 1960's. The combination of vigorous theoretical modeling and precise numerical inte- gration of the orbits have permitted fits to new data types with unprecedented accuracy. The range data are fit with typical accuracies of 0.3 m (lunar laser), 8 m .

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270 (Mars Viking), kilometers (Mercury and Venus radar), and tens of kilometers (Jupiter Pioneer). Optical positions remain the only data type for Saturn through Pluto and the smallest bodies of the solar system, while occulta- tions are important for the Moon. Accurate very-long- baseline interferometric angular positions exist for several inner bodies. The future holds the promise of further improvements in measurement techniques plus the spread of accurate ranging to more distant bodies. The solutions with these data permit stringent tests of rela- tivistic gravitational theories, the determination of masses and radii, the measurement of catalog errors, and the determination of rotational variations. Final Stages of Stellar Evolution. In the 1970's the evolution of a massive star, from hydrogen burning on the main sequence to the collapse of the central region to greater than nuclear density, has been delineated. This problem was posed in the 1930's, but its solution required an understanding of neutral current processes in the weak interaction, detailed knowledge of nucleosynthesis pro- cesses in stars, and numerical hydrodynamics and stellar evolution performed by computer. This is leading to a quantitative understanding of the major features of observed supernovae in terms of models of an exploding star and to quantitative examination of the sites of and yield from stellar nucleosynthesis. For the 1980's many further problems can be studied, for example, nonspherical collapse, the continued exploration of possible explosion mechanisms, and the nature of the gravitational radiation. Analytical Dynamics. There are still many dynamical problems that yield to analytical approaches. As an example it is now known that climatic changes on Mars may result from large changes in its obliquity brought about in turn by long-period changes in the orbit. Other prob- lems have benefited from the development of computer pro- grams that expand and manipulate long series, most par- ticularly Poisson series. These programs have generated analytical theories for the motion of the Moon, planets, and satellites and for the rotations of the Earth and the Moon. Such programs are in their infancy, and their future development and extension to the manipulation of general orthogonal functions will provide a tool with broad applications to theoretical astrophysics.

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271 C. Scientific Questions for the 1980's Scientific knowledge advances by solving old problems and by posing new ones. Theory develops best in a spontane- ous, unpredictable way, and prognostications for its future development are likely to be misleading. However, theorists have a responsibility to pose questions that focus attention on scientific issues. In this spirit we present, in random order, a list of questions for the next decade, chosen to illustrate the range and diversity of the science. We emphasize that this list is not intended to be complete in any sense. Some of the questions were apparent a decade ago; some are new. We expect that only a fraction of them will be answered in the next decade. Some will remain; others will be revised or replaced by more manageable subquestions. For many, the answers will be provided by new observations. And of course, many of the most important advances in theory will address ques- tions that we have not yet asked. How are stars formed, and what processes determine the initial mass function? Why are some white dwarf stars strongly magnetized and others not? How do accretion disks accrete? What powers the jets of active galactic nuclei? Is the solar system stable? Why is the galaxy correlation function a power law of slope minus 1.8? What determined the entropy-to-baryon ratio of the Universe? What was the origin of globular clusters, and what will be their fate? How are planetary nebulas formed? What accounts for the apparent deficiency of neutrinos from the Sun? Why are Jupiter's outer satellites retrograde? Where did Pluto come from? How can some isolated stars be x-ray sources? Under what circmstances does mixing occur within the stars? What accounts for the sunspot cycle? Why do solar magnetic fields clump to form sunspots? How did the Kirkwood gaps form? What accounts for the isotopic anomalies in meteorites, interstellar gas, and cosmic rays? What physical processes set the characteristic masses and radii of galaxies?

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272 What is the equation of state for dense nuclear matter? What accelerates cosmic rays, and how do they propagate? How should we compute the transport coefficients of astrophysical plasmas? How and where are interstellar grains formed? What powers the winds in the T-Tauri stars? How much energy is liberated as gravitational radiation during gravitational collapse? Why did the quasars appear at z = 3.5, and why are there practically none left today? Apart from General Relativity, are any theories of gravity consistent both with solar-system data and the observations of the binary pulsar? What accounts for the break in stellar rotation rate as a function of spectral type? How is the energy of a pulsar glitch stored in neutron star matter, and what determines its release? What fraction of stars ought to have planets? Earth- like planets? How is spiral structure in the galaxies maintained? What events triggered the formation of the solar system? How are power-law spectra produced in quasars? How are the coronas of the Sun and similar stars heated? What role does neutrino pressure play in supernovae? Can elliptical galaxies be formed from the merging of spirals? What is the global nature of convection in stars? What processes, in stars or elsewhere, have affected the deuterium abundance of the interstellar medium? What is the great Red Spot on Jupiter? What determines the fate of common-envelope binary stars? What is the prime mover of active galactic nuclei and quasars? Does differential diffusion play a role in determining stellar surface abundances? How are interstellar clouds supported against gravita- tional collapse? What is the nature and origin of the mass that surrounds galaxies? How do pulsars pulse? Apart from Freidmann models, are there any observa- tionally viable comological models of the big bang?

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275 The Panel on Theoretical Astrophysics of the Greenstein Committee also recommended the following: The strengthening of theoretical groups at the National Astronomy Centers (such as Kitt Peak and the National Radio Astronomy Observatory). These groups have actually decreased in size. We believe that the need to . . implement such a recommendation has become acute (see Recommendation 2 below). The establishment of a National Center for Theoretical Astrophysics, funded at a level of approximately $750,000 per year. This was not done. We do not recommend such a facility now (see Recommendations 2, 3, and 5 below). Providing an increase of computer power available to theorists in the form of an upgraded computing center at Kitt Peak National Observatory. This was not done. Cur- rent technology suggests a different solution (see Recom- mendation 3 below and Chapter 5 of this volume, containing the report of the Panel on Data Processing and Computa- tional Facilities). 2. Current Resources for Theoretical Astrophysics Here we assess the current support pattern and human resources for theoretical astrophysics. me quantitative data presented here are necessarily approximate, as their estimation requires judgments as to what research programs or fractions thereof are theory. Theoretical astrophysics research (excluding solar- system physics) is currently funded by NSF at a level of approximately $2.5 million/year (fiscal year 1978) and by NASA at the level of approximately $1.1 million/year (fiscal year 1979) through university grants programs. This funding is provided through approximately 60 grants by NSF and 30 grants by NASA. In addition to supporting these explicitly theoretical programs, NASA also supports a significant amount of interpretive theory related to specific missions. We have not been able to make a reli- able quantitative estimate of the level of this support. Theoretical astrophysics research is also supported by NSF and NASA through the employment of theorists at nationally prominent centers of astronomical research. The current staffing of theorists at a number of such centers is compared with the total (Ph.D. equivalent) scientific staffs in Table 4.1.

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276 TABLE 4.1 Staff Theorists at Nationally Prominent Centers Theorists Center Permanent Permanent Scientific Staff Postdocs Staff Kitt Peak National 1 2 35 Observatory Cerro Tololo Inter- 0 0 10 American Observatory National Astronomy 0 0 11 and Ionospheric (astronomers) Center National Radio 1.5 3 30 Astronomy Observatory Smithsonian 5.3 - 40 Astrophysical Observatory NASA-Goddard Space 4 6 40 Flight Center NASA-Goddard 3 5 5 Institute for Space Studies NASA Ames Research 6 3 14 Center The total NSF support of theoretical astrophysics, including the staff theorists at the National Astronomy Centers, represents approximately 5 percent of the NSF annual astronomy budget (approximately $57 million in fiscal year 1978). The NASA support of theoretical astrophysics including staff theorists at astronomical research centers represents approximately 1.2 percent of the NASA budget for astrophysical science (about $230 million in fiscal year 1979). Solar and solar-system research have been excluded from the above figures because these areas are funded in different ways. Most of the NSF funding of solar theory

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277 is through the atmospheric sciences section. The largest NSF program for solar theory is at the High Altitude Observatory (MAO) of the National Center for Atmospheric Research, which has 12 solar theorists and a total scien- tific staff of 20. NASA has a program for the support of theoretical studies in solar terrestrial physics, currently funded at a level of approximately $1.4 million/year (fiscal year 1979). As a result of the recommendations of the Space Plasma Physics report of the Space Science Board (National Academy of Sciences, Washington, D.C., 1978), this theo- retical effort has been augmented by a new program funded at a level of $2.2 million/year (fiscal year 1980). The Department of Energy (DOE) makes a significant con- tribution to theoretical astrophysics by supporting Part- time research in astrophysics by staff members at the Los Alamos and Livermore National Laboratories. These labora- tories also support some theorists at universities through visiting scientists programs, especially in those areas involving the special expertise of these laboratories, such as numerical hydrodynamics, stellar structure, super- nova explosions, nucleosynthesis, and gravitational collapse. The National Bureau of Standards (NBS) supports re- search in theoretical astrophysics at the Joint Institute for Laboratory Astrophysics (JILA) by three staff scien- tists and typically two or three visiting scientists. This support provides a valuable link between the NBS programs in atomic and molecular physics and astrophysics, particularly in the area of radiative transfer theory. The support of theory by NSF at the National Astronomy Centers contrasts sharply with that in other sciences. For example, in high-energy physics the major DOE labs have theory groups that are substantial fractions of the total scientific staffs: 15/103 at Brookhaven; 16/146 at Fermilab; and 26/120 at the Stanford Linear Accelerator Center (SLAC). These theory groups, particularly the one at SLAC, are comparable in quality with the best univer- sity departments and strengthen the interaction between experiment and theory and between the laboratories and university departments. This has occurred, for example, at the Smithsonian Astrophysical Observatory, where theorists interact effectively with the Harvard College Ohservatorv and with the observational programs in infra- red, ultraviolet, optical, and x-ray astronomy. We sug- gest (see, for example, Recommendations 1 and 2 below) _ ~ ~ ~ ~ . . ~

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278 that strong theory groups at the National Astronomy Centers should play a similar role. In our opinion, the support of theoretical astrophysics research at the universities has declined to a dangerously low level. The funding of theory at universities with substantial and excellent programs in astrophysics was once adequate to support lively groups of students, post- doctoral fellows, and visiting scientists. These programs are now underfunded, and the resulting lack of flexibility threatens their vigor. Of even greater concern to this panel are the examples we have seen of excellent theorists who have been unable to obtain funding or who have faced great delays in obtaining funding. A study of the 1979 issues of the Astrophysical Journal reveals the following statistics on the publication of theoretical papers: 533 different authors published one or more papers, 124 published two or more, and 48 pub- lished three or more papers. From these statistics, keeping in mind that many of the 533 authors publish less frequently than annually, we infer that some 100-200 people are active theorists. An additional 30 permanent theoretical positions would therefore have a significant impact on theoretical research. As discussed in the report by the Panel on Organiza- tion, Education, and Personnel (see Chapter 6), the rate at which universities appoint astronomers to faculty positions is expected to remain very low throughout the 1980's. We note also that the production of new Ph.D.'s in theoretical astrophysics has dropped precipitously in the past two years, and we expect this rate to decline further and remain low. But, as we have argued, the explosion of astronomical data creates an increasing need for theoretical research during the 1980's. There is now a reservoir of well-trained and highly qualified theorists who are in a "holding pattern" of temporary postdoctoral research positions. While one or two years of postdoctoral study has been a desirable tradition for young theorists, the shortage of permanent positions has forced many theorists into their second or even third postdoctoral appointment. A decade ago these people would have had no difficulty in finding a faculty position at a research-oriented university department, but now such positions are lacking. The existence of this holding pattern signals the prospect of a lengthy period of disruptive relocations and career uncertainty; it discourages excellent young scientists from pursuing a

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279 career in theoretical astrophysics. Yet a continuing production of theorists is required for the future bal- ance of the science. Therefore, we consider it imPera- tive that some 30 new permanent positions for theoretical astrophysicists be created within the next decade. It is sometimes suggested that since the best young theorists are still highly sought after, there is not a serious shortage of permanent positions for theorists. We believe that such suggestions are based on unwarranted confidence in the ability to predict future performance. Furthermore, we have noted several examples of outstand- ing young theorists with the potential to do excellent research who have not been able to find positions in research-oriented departments and have left the field entirely. E. Recommendations for Theoretical Astrophysics We believe that the astronomical community has an out- standing opportunity to remedy the deficiencies of the current program for theoretical astrophysics. At the time when unprecedented demands are confronting the science, we have a reservoir of well-trained and highly motivated young theorists. Likewise, the increasing role of computers in astrophysics coincides with a revolution in computer technology. Three principles have guided our suggested program for theory embodied in the recommenda- tions here: (1) An appropriate mix of theorists and observers should be involved in every astronomical enter- prise; an imbalance toward either observation or theory in institutions for astrophysical research is unhealthy. (2) We must create approximately 30 new permanent posi- tions for theorists to meet the challenge posed by the impending explosion of astronomical data. (3) We must establish a funding pattern adequate to ensure that the theoretical community can function effectively, by inter- acting with other theorists and by exploiting the power of modern computers. RECOMMENDATION II .1: The National Aeronautics and Space Administration should establish a strong, broad program of investment in theoretical astrophysics research. This program should relate to present and future space astron- omy missions and should include three components: (a) the support of theoretical astrophysics through contracts

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280 and grants, (b) a substantial increase in the number of staff theorists at the NASA centers; and (c) increased involvement of theorists in astronomical mission programs. m e NASA missions of the 1980's will be fully effec- tive only if they are developed on a firm scientific foun- dation involving a close interaction between observers and theorists. One of our primary recommendations is that the NASA practice of "exploration first, then extensive obser- vation, then theoretical analysis" be replaced by a more sophisticated and responsive recognition of the simultane- ous and complicated intellectual processes that actually occur in the interplay between observation and theory. New observational initiatives must be based on a healthy theoretical body of knowledge. It appears to us that in the 1970's this obvious point was neglected. NASA has a particular responsibility to support the growth of theory, for it has gained much from the existing knowl- edge. For example, the extraordinary fruitfulness of x-ray astronomy is in part due to the large applicable range of the previously developed plasma physics, the physics of matter at high temperatures, and General Relativity, which could be brought to bear on understand- ing the observational discoveries of the field. mese areas of theory were developed originally for other purposes. NASA has made no comparable investment in the development of these basic theoretical understandings, which will suggest new directions in space astronomy or be the analytic tools for understanding the results of the observational programs to which the agency is already committed. As a minimum that is not only consistent with NASA's mission agency status but also seems required by that status is the investment by NASA in basic astrophysical theory at a level compatible with the amount of theory tnat in effect it consumes. Otherwise we will see new ., . . _ _ . . . mission ideas that are less innovative, less productive, and less valuable scientifically. Space-based astronomy, ground-based observations, and theoretical astrophysics can be complementary. NASA should make its own investment in the broad development of astrophysical theory and in the 1980's, should develop a vigorous program of theo- retical support in order to provide intellectual and conceptual leadership for the 1990's and beyond. We applaud the recent decision by NASA to increase substantially the support of theoretical studies in solar- system plasma physics. We believe that this new program is an excellent response to the concerns raised in the

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281 Space Science Board report, Space Plasma Physics, and will greatly enhance the effectiveness of the NASA pro- gram of solar-system exploration. We suggest that a similar program to support theoretical astrophysics research would greatly strengthen the NASA program in space astronomy, and we make the following specific recommendations: (a) A NASA grant/contract program should be estab- lished specifically to support the investment in theoreti cal astrophysics that is needed now, if there is to be an innovative space observational program in the 1990's and beyond. This theory program should be separated from current specific mission objectives. All fields of theo- retical astrophysics should be represented since many of the theoretical advances most important to NASA's objec- tives cannot be foretold. The program should have a separate program director (cf. Recommendation 4), and proposals should be judged on the basis of peer review. We suggest an initial funding level of $4 million/year in fiscal year 1980 dollars. (b) m e NASA centers for astronomical research, and in particular the Space Telescope Science Institute, should acquire and maintain strong theoretical staffs (see Recommendation 2). In-house theoretical expertise, if of sufficient depth, breadth and quality, can play an important role in helping to create the ideas of future missions. Without being tied to specific missions, the NASA theorist can be a bridge between theory and NASA program objectives and provide effective communications between NASA and the external community of theoretical astrophysicists. (c) Theorists should be an integral part of astro- nomical mission programs. The purpose of a mission team is to make possible the application of diverse techniques and scientific backgrounds, in order to achieve increased understanding. We consider theoretical study to be an additional and distinct means of achieving that under- standing, and it should be thus represented as a matter of course. Theory often suggests the way to more mean- ingful measurements, and theorists should be involved in mission planning and definition as well as data analysis and interpretation of results. RECOMMENDATION II.2: The National Astronomy Centers (e.~., National Radio Astronomy Observatory, National Astronomy and Ionosphere Center, Kitt Peak National -

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282 Observatory) should acquire and maintain a theoretical scientific staff adequate to meet the demands imposed by their observational programs. As described earlier, the National Astronomy Centers are seriously undermanned with theorists. These facil- ities maintain large staffs of observers, and we believe it vital for the future health of astronomy that an equiv- alent theoretical component be established. Specifically, we advocate that the ratio of theorists to observers should be on the order of 1:5, roughly comparable with the ratio found in the larger university departments and high-energy physics laboratories. To be fully effective scientifically, the observational programs of the National Centers need the support of a strong theoretical research activity. Because the obser- vational programs are extremely varied, the theoretical support must have a breadth that cannot be provided by a small group, however competent. Effective theory, espe- cially of the interpretative or phenomenological kind, requires a minimum critical number of active theorists working closely in association with each other and with observers. Postdoctoral positions for theorists are a useful mechanism for introducing new ideas, but the occu- pants operate with high efficiency only with the presence of a permanent staff. Data analysis and software develop- ment should not be the primary responsibilities of the theory group. However, theorists can and should partici- pate in the planning of future observational programs and instruments. , The policy we recommend must be strongly supported by the Center Directors and the funding agencies; otherwise the positions will be subjected too readily to immediate economic pressures and sacrificed to short-term needs. We are opposed to the creation of a National Center for Theoretical Astrophysics, and we do not recommend that we follow the example of the physics community. A greater return for the funds would, in our view, be obtained by creating positions for theoretical astro- physicists at the National Centers, where they can di- rectly influence the observational programs and provide a mechanism for improved communication with theorists at universities (cf. Recommendation 5). Theorists have often grown into talented observers. The trend is not to be discouraged, but such losses to the theoretical staff should be balanced by new appoint- ments.

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283 If our recommendation is followed, we believe that the large portion of the nation's resources in observational astronomy that is employed at the National Centers will be applied with greatly enhanced effectiveness. We emphasize that this recommendation should not be implemented at the expense of theoretical research at the . universities. RECOMMENDATION I I .3: A concerted effort should be made to provide advanced computer technology to theoretical astrophysicists. As discussed earlier, the increasing role of computers in astrophysical theory predicted by the Greenstein report has now become a reality. However, access by theorists to computing power remains uneven and lags the develop- ment of the technology. Three recent developments create an outstanding opportunity for astrophysics. First, there has been an intellectual explosion in the interest among theorists in two- and three-dimensional problems in computational astrophysics. This is fueled both by intrinsically theoretical considerations and by the increasing spatial and spectral resolution of the observations. Examples of such problems are the dynamics of fluids, plasmas, or stellar systems in two and three space dimensions or of photons, neutrinos, cosmic rays, or stars in energy space and one or two space dimensions. The advance to higher dimension permits the investigation of qualitatively different phenomena that cannot be quan- titatively studied otherwise and are fundamental to our understanding of astrophysical systems. (For example, convection does not appear in one-dimensional hydro- dynamics.) Second, there has been a technological explosion in computer development. The decrease in cost and increase in power of computers is projected to continue unabated through the 1980's. Computational capability at a level that has existed only at a few major centers for computa- tion can now be brought to typical astrophysical research facilities. For astrophysical theory, the most spectac- ular improvement in cost-effectiveness has been not in supercomputers but rather in more modest, medium-scale machines with large virtual memories (especially when considered in conjunction with array processors). Third, there has been an explosion in the number of theorists interested in developing innovative uses of the new technology for theoretical problems. For the theorist the development of software for the new computers is

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284 analogous to instrument development for the observer; funding of such development is necessary to explore the possibilities presented by the new and rapidly changing technology. It is vital for the health of theoretical astrophysics that the opportunity to capitalize on these developments not be lost or delayed. We recommend a balanced and flexible program, including the installation of dedicated machines at existing centers for theoretical work, access to the most powerful comput- ers at national centers and DOE laboratories (cf. Recom- mendation 6), and continued use of university computing facilities as appropriate. We expect that such a program will involve a much larger fraction of the theoretical community in the formulation, analysis, and criticism of numerical theory. We need reliable theory to interpret the old and to guide the new observations, and a deeper interaction between numerical and analytical theorists is a vital step toward that goal. Visitor programs are important for scientists who lack good local facilities (cf. Recommendation 5). RECOMMENDATION II.4: The National Science Foundation should establish a separate program director with responsibility for theoretical astrophysics. Under the present arrangement, which divides astronomy artificially into the subdisciplines of "Stars and Stellar Evolution" and "Galactic and Extragalactic Astronomy," theory suffers. Good theory recognizes no arbitrary boundaries, and theoretical principles and techniques applicable in one area are valuable in the other; indeed, the research of many theorists can be placed equally in each category. m e recommendation for a new administra- tive arrangement, similar to that of the Physics Division, would increase the effectiveness of NSF support of astron- omy by providing a mechanism for the development of a balanced long-term theoretical program. It would also provide an identifiable point of contact for theorists, which would be of particular value for young theorists seeking support for the first time. RECOMMENDATION II.5: A network for close communication among theorists should be supported through summer institutes, workshops, and scientific visits. Theory plays a centripetal role in astronomy. The maintenance of the network recommended here provides an essential framework for the effective functioning of the theoretical community and requires a diverse and flexible

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285 support pattern. Developments in theoretical astrophysics tend to be both rapid and complex, so that face-to-face discussions with colleagues are the only fully adequate way of keeping abreast of many areas. Summer institutes, such as those at Aspen and Santa Cruz, have been extremely productive in inspiring both ideas and research collaborations. Programs to support visitors, at both universities and National Centers, can be very valuable. Extended visits are often most produc- tive, but short visits are usually easier to arrange, and both should be judiciously supported. We believe that well-planned visitors' programs at a number of institu- tions can achieve some of the goals of a national theory institute in a more effective and economical manner. Likewise, foreign-exchange programs and international workshops have been extremely valuable. The support of travel by theorists is a vital component of the mainte- nance of a vigorous communications network. Unfortu- nately, this fact has not been fully recognized by the funding agencies, because travel for theoretical research cannot be justified in such concrete and immediate terms as travel for observing. Support for the temporary relief from teaching duties should be available to capable theorists; this should be viewed as a routine device to initiate important projects and to increase scientific range and productivity. Highly talented theorists and promising theoretical programs should be supported wherever they are. The late 1970's and early 1980's have seen, and will continue to see, a dispersal of young astrophysicists at the highest level of ability among institutions that cannot maintain large research groups in this area. The problems faced by theorists at these institutions are particularly acute. The programs we suggest here offer a particularly cost- effective way to maintain high enthusiasm and to increase the scientific productivity of the theoretical community as a whole and should be supported as they serve that purpose. RECOMMENDATION II.6: The Department of Energy labora- tories should maintain a continued involvement with theoretical astrophysics. As noted earlier, the Livermore and Los Alamos National Laboratories make a significant and valuable contribution to theoretical astrophysics research. We urge continued and expanded support of this research by the Department of Energy and other federal agencies because of its value