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229 example at about 50 cm, would represent a major expansion of the scientific capabilities of the VLA. Even longer- wavelength capabilities are possible by constructing an independent Yagi array tied together with the present waveguide system. m e overall resolving power of the VLA could be increased by placing outrigger antennas well beyond the arms of the present Y and con- nested to it by microwave links. m e nature of the VLA system is well adapted to these improvements, which will ensure that it remains the state-of-the-art instrument for a continually growing area of observational radio astronomy. The Panel also recommends that serious consideration be given to upgrading the 300-m antenna at Arecibo, the nation's (and the world's) largest radio telescope in terms of collecting area. The highest priority is for a 300-m extension to the radius of the large fixed primary antenna, to a diameter of 360 m. The estimated cost is $11 million, or about 25 percent of the replacement cost of the entire telescope. The telescope would remain in operation throughout the construction. Extension of the reflector would increase the average signal-to-noise ratio of the instrument by 90 percent and would increase the number of objects of any given type that can be observed by a factor of about 2.5. The greatest use of Arecibo at present is in the observation of gas in distant galaxies; not only would the observable number be increased 2.5 times, but the red shifts that could be reached would be increased by 40 percent. The ability to detect radio spectral lines in very distant quasars would be greatly improved. Perhaps of most impor- tance, the speed at which observations could be made would be increased by a factor of 3.6--in effect tripling the number of 1000-ft telescopes available to U.S. scientists. In view of the heavy demand for this uniquely powerful instrument, this great increase in sensitivity would be a major benefit to American astronomers. IV. SCIENTIFIC PRIORITIES A. Cosmology The dream of understanding the origin and large-scale structure of the Universe is at least as old as civili- zation. The last three decades have seen an explosive growth in our scientific knowledge of the Universe on a grand scale, owing mostly to new astronomical instruments
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230 and observations. Large optical telescopes and the young science of radio astronomy have led the way by permitting astronomers to probe deeper into space and hence further back in time. The current picture of the Universe is that it is expanding and evolving from a tremendous explosion that occurred about 15 billion years ago. The origin of this incredible event, what (if anything) came before, and even whether the concept of previous existence has any meaning are questions about which nothing is known: science today stops short at the big bang. One of the first cosmological studies in radio astron- omy was to measure the number of extragalactic sources as a function of intensity. The goal was to look for departures from classical geometry and thereby to learn the rate by which gravitational forces are slowing the - expansion of the Universe as a whole. Is there enough attraction ultimately to stop the expansion and cause the Universe to collapse, or is the momentum so large that the Universe expands forever? By looking at fainter and fainter sources one is looking further and further back in time. Source counts versus intensity then measure the change in source density due to the changing rate of expansion. Unfortunately, this program failed for the same reason that similar work with optical sources failed: evolution in the intrinsic luminosity of the radio sources. The dramatic identification of the radio source 3C295 with a galaxy at the large red shift z = 0.5 was a major step out into the Universe. Galaxies at this distance and beyond are so faint that optical spectra require long observations even with the largest telescopes. So, to increase the probability of finding a high-red-shift galaxy, optical observers search the region of otherwise unidentified radio sources. Such searches have identi- fied galaxies at red shifts with z about 1, and show promise at even larger red shifts. Galaxies with large red shifts are the best understood bright objects in the Universe and the most promising probes of large-scale space-time structure. Radio astronomy also pointed the way to quasars. These enigmatic sources have high red shifts and, if at the distances implied by their red shifts, their output of light and radio waves is enormous. The high red shifts of quasars (up to z about 3.5) make them extremely interesting as cosmological probes. Again one needs to know more about the intrinsic source mechanisms and evolution, before quasar red shifts can be used as probes
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231 of the size and history of the Universe, but already they are giving important clues to the energetics of galaxies, the composition of the intergalactic medium, and the coarse distribution of matter beyond z = 1. By far our deepest probe into the Universe is the 3 K background radiation, accidentally discovered in 1965, though it had been predicted many years earlier. me 3 K background is the thermal radiation left over from the compact and hot early Universe. The subsequent expansion has cooled the radiation (by red shifting wavelengths) from more than 101° K to the current temperature of 3 K. Because thermal radiation at this temperature has wave- lengths in the centimeter and millimeter ranges, microwave and radio techniques are essential to its study. m e radiation has had practically no interaction with matter for billions of years, coming to us from a very early epoch of the Universe. The detection of the 3 K radiation has caused most cosmologists to reject the steady-state and other non- evolving cosmological models, since these theories cannot plausibly account for such a large quantity of thermal radiation. Assuming the big-bang model to calculate helium production in the early Universe, one obtains the temperature, density, and expansion rate; these values are just right to give the currently observed helium abundance--20 percent to 25 percent by mass, thus sup- oortino the big-bang model. Over the last 15 years, the spectrum and spatial distribution of the 3 K radiation have been observed extensively and are in substantial agreement with theory at the current level of experi- mental accuracy. We can learn a great deal about processes in the very early Universe by measuring the 3 K radiation with the highest possible precision. A question that still confounds cosmologists is how galaxies manage to form in an expanding Universe. When were they produced? How long did it take? What was the sequence of events? Almost nothing is known about this process, which ranks among the most interesting and important in natural science. One possibility is that the 3 K photons suffered some scattering when passing through the hot, dense plasma that should have existed around collapsing galaxies and clusters of galaxies. If so, the spatial distribution of the 3 K radiation may possess very weak ripples on small angular scales (10 arcsec to 30 arcmin), but so far none have been observed. Another clue to the behavior of the Universe at high red shift comes from the "large-scale anisotropy" observations ~ _
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232 of the background radiation, which look for irregularities on angular scales from 10° to 180°. These observations have detected a "dipole" effect (variation with the cosine of the angle in the sky), which is thought to result from the motion of the Galaxy through the 3 K radiation. This dipole variation is an intriguing result in itself, but cosmologists are even more interested in whether the radi- ation possesses any intrinsic structure on large angular scales. A rotating Universe, very-large-scale shear motion, or a slightly nonspherical expansion could each produce large-scale anisotropy in the 3 K background. The opportunity to look for such large-scale structure in the Universe was beyond our means before the discovery of the 3 K radiation. The following are among the key questions in cosmology that radio observations during the coming decade may help to answer: · Is the Universe homogenous and isotropic on large scales? m is was Einstein's fundamental postulate in applying General Relativity to the Universe and seems to be roughly true from galaxy counts. However, the best evidence is the remarkable isotropy of the 3 K radiation, the dipolar effect of the Earth's peculiar motion being only 0.1 percent. More measurements, with better receivers and sky coverage, will greatly improve the current result. The Cosmic Background Explorer (COBE) satellite, to be launched in the 1980's, will reach an accuracy of 0.01 percent in this fundamental observation. · Is the Universe open or closed? One way to find out is to measure the mass density of the Universe. If it is greater than 2 X 10-29 g/cm3, then gravitational attraction will overcome the momentum of expansion and the Universe will eventually collapse. Known luminous matter falls short by a factor of 10, but much of the matter in the Universe could be dark. Galaxies may have large halos made up of very-low-luminosity stars or black holes. Plots of radial velocity as a function of posi- tion for galaxies are flat out to very large radii, indicating high mass density inside--more than can be accounted for by visible starlight, and such "rotation curves" give a means of probing for dark matter in the halos of galaxies. Radio measurements of the 21-cm hydrogen line are particularly useful on large nearby galaxies, where the rotation curve can be followed out beyond the visible edge of the galaxy. The motion of hydrogen clouds around galaxies is another promising way
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233 to study the amount of dark matter they contain. Here again, sensitive 21-cm measurements are needed. · When and how did galaxies form? If current theo- ries are correct, galaxies must have formed sometime after the cosmic temperature had dropped low enough for protons and electrons to combine to neutral hydrogen. Then grav- itational forces overcame radiation drag, and matter con- densed into clouds. Galaxy formation could have taken place at any red shift between z = 1000 (when the temper- ature dropped below 3000 K) and z = 3 (where galaxies in the process of formation would have been detected at visible wavelengths). The formation of galaxies should perturb the 3 K back- ground radiation and show up as ripples or granulation on small angular scales. Small-scale anisotropy measurements are already very close to the levels predicted, and better measurements will soon be made. Quiet receivers, well- shielded antennas, and stable atmospheric conditions are ~ ~ ~ ~ ~ ~. A positive measurement of the intensity and angular scale of small-scale anisot- requlred to detect this effect ropy would be a major advance toward understanding galaxy formation. Two other effects of galaxy formation may be observable at radio frequencies. Theorists have wondered whether star formation in young galaxies, much more intense than that which occurs today, might re-ionize the remaining hydrogen gas. ~ bremsstrahlung (thermal) emission. The resulting Plasma would radiate by _ The detection of this radiation--probably now at decimeter wavelengths--would give us a direct view of the epoch of galaxy formation. Another possibility is to look for 21-cm line emission from neutral hydrogen before it collapse into stars or is re-ionized. Red shifting would have moved this line to long wavelengths, so the search must be done against an intense background from the Galaxy and man-made inter- ference. Some observational work has already been done on this difficult, but important, problem. · How big and how old is the Universe? If one assumes that the current picture of an expanding, evolv- ing Universe is correct, then by measuring the velocities and distances of galaxies one can estimate the age of the Universe. The hard observational problem is to measure A new radio method is the distances to other galaxies. extremely promising. It uses the apparent tight correla- tion between infrared brightness and rotational speed of spiral galaxies, the rotational speed being determined from the 21-cm line. The sensitivity of large antennas
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234 allows hundreds of galaxies at large distances to be measured. Not only is the technique giving Hubble's constant, but it shows promise for answering old ques- tions about the dynamics of galaxies in our neighborhood. For example, is the Galaxy falling toward the Virgo cluster? The early results of these measurements are affirmative. An important lesson from the history of cosmology is that even over a decade one cannot expect to forecast progress accurately. In fact, quite the opposite is true; one expects major discoveries, brand new insights, and perhaps complete changes of course in such a young and innovative science. How then does one plan? Again, history tells us that a good strategy is to extrapolate from current research and ideas--plan for the progress that seems possible. The process of pushing back the frontiers with good ideas and careful science will trigger the important discoveries. Except for the COBE satellite, there are no major new radio facilities whose main justification is cosmological. Because nearly all of astronomy interacts with cosmology, the field will be advanced by implementation of many of the Panel's recommendations. However, some general char- acteristics of the field place emphasis on certain areas of planning for the 1980's. First, there is always a need to observe fainter sources, with a premium on larger telescopes, lower-noise receivers, and more angular and spectral resolution. The high cost of building and operating such instruments has always been a major jus- tification for the National Astronomy Centers, and it is hence important to cosmology to keep these Centers healthy and productive. Second, as with any rapidly developing field, it is important to encourage new ideas and novel instrumentation. Adequate support of good theoretical and experimental research by small groups in university, governmental, and industrial laboratories is essential to progress in a field that puts a high premium on innova- tion. The exciting problems and opportunities in cos- mological research have attracted many young scientists, as well as established scientists from other fields. It is important that the level of research support be commen- surate with the level of quality and interest in this area.
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235 B. Galaxies With unaided eyes we can see only a small fraction of our Galaxy; its local features appear as the Milky Way. Our solar system is imbedded in this structure at a location not particularly favored or unique. We lie beyond its most prominent spiral features, toward an edge that we are as yet unable to define precisely. Galactic studies describe the properties of this system--its size, mass, dynamics, content, and evolutionary history--a descrip- tion of our environment in its broadest sense. This environment is far from static, for the Galaxy is differ- entially rotating: than do the outer regions. changing. . the inner parts circle more often Our neighbors are constantly ~ _ _ ~ _ _ _ A _ Shocks perhaps driven by density waves give rise to regions of star formation on a Galactic scale, and these in turn give rise to the spiral arms. Shocks may also be triggered by supernova explosions. Chemically evolved material is violently ejected and mixed through differ- ential rotation and turbulence. We observe this chemical evolution as an abundance gradient across our Galaxy. In addition to these explosive events we find that there is also a steady stream of matter from the surfaces of stars entering the interstellar medium. m e mechanism here, a stellar wind, is similar to the solar wind. The life cycle continues through the continual formation of new stars from this interstellar material. At the very center of our Galaxy are massive molecular clouds in the environ- ment of a strong radiation field. Various aspects of these events are seen in other galaxies, and we are often guided in the study of our Galaxy by what we see in these other systems. That our Milky Way is a spiral system was first sug- gested over a century ago, but observational evidence supporting large-scale spiral structures has been obtained only in the second half of this century. Application of radio spectral-line techniques to the distribution of interstellar matter supplied the necessary information. The first indications of spiral structure came from 21-cm hydrogen-line studies and more recently from radio studies of the carbon monoxide molecule. Hydrogen-line studies have also shown that the outer regions of a spiral galaxy are often bent or warped. m ere are a number of competing explanations: tidal perturbation by nearby galaxies, gal- actic winds, precession--but none is entirely adequate. The understanding of such warps may be relevant to the
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236 possible existence and form of dark massive halos sur- rounding galaxies. The last decade has seen the discovery of several dozen molecules, organic and inorganic, in the inter- stellar medium. A new discipline, astrochemistry, has developed, in which radio astronomers, chemists, and physicists are collaborating profitably. The growing number of molecular radio lines in external galaxies, now half a dozen, offers promise of a new tool for extraga- lactic research. m e enigmatic properties of galactic nuclei have been extensively studied at optical and at radio wavelengths. However, we lack detailed knowledge of the structure of the central energy source--not only in normal galaxies such as our own but also in the active nuclei of radio galaxies and quasars. Similarly, we lack knowledge of the fundamental physical processes involved in the energy release from these nuclei. m e study of other galaxies has guided us to a better understanding of the kinematics within our own Galaxy. Hydrogen-line studies have shown a flat rotation curve; that is, the rotational velocity in other galaxies remains essentially constant as far from the galactic center as can be measured. This is in sharp contrast to the pre- viously predicted Keplerian decrease, a prediction pat- terned after the decrease of surface brightness toward the edge of a galaxy. Our own Galaxy has recently been shown to have such a flat rotation curve. The shape of the galactic rotation curve is determined by the mass distribution. The failure of rotational vel- ocities to decrease at large distances from the center implies a significant mass distribution exterior to the visible galaxy. Together with the decreasing light at large distances, this requires a significant change in the stellar population of the galaxy with distance from the center. We are abysmally ignorant of the amount, extent, and nature of the mass that surrounds a normal isolated galaxy, so our knowledge of galactic structure in a fundamental sense remains extremely primitive. The quest for intergalactic matter is ongoing. X-ray studies indicate a hot component, but one that appears to involve only a moderate amount of material--an amount far from sufficient to explain why galaxies appear to be gravitationally bound in clusters--or to close the Uni- verse. There have been many searches for a cooler com- ponent, presumably some form of hydrogen, but these searches have been unsuccessful, placing serious con-
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237 straints on the amount of such cool gas either in clus- ters or distributed along the line of sight. No isolated hydrogen clouds have been found in the intergalactic medium, but near some galaxies there is 21-cm evidence for tidal disruption--material pulled out by the gravity of neighboring galaxies. Many such examples are known, the nearest and most conspicuous being the Magellanic Stream. Such 21-cm plumes and streamers may ultimately become valuable probes of the intergalactic medium. Twenty years after their discovery, the high-velocity hydrogen clouds widely and prominently distributed in our own Galaxy are still poorly understood. Are they Galactic or extragalactic? Are they primordial or secondary in nature? What is the relationship, if any, between them and the Magellanic Stream? A quantitative description of the color, H I content, luminosity, and other gross properties of galaxies can be obtained with present instruments. Many of these proper- ties, and especially the intrinsic dispersion, are poorly understood. At least some of these properties must be continually changing, such as the composition of the in- terstellar medium within a galaxy. Some changes must result from the continual evolution of the stellar popula- tion, others appear related to environment. The significant data that come from radio studies often suffer from poor angular resolution, a property inherent in the relatively long wavelength of such electromagnetic radiation. However, interferometric techniques on a large scale such as with the VLA, or on an even larger scale with VLBI, yield a very high resolution, often better than at optical wavelengths. We are thus able to use the radio window as a probe with exceedingly fine resolution, giving detail in our own Galaxy and others on a scale previously unattainable. A relatively unexplored window still remains--the submillimeter region. Both continuum and spectral-line studies will profit greatly from work in this unexplored part of the radio spectrum. C. Quasars and Galactic Nuclei One of the most challenging scientific problems of the past 20 years has been the nature of quasars and their relationship to galaxies in the scheme of cosmic evolu- tion. After the discovery of quasars in the early 1960's, it was clear that their remarkable properties raised fun- damental questions in physics and cosmology. Their large
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238 red shifts implied large distances, placing them near the edge of the observable Universe. Quasars were possible tracers of the large-scale geometry of space and might provide decisive clues about the past and future of the Universe. At the same time, at the distances implied by their red shifts, their energy output had to be enormous, exceeding that of normal galaxies by several orders of magnitude. The energy source would have to be quite unlike those in any well-understood object. If the red shifts were not of cosmological origin, but perhaps caused by gravitational or ballistic effects, other puzzles arose. In recent years the red-shift puzzle has come close to resolution. A number of low-red-shift quasars have been found to reside in groups of galaxies having the same red shift. Furthermore, some BL Lacertae objects (compact sources thought to be closely related to quasars) have been found embedded in galaxies, again with the same red shift. Because the galactic red shifts are presumably cosmological, both discoveries strongly support the hy- pothesis that the red shifts of quasars are also cosmo- logical and that most quasars are indeed very far away. One of the main goals today is to use quasars as cosmo- logical probes and to understand the physical processes that generate such enormous quantities of energy. At the center of most galaxies (including our own) is a concentrated nucleus that seems to cause a variety of phenomena. Because gas motions in galactic nuclei are so irregular, one suspects that large quantities of energy are explosively released from time to time. VLBI at radio wavelengths has shown that the nucleus is typically only a few tens of astronomical units in size. So far, these size measurements seem to be limited to scattering of the radio emission from surrounding clouds of ionized gas; VLBI measurements at wavelengths of 1 cm and shorter are under way to avoid the scattering effects and reveal the actual size and shape of the active region. Seyfert galaxies represent the next step up the energy scale in activity, their nuclei showing evidence of abun- dant quantities of ionized gas in violent agitation. m e radio emission generally exceeds that of our own Galaxy by several factors of 10, and the nucleus is clearly the seat of the activity. No clear division may exist between the nuclei of normal galaxies and those of Seyferts; from high-resolution studies of the structure of such nuclei we hope to understand the similarities and differences. Activity in the nuclei of galaxies is probably an episodic
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239 affair, and Seyfert galaxies may represent the active phase of the nuclear outburst. Giant radio galaxies, whose radio luminosities may exceed that of our own Galaxy by a factor of 106, also have active nuclei. m ese are generally unresolved with the longest existing interferometers and again seem to be the seat of the energetic events. A combination of syn- thesis mapping and long-baseline interferometry has revealed in some radio Galaxies a striking hierarchy of structures: - first an unresolved radio nucleus of less than 10 light-years across, then jets from the nucleus several thousand light-years long, finally enormous radio lobes stretching millions of light-years into space. In several cases, the structures are remarkably well aligned over size scales that differ by a million or more. The alignment constitutes fairly direct evidence that the energy source lies in the compact nucleus. Quasars often exhibit similar hierarchies of scale and alignment, suggesting that quasars and active galactic nuclei are different aspects of the same fundamental mechanism. Statistical studies at both radio and optical wavelengths show that quasars were much more common in the early Universe. Perhaps the activity seen in nearby galaxies (and our own) is the relic of earlier, more vigorous activity. The jury is still out, and further observation of angular structure, time variations in the radio spectrum, and the visual properties of quasars are required to reach a verdict. At present, VLBI explores angular sizes from 3 X 10-4 to 1 X 10-2 arcsec. At such resolution the radio emis- sion from both quasars and the nuclei of radio galaxies often comes from several independent "hot spots" that move about with time. In several cases, the apparent relative velocities of the spots exceed the speed of light. Such velocities are forbidden by the present laws of physics, and one must, of course, examine all alternatives very carefully. It may be that we are not observing a true velocity--the spot from a searchlight beam moving across clouds, for example, can appear to move more rapidly than the speed of light, but it is only an apparent or "phase" velocity. In a similar way one can imagine that the superluminal velocities in quasars arise from moving "searchlight beams" of relativistic particles that only radiate when they encounter surrounding clouds of gas and magnetic fields. Many models have been proposed, but to test them requires very accurate, high-contrast maps from a dedicated VLBI array.
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247 providing detailed clues about the way in which massive stars are born. However, we have only vague observational clues regarding the general distribution of gas within the clouds or the way in which stars of lower mass, like our Sun, condense. Maser emission from OH and especially H2O imply small regions of high density, but detailed interpretation of the maser process is difficult. Infra- red studies show pointlike objects, which are presumably protostars. Until recently, the angular resolution of radio lines has been too poor to reveal sufficient detail, and it will be the task of the new millimeter interfer- ometers and the 25-m dish to provide essential high- resolution data during the 1980's. Many questions need to be answered. For example, can the effects of magnetic field be seen in the collapse of gas into stars? How does the fraction of mass that ends up in stars depend on their mass? What triggers star formation--expanding H II regions, supernova remnants, galactic shocks? Do many stars form at once, or is it largely a sequential process? Do cloudless condense into rings followed by further collapse into binary or multiple star systems as suggested by theory? mese are examples of the many questions for which the tools are available for the decade ahead. Radio studies continue to expand our understanding of the interstellar medium. Early observations of the polarized low-frequency radio emission from the Milky Way demonstrated the pervasive character of the cosmic-rays and magnetic fields in the Galaxy. During the 1970's the detection of the Zeeman effect (the splitting of emission or absorption lines into several components by a magnetic field) has yielded definitive values for the field strength in a few places. The field is surprisingly weak in regions of low density and, on the other hand, strik- ingly strong where it has been observed in molecular clouds. Recent improvements in receiver sensitivity at centimeter wavelengths may make lame-scale mapping of the magnetic field possible in the 1980's. Depending on its strength, the field may have important dynamical effects on the large-scale structure of the Galaxy and on the way in which the molecular clouds give birth to stars. The emission from many molecular species in galactic clouds exhibits departures from local thermodynamic equi- librium. The most extreme examples are the interstellar masers, which show emission from narrow, highly amplified spectral lines. In these sources, VLBI reveals that the radiation comes from a cluster of bright spots having
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248 angular diameters of a 10-3 arcsec or less. The masers often occur in regions of star formation, near newly formed O and B stars. It is plausible that every newly formed massive star may pass through a maser emission stage lasting 104 years or so. Because newly formed stars are often optically obscured by dense dust clouds, and because their surrounding ionization shells are small, the maser radiation may be the best way of studying their earliest stages of development. The velocities and loca- tion of the bright spots serve to outline the dynamical structure of the circumstellar envelopes of the stars, and Zeeman splitting of the lines can reveal the spatial structure of the magnetic field. In many cases, evolution of the emitting region is indicated by variations in the source structure over times as short as a week. As yet we do not know how the masers are formed or how they are "pumped" by a nearby energy source. Further observations with VLBI techniques are necessary for progress on these important questions. During the 1970's our notion of the overall equi- librium of the interstellar medium has changed consider- ably. Observations a decade ago seemed consistent with the view that two phases were in equilibrium, one dense and cold (10-100 K) and the other thin and warm (10,000 K). The discovery of highly ionized oxygen in the W , the discovery of soft x-ray background radiation, and the identification of large gas bubbles in radio pictures of neutral hydrogen have led to the view that there is also a large-scale component that is very hot--near a million degrees. To elucidate this view, the crucial radio studies in the 1980's will be very sensitive hydrogen absorption observations and accurate pulsar distance determinations to find the electron density. From a technological standpoint, the discovery of interstellar molecules has been part of one of the major themes of radio astronomy and indeed radio science--an advance to ever higher frequencies. Experience has shown that the millimeter-wave band--the short-wavelength or high-frequency end of the radio "window" in the terres- trial atmosphere--is the most profitable region in which to find new molecules and study molecular clouds. In response to this finding, a worldwide effort began in the 1970's to construct new millimeter-wave telescopes and to provide the few existing telescopes with better receivers and spectrometers. Over one half of the molecules dis- covered to date were found with one instrument, the 11-m telescope of the National Radio Astronomy Observatory
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249 (NRAO) at Kitt Peak. A successor to this productive facility, a 25-m telescope to be located at high altitude so that it can work as far as possible in the infrared region, has been proposed by NRAO and if constructed will constitute the most powerful U.S. facility for millimeter- wave astronomy. E. Stars and Pulsars Many of the important discoveries of radio astronomy over its 40-year history have been of objects that even by astronomical standards are extremely large: the remains of exploded stars filling regions of space tens of light- years in diameter, the gaseous components of whole gal- axies at least a hundred thousand light-years across, and the giant radio galaxies stretching tens of millions of light-years. On the other hand, in the past decade many of the important astrophysical advances have involved radio-frequency observations of very small objects, rang- ing downward from the compact active nuclei of galaxies-- perhaps only light-weeks in size--to circumstellar envel- opes, closely spaced pairs of orbiting stars, stars undergoing mass loss and other types of surface activity, and the minute, ultradense pulsars with diameters of only a few miles. Enormous advances were made in understanding these phenomena during the 1970's, in part because radio- frequency observations of objects traditionally studied in other wavelength regions have shed new light on their structure, their physical conditions and environment, and their places in the scheme of galactic evolution. For some time now, our understanding of the life cycles of stars has been generally satisfactory, except for the earliest and latest phases: star birth and star death. As described elsewhere in this chapter, star formation takes place in the cold, neutral gas clouds of inter- stellar space, and radio observations of the more abundant molecular species are very effective probes of this intri- cate process. Once formed, a stable, isolated star radi- ates most of its energy at (or near) visible wavelengths. Even so, much detailed information concerning stellar surface phenomena has come from radio observations of the Sun and the solar wind, both from the ground and from Earth-orbiting spacecraft. Observations of red dwarf flare stars, both at radio and optical wavelengths, have demonstrated the existence of similar (though much more violent) surface phenomena on other stars as well.
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250 When a star exhausts its original supply of fuel for nuclear reactions and enters the last phases of its life, other important changes occur. Many of these changes involve the ejection of large quantities of material from the outer layers of the star, and these processes and their consequences are often best observed at radio wave- lengths. Several major new classes of stellar phenomena were first recognized and widely studied with radio tech- niques during the 1970's, including the expanding shells around stars that recently have undergone nova outbursts, shells of relatively cool gas and dust in circumstellar envelopes around red giant stars, mass exchange processes between members of close binary pairs, steady mass outflow from highly luminous, isolated stars of different types, and pulsars or neutron stars, thought to be remnants of supernova explosions. Much of this work involves the ways in which stars shed substantial fractions of their mass before collaps- ing to form white dwarfs or neutron stars. For example, stellar astronomers have recently discovered that young, rapidly evolving stars lose much of their mass in power- ful stellar winds driven by radiation pressure. m is discovery has influenced our understanding of star for- mation and evolution and has had an impact on theories of H II regions, supernova remnants, and binary x-ray sys- tems. Observations of stellar winds and their effects are now possible in the radio, infrared, optical, ultra- violet, and x-ray spectral ranges, and such data provide the basic information needed to construct a model of the underlying physical processes. Accurate mass-loss rates for individual stars are probably best determined from measurements of radio flux, together with estimates of wind velocity obtained from ultraviolet, infrared, or Balmer line observations. Only a small number of young, early-type stars were detectable with the radio tele- scopes of the 1970's, but this situation is now rapidly changing. The VLA, for example, is probably able to detect every Galactic Wolf-Rayet star in its field of view. High-resolution radio interferometry will enable direct measurement of the angular sizes of stellar winds, from which the gas temperature can be directly calculated. The mass lost by stars in stellar winds has implica- tions for many other fields of astronomy. For example, the more luminous early-type stars lose mass at a rate sufficient to modify drastically their evolution, and this effect must be considered when such stars are used as extragalactic distance standards. In a dense inter-
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251 stellar cloud, a stellar wind will modify the radio and infrared emission from the associated compact H II region, causing shocks and x-ray emission and perhaps affecting the formation and excitation of molecules. Winds may even influence star formation, initiating the process by gas compression or terminating it by disrupting the gas cloud. The huge cavities created by stellar winds in diffuse interstellar gas must be taken into account when interpreting the size and structure of giant H II regions, both Galactic and extragalactic, and also when interpret- ing the structure and evolution of supernova shells. These shells have characteristics determined in part by the dynamical action of a stellar wind on the surrounding interstellar gas before the supernova occurred. The same physical processes involved in the gas dynam- ics and radiation transfer within stellar winds are thought to be important in the emission line regions of quasars and active galaxies. Since stellar winds are easier to observe than are quasars, the development of a physical theory to explain stellar winds may contribute to understanding quasars as well. Another exciting discovery of the past few years, a result of x-ray observations, is the detection of stellar coronas around a wide variety of stars. It had been thought that coronas surround only solar-type stars with shallow convection zones and sufficient mechanical energy to heat the outer atmosphere. For the Sun, heating by acoustic waves is now believed to be negligible, and dis- sipation of magnetic energy is the favored energy source. Magnetic fields in the hot coronas of other stars can be studied by observing their radio emission. As the x rays are sensitive to the temperature and emitting volume and the radio emission to magnetic fields, the combination can probe the stellar coronas in great detail. Such informa- tion is necessary to understand how much the coronas affect the stellar winds and ultimately to understand the origin of magnetic activity in stars. In addition to mass outflow from early-type stars, observations reveal the existence of extended circum- stellar gas and dust shells around a variety of other stars, most of which seem to be losing mass at very high rates. Knowledge of the chemical composition of the shells, as well as an understanding of the subsequent injection of this material into the interstellar medium, is crucial to unraveling the mystery of how the galactic material is enriched in heavy elements and complex molecules.
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252 A notable class of stars with dense circumstellar shells consists of the luminous, highly evolved red giants, which may be the principal source of "processed" interstellar material--gas that has been enriched in heavy elements. Density and temperature are such that the shells are best observed at millimeter, submillimeter, and infrared wavelengths. At millimeter wavelengths, observations have shown that many of these stars produce maser emission from SiO, OH, and H2O. Further studies of these maser sources with VLBI techniques will provide information about physical conditions in the shells. As is the case with the shells surrounding early-type stars, spatial mapping of the continuum emission is also of high priority, because one needs shell sizes to determine opacities and masses. Pushing interferometric techniques toward shorter radio wavelengths and into the infrared region is needed to achieve this goal. One of the most important discoveries of modern astron- omy was the detection of the long-predicted neutron stars, first observed as sources of pulsed radio emission. Pul- sars have provided the first (and only) example of macro- scopic matter at nuclear density; with masses comparable with that of the Sun, but radii 105 times smaller, the densities are as large as 1015 times that of normal solid matter. The equation of state of matter in neutron star interiors requires relativistic many-body theory and the most advanced concepts of particle physics. Theoretical advances over the past decade have emphasized the impor- tance of comparing the calculated masses of neutron stars with observations and the possibility of determining the structure of neutron stars from observations of pulsar periods over extended intervals of time. Radio-frequency timing measurements, and recently hard x-ray spectral observations, have shows that pulsars possess huge mag- netic fields (up to 101 times that of the Earth). It is apparent from all these remarkable properties that neutron stars offer a unique testing ground for our under- standing of the fundamental laws of nature. In addition, pulsars have provided new insights into the late stages of stellar evolution and the complex in- terrelations among the constituents of the Galaxy. Since pulsars are thought to be "born" in the most spectacular stellar deaths--supernova explosions--they occupy a well- defined place in the Galactic hierarchy of things. Sys- tematic pulsar surveys, followed by a reckoning of the implied galactic distribution and number density of the pulsar population, have led to an estimate of the pulsar
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253 birth rate, apparently in conflict with the rate of super- novae. Because of its implications for stellar and ga- lactic evolution, this conflict is likely to be a major focus of theoretical effort during the next few years. The problem may be resolved by better determining the mass range of stars that end their lives as supernovae. It is clear that observations of pulsars will continue to yield scientific rewards in the 1980's. A number of obvious problems need to be attacked. The pulsar emission mechanism is still not understood, nor is the process that seems to cause pulsars to cease emitting radio waves when they are about a million years old. More data on old pul- sars are needed, and there is a need for further measure- ments of distances and proper motions to improve the accuracy of inferences based on positions and kinematics. Pulsar observations in general require large telescopes working at wavelengths longer than about 10 cm and (for astrometric purposes) Very-Long-Baseline Interferometry. A unique and fundamental testing ground for theories of gravitation became available in 1974 with the discovery of a binary pulsar--a pulsar orbiting another star. The General Theory of Relativity predicts that two masses orbiting each other will emit gravitational waves, but until recently this fundamental prediction of the theory had never been tested. Gravitational waves radiated by the binary-pulsar system should carry energy away at the expense of orbital kinetic energy, and therefore the pul- sar and its companion should slowly spiral together. Five years of careful timing measurements of the pulsar have revealed just such an effect: the orbital period is gradually decreasing. These data thus furnish the first evidence in support of the existence of gravitational waves, as well as verification of a previously untested aspect of General Relativity. It is essential that obser- vations of this binary pulsar be continued for at least 5 more years. F. The Sun Over the past decade, solar radio astronomy made many advances: the detection of coronal holes, the obser- vation of very small structures associated with the primary energy release in solar flares, and both obser- vational and theoretical advances in the understanding of the complex plasma processes caused by electron streams moving from the Sun through interplanetary space.
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254 The problems of solar radio astronomy can be divided into two broad categories: what we can learn about the Sun itself, i.e., the problems of solar and stellar physics, and what we can learn about fundamental physical processes, especially in plasma physics. m e corona and solar wind provide a unique plasma laboratory with dimen- ~, _ ~ signs of 10t km and more. Many of the phenomena that occur in the corona are not observed under laboratory conditions because they become important only on a large scale. Of all astronomical sources studied, the solar corona and wind have probably yielded the greatest wealth of observational data, and to understand them requires the most advanced ideas of plasma physics. As an example, the most extensively studied problem in astrophysical plasma physics is probably the Type III solar radio burst. This common type of radio emission is caused by a stream of electrons moving from the inner cor- ona to the orbit of the Earth and beyond. This stream excites density waves in the coronal plasma, at first high radio frequencies in the lower corona, then lower frequencies in the outer corona and the interplanetary medium. The radiation (emitted at the local plasma frequency or its second harmonic) arises as nonlinear emission from a region of turbulence. A long-standing problem has been that theory and laboratory experiments predict that instabilities should destroy the streaming motion within a few kilometers, but this is not observed. Two theories have been developed in the past five years to explain the continued propagation of the electron stream and the Type III bursts. In one, the "quasi- linear" theory, there is a backreaction of the plasma waves on the electron velocity distribution that subverts the instability, followed by the reabsorption of most of the wave energy by trailing parts of the stream. In the other, the "nonlinear theory, the plasma waves undergo self-focusing, which concentrates the wave energy into very small volumes, at the same time expelling the plasma particles and subverting the instability by altering the wave spectrum. During the collapse, plasma standing waves would radiate at the local plasma frequency and its second harmonic, with an intensity comparable with that observed. The recent progress on this difficult problem comes mainly from combining sophisticated plasma theory with extensive measurements not only of the radio bursts but also of the fast-electron distribution functions and the plasma wave intensity. Such a combination of diverse information is essential for progress because of the large number of possible solutions to the relevant equations.
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255 The solar flare is another complex interaction of many processes that, despite a century of study, remains poorly understood. The energy for a flare is thought to be stored as electric currents and magnetic fields in the corona and to be released when sudden changes, perhaps caused by shearing motions, occur within the magnetic- field structure. The regions of energy release are probed most directly by centimeter-wavelength radio emissions and hard x rays. Together, the two types of data provide detailed information on the densities and particle ener- gies in the emitting region and the magnetic-field strength and topology. Recent observations, for example, suggest that the plasma in the region of a solar flare is largely thermal at a few hundred million kelvins, rather than being nonthermal and dominated by highly organized beams of electrons. Nonthermal and relativistic particle populations are probably of secondary importance in the energetics of the flare; nevertheless, they are of great interest because of their geophysical effects such as auroras and geomagnetic storms. Further progress in understanding solar flares will require very high spatial and temporal resolution at several radio wavelengths, accurate polarization mea- surements, concurrent hard x-ray data with high spatial resolution, and better theory. Before and after flares we need accurate maps in circular polarization to deter- mine the strength and structure of the coronal magnetic field. m e VLA is a unique and powerful tool for pro- viding the necessary spatial resolution, though improve- ments are required in time resolution polarization our- ity, and map processing. Unfortunately, comparable spatial resolution Is unachievable with present x-ray telescopes. Another facility, the Owens Valley Observa- tory, is developing powerful tools for observing details of the spectrum of bursts with high time resolution (50 msec), though with limited information on spatial structure. Over the past decade it has been learned that changes in the solar corona are not slow and steady but are domi- nated by violent events or "coronal transients, n which are strikingly revealed in white-light pictures. Tran- sients involve the ejection of plasma and magnetic field into the corona, usually in association with a flare or the eruption of a prominence. The mechanical, kinematic, and magnetic energy of the faster, flare-associated tran- sients may equal or exceed that released by the flares in all other forms. Rearrangements of magnetic fields, whose strengths can at present be derived only from meter-
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256 wavelength radio observations, probably provide the driv- ing forces for the transients. As yet we have no convinc- ing theory for the transient mechanisms or their relation to the basic cycle of solar activity. For many transients, it appears that the moving matter and magnetic field act as a piston that generates a shock wave accompanied by nonthermal radio emission. The shock sometimes reaches the Earth, causing auroras and magnetic storms, and is observable from near-Earth spacecraft. Detailed measurements of the shock parameters are just becoming possible: particle densities, field variations, electron distribution functions, plasma wave intensities, and kilometer-wavelength radio emissions. From these mea- surements we can expect much better theories of the struc- ture of collisionless shocks, particle acceleration with- in them, and the nonlinear wave-particle interactions that give rise to the varied phenomena. The theories can then be applied to similar phenomena (of possibly much greater magnitude) that occur in other stars and galaxies, where detailed observations of the plasma parameters cannot be made. Radio observations are also needed to advance our understanding of the quiet solar atmosphere, especially the transition region between the chromosphere and the inner corona. At present there is a discrepancy between radio and extreme ultraviolet observations, with the observed radio brightness at decimeter wavelengths being a factor of 10 lower than expected from models derived from ultraviolet line intensities. It is likely that important physical processes have been omitted when deriv- ing these models, for example, fast electrons from the corona may penetrate deep into the transition region giv- ing nonequilibrium ionization of heavy elements and enhanced line radiation--or heavy ions may diffuse toward higher temperatures, giving anomalous line intensities-- or the continual interchange of mass and energy between chromosphere and corona may invalidate the assumptions of hydrostatic and thermal equilibrium. Similar effects may occur in other stars, but until we understand the pro- cesses occurring in the Sun it is impossible to know. G. The Planets Radar astronomy has produced a host of important results over the last few decades. Detailed maps of a large portion of the surface of Venus have been made, revealing
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257 a variety of geologic structures. The nature of the par- ticles composing Saturn's rings has been largely under stood and the small-scale qualities of the surfaces of the Galilean satellites determined. In this decade, it is expected that new pictures of the surface of Venus will be obtained with oreatlv improved resolution. - _ In addition, there will be a large number of opportunities to observe asteroids, yielding very precise information about their sizes, rotation rates, and surface characteristics. Existing data suggest that some asteroids have surfaces quite different from those of other solar-system objects. It will be of par- ticular interest to see if radar shows that some of these objects are metallic. The above observations can be done well with existing equipment, in particular the large telescope at Arecibo. Although it would be possible to improve the radar capa- bilities at Arecibo by adding new transmitters of higher power or shorter wavelength, at present it would not be cost effective to do this. On the other hand, increasing the radius of the Arecibo reflector by 30 m would improve the radar signal-to-noise ratio about 3.5 times. This would provide a much greater improvement in sensitivity for objects near the extremes of the Arecibo declination coverage. From the standpoint of radar astronomy, it would be extremely cost effective to carry out this extension. During the last decade, centimeter- and millimeter-wave passive radio observations of the planets have produced data important to understanding the atmospheres of the _ , ~ _ , _ _ _ _ ~ _ _ _ nearby planets. Except at centimeter wavelengths, most of the studies have not been made with enough angular resolution to distinguish spatial features. Nevertheless, large-scale properties such as the cloud-top pressure in the Jovian atmosphere and the CO2 partial pressure at the surface of Venus have been deduced. Further progress will be made in the 1980's with millimeter-wave interfer- ometers. Angular resolution of about 1 arcsec is now pos- sible, so individual surface features can be studied with a resolution comparable with that of an optical photo- graph. The most valuable work will probably be studies of distant objects that have not received close scrutiny by spacecraft. Examples include investigation of the surface features of Mercury (a planet without an atmo- sphere) and determination of the abundance of ammonia in the atmospheres of the Jovian planets. Insight into the distribution of nitrogen in the solar system yielded by
Representative terms from entire chapter: