Annual Review of Astronomy and Astrophysics - Volume 37, 1999
Volume 37, 1999
- Preface
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- Review Articles
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Adventures in Cosmogony
Vol. 37 (1999), pp. 1–36More Less▪ AbstractI was born and educated in Canada, obtaining my PhD in experimental nuclear physics. When I learned that technetium had been found in stellar spectra, I taught myself some astrophysics and began to study stellar nucleosynthesis. This is an account of those studies and of the pathway through much of theoretical astrophysics and planetary physics that was a natural outgrowth of the pursuit of nucleosynthesis problems. I also discuss my experiences in government service and in academia, in organization of conferences, in governmental advising, and in academic administration. In particular, I emphasize the logical connections among the various scientific themes that I have pursued.
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A Critical Review of Galactic Dynamos
Vol. 37 (1999), pp. 37–64More Less▪ AbstractI present the general picture of how galactic magnetic fields grow in disks according to the alpha-Omega dynamo theory. Emphasis is placed on following the lines of force during the dynamo process. The dynamo equation is presented together with a simple growing solution for the galactic disk. Then I take up the various critical questions that have been raised concerning the galactic dynamo theory. These are (1) the importance of the escape of flux from the disk in order for the magnetic field to grow; (2) the physics of turbulent diffusion and its mixing of field lines together so that the rms field is possibly greater than the mean field; (3) whether magnetic reconnection plays a role in the galactic dynamo; (4) whether small-scale fields can grow large enough to swamp the dynamo. Then I discuss the possible seed fields from which the dynamo starts and their relation to the primordial hypothesis. Finally I take up the question of the final evolution of the galactic field after the alpha effect saturates. My conclusion is that all these problems warrant attention but none of them seem to be serious enough to cast any real doubt on the dynamo as the most likely generator of galactic fields.
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Frequency Allocation: The First Forty Years
Vol. 37 (1999), pp. 65–96More Less▪ AbstractIn 1960 ICSU set up an Inter-Union Commission (IUCAF) on the Allocation of Frequencies for Space Research and Radio Astronomy, to keep key parts of the radio spectrum clear for passive, scientific use. IUCAF represents URSI, IAU and COSPAR at World Radio Conferences (WRCs) convened by the International Telecommunications Union (ITU) in Geneva; the WRCs establish the international law which governs users of the radio spectrum. This review recounts many serious threats posed to passive scientific research by commercial and military operations, particularly those involving radio emissions from aircraft and spacecraft. The continual conflict between commercial greed and scientific curiosity has often put the future of radio astronomy, space research, and earth exploration in jeopardy. The conflict increases as we move into the Information Age.
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Reference Frames in Astronomy
Vol. 37 (1999), pp. 97–125More Less▪ AbstractAdvances in wide-angle astrometric measurements of three to four orders of magnitude in the last thirty years have resulted in a redefinition of the fundamental astronomical reference frame. This new frame, the International Celestial Reference Frame (ICRF), is based on the radio positions of 212 compact extragalactic radio sources. The ICRF defines the direction of the axes of the International Celestial Reference System (ICRS) with a precision of approximately 20 μas. At optical wavelengths, the Hipparcos catalog is the realization of this frame. The precision with which the ICRF is now determined requires that the ICRS models for precession, nutation, and others, be revised. Increases in the precision of measurements from astrometric space missions will further improve the celestial reference frame and may require its redefinition within the next ten years. These improvements will again challenge the models for the celestial reference system.
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Probing the Universe with Weak Lensing
Vol. 37 (1999), pp. 127–189More Less▪ AbstractGravitational lenses can provide crucial information on the geometry of the Universe, on the cosmological scenario of formation of its structures as well as on the history of its components with look-back time. In this review, I focus on the most recent results obtained during the last five years from the analysis of the weak lensing regime. The potential of weak lensing as a probe of dark matter and the study of the coupling between light and mass on scales of clusters of galaxies, large-scale structures and galaxies is discussed first. Then I present the impact of weak lensing for the study of distant galaxies and of the population of lensed sources as a function of redshift. Finally, I discuss the potential of weak lensing to constrain the cosmological parameters, either from pure geometrical effects observed in peculiar lenses, or from the coupling of weak lensing with the CMB.
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The HR Diagram and the Galactic Distance Scale After Hipparcos
Vol. 37 (1999), pp. 191–237More Less▪ AbstractThe completion and publication of the Hipparcos astrometric catalogue has revitalized studies in many fundamental areas of Galactic structure and stellar evolution. This article reviews the impact of the new parallax results on our understanding of the location of the main-sequence as a function of abundance, of the luminosity calibration of primary distance indicators and of the Galactic distance scale. Many of these issues remain to be resolved.
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Nucleosynthesis in Asymptotic Giant Branch Stars: Relevance for Galactic Enrichment and Solar System Formation
Vol. 37 (1999), pp. 239–309More Less▪ AbstractWe present a review of nucleosynthesis in AGB stars outlining the development of theoretical models and their relationship to observations. We focus on the new high resolution codes with improved opacities, which recently succeeded in accounting for the third dredge-up. This opens the possibility of understanding low luminosity C stars (enriched in s-elements) as the normal outcome of AGB evolution, characterized by production of 12C and neutron-rich nuclei in the He intershell and by mass loss from strong stellar winds. Neutron captures in AGB stars are driven by two reactions: 13C(α,n)16O, which provides the bulk of the neutron flux at low neutron densities (Nn ≤ 107 n/cm3), and 22Ne(α,n)25Mg, which is mildly activated at higher temperatures and mainly affects the production of s-nuclei depending on reaction branchings. The first reaction is now known to occur in the radiative interpulse phase, immediately below the region previously homogenized by third dredge-up. The second reaction occurs during the convective thermal pulses. The resulting nucleosynthesis phenomena are rather complex and rule out any analytical approximation (exponential distribution of neutron fluences). Nucleosynthesis in AGB stars, modeled at different metallicities, account for several observational constraints, coming from a wide spectrum of sources: evolved red giants rich in s-elements, unevolved stars at different metallicities, presolar grains recovered from meteorites, and the abundances of s-process isotopes in the solar system. In particular, a good reproduction of the solar system main component is obtained as a result of Galactic chemical evolution that mixes the outputs of AGB stars of different stellar generations, born with different metallicities and producing different patterns of s-process nuclei. The main solar s-process pattern is thus not considered to be the result of a standard archetypal s-process occurring in all stars. Concerning the 13C neutron source, its synthesis requires penetration of small amounts of protons below the convective envelope, where they are captured by the abundant 12C forming a 13C-rich pocket. This penetration cannot be modeled in current evolutionary codes, but is treated as a free parameter. Future hydrodynamical studies of time dependent mixing will be required to attack this problem. Evidence of other insufficiencies in the current mixing algorithms is common throughout the evolution of low and intermediate mass stars, as is shown by the inadequacy of stellar models in reproducing the observations of CNO isotopes in red giants and in circumstellar dust grains. These observations require some circulation of matter between the bottom of convective envelopes and regions close to the H-burning shell (cool bottom processing). AGB stars are also discussed in the light of their possible contribution to the inventory of short-lived radioactivities that were found to be alive in the early solar system. We show that the pollution of the protosolar nebula by a close-by AGB star may account for concordant abundances of 26Al, 41Ca, 60Fe, and 107Pd. The AGB star must have undergone a very small neutron exposure, and be of small initial mass (
). There is a shortage of 26Al in such models, that however remains within the large uncertainties of crucial reaction rates. The net 26Al production problem requires further investigation.
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Physical Conditions in Regions of Star Formation
Vol. 37 (1999), pp. 311–362More Less▪ AbstractThe physical conditions in molecular clouds control the nature and rate of star formation, with consequences for planet formation and galaxy evolution. The focus of this review is on the conditions that characterize regions of star formation in our Galaxy. A review of the tools and tracers for probing physical conditions includes summaries of generally applicable results. Further discussion distinguishes between the formation of low-mass stars in relative isolation and formation in a clustered environment. Evolutionary scenarios and theoretical predictions are more developed for isolated star formation, and observational tests are beginning to interact strongly with the theory. Observers have identified dense cores collapsing to form individual stars or binaries, and analysis of some of these cores support theoretical models of collapse. Stars of both low and high mass form in clustered environments, but massive stars form almost exclusively in clusters. The theoretical understanding of such regions is considerably less developed, but observations are providing the ground rules within which theory must operate. The richest and most massive star clusters form in massive, dense, turbulent cores, which provide models for star formation in other galaxies.
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High-Energy Processes in Young Stellar Objects
Vol. 37 (1999), pp. 363–408More Less▪ AbstractObservational studies of low-mass stars during their early stages of evolution, from protostars through the zero-age main sequence, show highly elevated levels of magnetic activity. This activity includes strong fields covering much of the stellar surface and powerful magnetic reconnection flares seen in the X-ray and radio bands. The flaring may occur in the stellar magnetosphere, at the star-disk interface, or above the circumstellar disk. Ionization from the resulting high-energy radiation may have important effects on the astrophysics of the disk, such as promotion of accretion and coupling to outflows, and on the surrounding interstellar medium. The bombardment of solids in the solar nebula by flare shocks and energetic particles may account for various properties of meteorites, such as chondrule melting and spallogenic isotopes. X-ray surveys also improve our samples of young stars, particularly in the weak-lined T Tauri phase after disks have dissipated, with implications for our understanding of star formation in the solar neighborhood.
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Sources of Relativistic Jets in the Galaxy
Vol. 37 (1999), pp. 409–443More Less▪ AbstractBlack holes of stellar mass and neutron stars in binary systems are first detected as hard X-ray sources using high-energy space telescopes. Relativistic jets in some of these compact sources are found by means of multiwavelength observations with ground-based telescopes. The X-ray emission probes the inner accretion disk and immediate surroundings of the compact object, whereas the synchrotron emission from the jets is observed in the radio and infrared bands, and in the future could be detected at even shorter wavelengths. Black-hole X-ray binaries with relativistic jets mimic, on a much smaller scale, many of the phenomena seen in quasars and are thus called microquasars. Because of their proximity, their study opens the way for a better understanding of the relativistic jets seen elsewhere in the Universe. From the observation of two-sided moving jets it is inferred that the ejecta in microquasars move with relativistic speeds similar to those believed to be present in quasars. The simultaneous multiwavelength approach to microquasars reveals in short timescales the close connection between instabilities in the accretion disk seen in the X-rays, and the ejection of relativistic clouds of plasma observed as synchrotron emission at longer wavelengths. Besides contributing to a deeper understanding of accretion disks and jets, microquasars may serve in the future to determine the distances of jet sources using constraints from special relativity, and the spin of black holes using general relativity.
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The First 50 Years at Palomar: 1949–1999 The Early Years of Stellar Evolution, Cosmology, and High-Energy Astrophysics
Vol. 37 (1999), pp. 445–486More Less▪ AbstractPROLOGUE
In 1999 we celebrate the 50th anniversary of the initial bringing into operation of the Palomar 200-inch Hale telescope. When this telescope was dedicated, it opened up a much larger and clearer window on the universe than any telescope that had gone before.
Because the Hale telescope has played such an important role in twentieth century astrophysics, we decided to invite one or two of the astronomers most familiar with what has been achieved at Palomar to give a scientific commentary on the work that has been done there in the first fifty years.
The first article of this kind which follows is by Allan Sandage, who has been an active member of the staff of what was originally the Mount Wilson and Palomar Observatories, and later the Carnegie Observatories for the whole of these fifty years.
The article is devoted to the topics which covered the original goals for the Palomar telescope, namely observational cosmology and the study of galaxies, together with discoveries that were not anticipated, but were first made at Palomar and which played a leading role in the development of high energy astrophysics.
The Palomar work first showed how optical astronomy would be the key to our understanding of observations made in other parts of the electromagnetic spectrum, particularly at radio wavelengths and at X-ray energies.
—Geoffrey Burbidge, Editor
An account is given of the history of two observational programs set for the Palomar 200-inch telescope, one by Walter Baade and the other by Edwin Hubble near the start of the scheduled operation of the telescope 50 years ago. The review is partly an assessment of whether, and how well, these programs have been carried to completion, and partly an account of the response of Palomar to new discoveries and developments not foreseen in 1950. Stellar evolution, the discovery of variations in the metallicity of stars of different populations, the chemical evolution of the Galaxy, the Cepheid P-L relation, the redshift-distance relation of the expanding universe, and the extragalactic distance scale are discussed as they relate to the predictions for progress on the programs set out by Baade and Hubble. Not foreseen was the invention and development of radio astronomy and high energy astrophysics, leading to the discovery of radio galaxies, quasars, and the gradual realization of violent events, both in stars and in galaxies. The review is highly restricted to these subjects, covering only three areas among the totality of the work in observational astrophysics studied during the first 50 years at Palomar.
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Elemental Abundances in Quasistellar Objects: Star Formation and Galactic Nuclear Evolution at High Redshifts
Fred Hamann, and Gary FerlandVol. 37 (1999), pp. 487–531More Less▪ AbstractQuasar (QSO) elemental abundances provide unique probes of high-redshift star formation and galaxy evolution. There is growing evidence from both the emission and intrinsic absorption lines that QSO environments have roughly solar or higher metallicities out to redshifts >4. The range is not well known, but solar to a few times solar metallicity appears to be typical. There is also evidence for higher metallicities in more luminous objects and for generally enhanced N/C and Fe/α abundances compared with solar ratios.
These results identify QSOs with vigorous, high-redshift star formation—consistent with the early evolution of massive galactic nuclei or dense protogalactic clumps. However, the QSOs offer new constraints. For example, (a) most of the enrichment and star formation must occur before the QSOs “turn on” or become observable, on time scales of
1 Gyr at least at the highest redshifts. (b) The tentative result for enhanced Fe/α suggests that the first local star formation began at least ∼1 Gyr before the QSO epoch. (c) The star formation must ultimately be extensive to reach high metallicities; that is, a substantial fraction of the local gas must be converted into stars and stellar remnants. The exact fraction depends on the shape of the initial mass function (IMF). (d) The highest derived metallicities require IMFs that are weighted slightly more toward massive stars than in the solar neighborhood. (e) High metallicities also require deep gravitational potentials. By analogy with the well-known mass–metallicity relation among low-redshift galaxies, metal-rich QSOs should reside in galaxies (or protogalaxies) that are minimally as massive (or as tightly bound) as our own Milky Way.
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Far-Ultraviolet Radiation from Elliptical Galaxies
Vol. 37 (1999), pp. 603–648More Less▪ AbstractFar-ultraviolet radiation is a ubiquitous, if unanticipated, phenomenon in elliptical galaxies and early-type spiral bulges. It is the most variable photometric feature associated with old stellar populations. Recent observational and theoretical evidence shows that it is produced mainly by low-mass, small-envelope, helium-burning stars in extreme horizontal branch and subsequent phases of evolution. These are probably descendants of the dominant, metal rich population of the galaxies. Their lifetime UV outputs are remarkably sensitive to their physical properties and hence to the age and the helium and metal abundances of their parents. UV spectra are therefore exceptionally promising diagnostics of old stellar populations, although their calibration requires a much improved understanding of giant branch mass loss, helium enrichment, and atmospheric diffusion.
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Previous Volumes
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Volume 62 (2024)
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Volume 61 (2023)
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Volume 60 (2022)
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Volume 59 (2021)
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Volume 58 (2020)
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Volume 57 (2019)
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Volume 56 (2018)
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Volume 55 (2017)
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Volume 54 (2016)
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Volume 53 (2015)
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Volume 52 (2014)
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Volume 51 (2013)
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Volume 50 (2012)
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Volume 49 (2011)
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Volume 48 (2010)
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Volume 47 (2009)
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Volume 46 (2008)
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Volume 45 (2007)
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Volume 44 (2006)
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Volume 43 (2005)
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Volume 42 (2004)
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Volume 41 (2003)
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Volume 40 (2002)
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Volume 39 (2001)
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Volume 38 (2000)
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Volume 37 (1999)
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Volume 36 (1998)
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Volume 35 (1997)
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Volume 34 (1996)
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Volume 33 (1995)
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Volume 32 (1994)
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Volume 31 (1993)
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Volume 30 (1992)
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Volume 29 (1991)
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Volume 28 (1990)
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Volume 27 (1989)
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Volume 26 (1988)
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Volume 25 (1987)
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Volume 24 (1986)
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Volume 23 (1985)
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Volume 22 (1984)
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Volume 21 (1983)
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Volume 20 (1982)
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Volume 19 (1981)
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Volume 18 (1980)
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Volume 17 (1979)
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Volume 16 (1978)
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Volume 15 (1977)
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Volume 14 (1976)
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Volume 13 (1975)
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Volume 12 (1974)
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Volume 11 (1973)
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Volume 10 (1972)
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Volume 9 (1971)
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Volume 8 (1970)
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Volume 7 (1969)
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Volume 6 (1968)
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Volume 5 (1967)
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Volume 4 (1966)
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Volume 3 (1965)
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Volume 2 (1964)
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Volume 1 (1963)
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Volume 0 (1932)