Annual Review of Astronomy and Astrophysics - Volume 38, 2000

During the last decade, the understanding of fine features of the structure and evolution of stars has become possible as a result of enormous progress made in the acquisition of high-quality observational and experimental data, and of new developments and refinements in the theoretical description of stellar plasmas. The confrontation of high-quality observations with sophisticated stellar models has allowed many aspects of the theory to be validated, and several characteristics of stars relevant to Galactic evolution and cosmology to be inferred. This paper is a review of the results of recent studies undertaken in the context of the Hipparcos mission, taking benefit of the high-quality astrometric data it has provided. Successes are discussed, as well as the problems that have arisen and suggestions proposed to solve them. Future observational and theoretical developments expected and required in the field are also presented.

We review the research on a wide variety of topics using data obtained with the 200-inch Hale telescope. Using state-of-the-art spectrographs, photometers, spectrometers and infrared detectors, the Palomar astronomers investigated the spectra of stars, interstellar matter, AGNs and quasars in great detail. Spectral resolutions ranged from 1000 A for broad-band photometry to 0.04 A using interferometric techniques.

The common envelope phase of binary star evolution plays an essential role in the formation of short period systems containing a compact object. In this process, significant mass and angular momentum are lost, transforming a wide progenitor system into a close remnant binary. The pathways leading to this phase and the outcomes are described. Emphasis is placed on the conditions that are required for survival of the binary according to the results of three-dimensional hydrodynamics calculations. The evolution of high-mass systems containing neutron stars is discussed, including double neutron stars, binary pulsars, Thorne-Zytkow objects, and high- and low-mass X-ray binaries.

In this article we first review the main physical effects to be considered in the building of evolutionary models of rotating stars on the Upper Main-Sequence (MS). The internal rotation law evolves as a result of contraction and expansion, meridional circulation, diffusion processes, and mass loss. In turn, differential rotation and mixing exert a feedback on circulation and diffusion, so that a consistent treatment is necessary.

We review recent results on the evolution of internal rotation and the surface rotational velocities for stars on the Upper MS, for red giants, supergiants, and W-R stars. A fast rotation enhances the mass loss by stellar winds and, conversely, high mass loss removes a lot of angular momentum. The problem of the breakup or Ω-limit is critically examined in connection with the origin of Be and LBV stars. The effects of rotation on the tracks in the HR diagram, the lifetimes, the isochrones, the blue-to-red supergiant ratios, the formation of Wolf-Rayet stars, and the chemical abundances in massive stars as well as in red giants and AGB stars are reviewed in relation to recent observations for stars in the Galaxy and Magellanic Clouds. The effects of rotation on the final stages and on the chemical yields are examined, along with the constraints placed by the periods of pulsars. On the whole, this review points out that stellar evolution is not only a function of mass M and metallicity Z, but of angular velocity Ω as well.

Because calibrated light curves of type Ia supernovae have become a major tool to determine the local expansion rate of the universe and also its geometrical structure, considerable attention has been given to models of these events over the past couple of years. There are good reasons to believe that perhaps most type Ia supernovae are the explosions of white dwarfs that have approached the Chandrasekhar mass, , and are disrupted by thermonuclear fusion of carbon and oxygen. However, the mechanism whereby such accreting carbon-oxygen white dwarfs explode continues to be uncertain. Recent progress in modeling type Ia supernovae as well as several of the still open questions are addressed in this review. Although the main emphasis is on studies of the explosion mechanism itself and on the related physical processes, including the physics of turbulent nuclear combustion in degenerate stars, we also discuss observational constraints.

Astronomical studies in the extreme ultraviolet (EUV) band of the spectrum were dismissed during the early years of space astronomy as impossible, primarily because of the mistaken view that radiation in this band would be absorbed by the interstellar medium. Observations in the 1980s from sounding rockets and limited duration orbital spacecraft began to show the potential of this field and led to the deployment of two spacecraft devoted to EUV astronomy: the UK Wide Field Camera and the Extreme Ultraviolet Explorer. The instrumentation in these missions, although quite limited in comparison with instrumentation in other fields of space astronomy, provided unique and far-reaching results. These included new information on solar system topics, stellar chromospheres and corona, white dwarf astrophysics, cataclysmic variables, the interstellar medium, galaxies, and clusters of galaxies. We summarize these findings herein.

ROSAT observations indicate that approximately half of all nearby groups of galaxies contain spatially extended X-ray emission. The radial extent of the X-ray emission is typically 50–500 h⁻¹100 kpc or approximately 10–50% of the virial radius of the group. Diffuse X-ray emission is generally restricted to groups that contain at least one early-type galaxy. X-ray spectroscopy suggests the emission mechanism is most likely a combination of thermal bremsstrahlung and line emission. This interpretation requires that the entire volume of groups be filled with a hot, low-density gas known as the intragroup medium. ROSAT and ASCA observations indicate that the temperature of the diffuse gas in groups ranges from approximately 0.3 keV to 2 keV. Higher temperature groups tend to follow the correlations found for rich clusters between X-ray luminosity, temperature, and velocity dispersion. However, groups with temperatures below approximately 1 keV appear to fall off the cluster LX-T relationship (and possibly the LX-σ and σ-T cluster relationships, although evidence for these latter departures is at the present time not very strong). Deviations from the cluster LX-T relationship are consistent with preheating of the intragroup medium by an early generation of stars and supernovae.

There is now considerable evidence that most X-ray groups are real, physical systems and not chance superpositions or large-scale filaments viewed edge-on. Assuming the intragroup gas is in hydrostatic equilibrium, X-ray observations can be used to estimate the masses of individual systems. ROSAT observations indicate that the typical mass of an X-ray group is ∼10¹³ h⁻¹100 M out to the radius to which X-ray emission is currently detected. The observed baryonic masses of groups are a small fraction of the X-ray determined masses, which implies that groups are dominated by dark matter. On scales of the virial radius, the dominant baryonic component in groups is likely the intragroup medium.

Since the discovery of the first bona-fide brown dwarfs and extra-solar planets in 1995, the field of low-mass stars and substellar objects has progressed considerably, both from theoretical and observational viewpoints. Recent developments in the physics entering the modeling of these objects have led to significant improvements in the theory and to a better understanding of these objects' mechanical and thermal properties. This theory can now be confronted with observations directly in various observational diagrams (color-color, color-magnitude, mass-magnitude, mass-spectral type), a stringent and unavoidable constraint that became possible only recently with the generation of synthetic spectra. In this paper we present the current state-of-the-art general theory of low-mass stars and sub stellar objects, from one solar mass to one Jupiter mass, regarding primarily their interior structure and evolution. This review is a natural complement to the previous review by Allard et al (1997) on the atmosphere of low-mass stars and brown dwarfs. Special attention is devoted to the comparison of the theory with various available observations. The contribution of low-mass stellar and sub stellar objects to the Galactic mass budget is also analyzed.

The discovery of counterparts in X-ray and optical to radio wavelengths has revolutionized the study of γ-ray bursts, until recently the most enigmatic of astrophysical phenomena. We now know that γ-ray bursts are the biggest explosions in nature, caused by the ejection of ultrarelativistic matter from a powerful energy source and its subsequent collision with its environment. We have just begun to uncover a connection between supernovae and γ-ray bursts, and are finally constraining the properties of the ultimate source of γ-ray burst energy. We review here the observations that have led to this breakthrough in the field; we describe the basic theory of the fireball model and discuss the theoretical understanding that has been gained from interpreting the new wealth of data on γ-ray bursts.

Our understanding of the evolution of organic molecules, and their voyage from molecular clouds to the early solar system and Earth, has changed dramatically. Incorporating recent observational results from the ground and space, as well as laboratory simulation experiments and new methods for theoretical modeling, this review recapitulates the inventory and distribution of organic molecules in different environments. The evolution, survival, transport, and transformation of organics is monitored, from molecular clouds and the diffuse interstellar medium to their incorporation into solar system material such as comets and meteorites. We constrain gas phase and grain surface formation pathways to organic molecules in dense interstellar clouds, using recent observations with the Infrared Space Observatory (ISO) and ground-based radiotelescopes. The main spectroscopic evidence for carbonaceous compounds in the diffuse interstellar medium is discussed (UV bump at 2200 Å, diffuse interstellar bands, extended red emission, and infrared absorption and emission bands). We critically review the signatures and unsolved problemsrelated to the main organic components suggested to be present in the diffuse gas, such as polycyclic aromatic hydrocarbons (PAHs), fullerenes, diamonds, and carbonaceous solids. We also briefly discuss the circumstellar formation of organics around late-typestars.

In the solar system, space missions to comet Halley and observations of the bright comets Hyakutake and Hale-Bopp have recently allowed a reexamination of the organic chemistry of dust and volatiles in long-period comets. We review the advances in this area and also discuss progress being made in elucidating the complex organic inventory of carbonaceous meteorites. The knowledge of organic chemistry in molecular clouds, comets, and meteorites and their common link provides constraints for the processes that lead to the origin, evolution, and distribution of life in the Galaxy.

The brown dwarfs occupy the gap between the least massive star and the most massive planet. They begin as dimly stellar in appearance and experience fusion (of at least deuterium) in their interiors. But they are never able to stabilize their luminosity or temperature and grow ever fainter and cooler with time. For that reason, they can be viewed as a constituent of baryonic “dark matter.” Indeed, we currently have a hard time directly seeing an old brown dwarf beyond 100 pc. After 20 years of searching and false starts, the first confirmed brown dwarfs were announced in 1995. This was due to a combination of increased sensitivity, better search strategies, and new means of distinguishing substellar from stellar objects. Since then, a great deal of progress has been made on the observational front. We are now in a position to say a substantial amount about actual brown dwarfs. We have a rough idea of how many of them occur as solitary objects and how many are found in binary systems. We have obtained the first glimpse of atmospheres intermediate in temperature between stars and planets, in which dust formation is a crucial process. This has led to the proposal of the first new spectral classes in several decades and the need for new diagnostics for classification and setting the temperature scale. The first hints on the substellar mass function are in hand, although all current masses depend on models. It appears that numerically, brown dwarfs may well be almost as common as stars (though they appear not to contain a dynamically interesting amount of mass).

Broad emission lines hold fundamental clues about the kinematics and structure of the central regions in AGN. In this article we review the most robust line profile properties and correlations emerging from the best data available. We identify fundamental differences between the profiles of radio-quiet and radio-loud sources as well as differences between the high- and low-ionization lines, especially in the radio-quiet majority of AGN. An Eigenvector 1 correlation space involving FWHM Hβ, W(FeIIopt)/W(Hβ), and the soft X-ray spectral index provides optimal discrimination between all principal AGN types (from narrow-line Seyfert 1 to radio galaxies). Both optical and radio continuum luminosities appear to be uncorrelated with the E1 parameters. We identify two populations of radio-quiet AGN: Population A sources (with FWHM(Hβ) 4000 km s⁻¹, generally strong FeII emission and a soft X-ray excess) show almost no parameter space overlap with radio-loud sources. Population B shows optical properties largely indistinguishable from radio-loud sources, including usually weak FeII emission, FWHM(Hβ) 4000 km s⁻¹ and lack of a soft X-ray excess. There is growing evidence that a fundamental parameter underlying Eigenvector 1 may be the luminosity-to-mass ratio of the active nucleus (L/M), with source orientation playing a concomitant role.

This review emphasizes the mass loss processes that affect the fates of single stars with initial masses between one and nine solar masses. Just one epoch of mass loss has been clearly demonstrated to be important for these stars; that is the episode that ends their evolution up the asymptotic giant branch. Quite a clear picture of this evolutionary stage is emerging from current studies. Mass loss rates increase precipitously as stars evolve toward greater luminosity and radius and decreased effective temperature. As a result, empirical relationships between mass loss rates and stellar parameters are determined mostly by selection effects and tell us which stars are losing mass rather than how stars lose mass. After detailed theoretical models are found to match observational constraints, the models may be used to extrapolate to populations not available for study nearby, such as young stars with low metallicity. The fates of stars are found to depend on both their initial masses and their initial metallicities; a larger proportion of low-metallicity stars should end up with core masses reaching the Chandrasekhar limit, giving rise to Type 1.5 supernovae, and the remnant white dwarfs of low-Z populations will be both fewer and more massive than those in Population I. There are also clear indications that some stars lose one to several tenths of a solar mass during the helium core flash, but neither models nor observations reveal any details of this process yet. The observational and theoretical bases for a variety of mass loss formulae in current use are also reviewed in this article, and the relations are compared in a series of figures.

This review deals with the winds from “normal” hot stars such as O-stars, B- and A-supergiants, and Central Stars of Planetary Nebulae with O-type spectra. The advanced diagnostic methods of stellar winds, including an assessment of the accuracy of the determinations of global stellar wind parameters (terminal velocities, mass-loss rates, wind momenta, and energies), are introduced and scaling relations as a function of stellar parameters are provided. Observational results are interpreted in the framework of the stationary, one-dimensional (1-D) theory of line-driven winds. Systematic effects caused by nonhomogeneous structures, time dependence, and deviations from spherical symmetry are discussed. The review finishes with a brief description of the role of stellar winds as extragalactic distance indicators and as tracers of the chemical composition of galaxies at high redshift.

The Hubble space telescope observations of the northern Hubble deep field, and more recently its counterpart in the south, provide detections and photometry for stars and field galaxies to the faintest levels currently achievable, reaching magnitudes V ∼ 30. Since 1995, the northern Hubble deep field has been the focus of deep surveys at nearly all wavelengths. These observations have revealed many properties of high redshift galaxies and have contributed to important data on the stellar mass function in the Galactic halo.

The first millisecond X-ray variability phenomena from accreting compact objects have recently been discovered with the Rossi X-ray Timing Explorer. Three new phenomena are observed from low-mass X-ray binaries containing low-magnetic-field neutron stars: millisecond pulsations, burst oscillations, and kilohertz quasi-periodic oscillations. Models for these new phenomena involve the neutron star spin and orbital motion close around the neutron star, and rely explicitly on our understanding of strong gravity and dense matter. I review the observations of these new neutron-star phenomena and some possibly related phenomena in black-hole candidates, and describe the attempts to use these observations to perform measurements of fundamental physical interest in these systems.

More than a decade ago the IRAS satellite opened the realm of external galaxies for studies in the 10 to 100 μm band and discovered emission from tens of thousands of normal and active galaxies. With the 1995–1998 mission of the Infrared Space Observatory¹, the next major steps in extragalactic infrared astronomy became possible: detailed imaging, spectroscopy, and spectrophotometry of many galaxies detected by IRAS, as well as deep surveys in the mid- and far-IR. The spectroscopic data reveal a wealth of detail about the nature of the energy source(s) and about the physical conditions in galaxies. ISO's surveys for the first time explore the infrared emission of distant, high-redshift galaxies. ISO's main theme in extragalactic astronomy is the role of star formation in the activity and evolution of galaxies.