Annual Review of Astronomy and Astrophysics - Volume 35, 1997

Eta Carinae (Eta) is one of the most remarkable of all well-studied stars and perhaps the most poorly understood. Observations with the Hubble Space Telescope and other modern instruments have solved a few of the mysteries concerning this object while opening a comparable number of new ones. In this review we first recount some essential background information concerning Eta, then we sketch most of the observational developments of the past few years, related to the star itself and to its ejecta. Throughout, we propose a series of specific unsolved observational and theoretical problems that seem especially interesting or important at this time.

To what extent are changes in the Earth's global environment linked with fluctuations in its primary energy source, the radiation from a variable star, the Sun? A firm scientific basis for policy making with regard to anthropogenic greenhouse warming of climate and chlorofluorocarbon depletion of ozone requires a reliable answer to this question. Reduction of the vulnerability of spacecraft operations and communications to space weather necessitates knowledge of solar induced variability in Earth's upper atmosphere. Toward these goals, solar radiation monitoring and studies of solar variability mechanisms facilitate an understanding of the sources and amplitudes of the Sun's changing radiation. Interdisciplinary studies that link these changes with a wide array of terrestrial phenomena over the longer time scales of global change and the shorter time scales of space weather address the relevance of solar radiation variability for Earth. However, although numerous associations are apparent between solar and terrestrial fluctuations, full comprehension of the physical mechanisms responsible for the many facets of radiative Sun-Earth coupling remains to be accomplished.

Luminous supersoft X-ray sources were discovered with the Einstein observatory and have been established as an important new class of X-ray binaries on the basis of observations with the Roentgen Satellite (ROSAT). They have extremely soft spectra (equivalent blackbody temperatures of ∼15–80 eV) and are highly luminous (bolometric luminosities of 10³⁶–10³⁸ erg s⁻¹). Correcting for the heavy extinction of soft X rays by interstellar neutral hydrogen, their numbers in the disks of ordinary spiral galaxies like our own and M31 are estimated to be of the order of 10³. Their observed characteristics are consistent with those of white dwarfs, which are steadily or cyclically burning hydrogen-rich matter accreted onto the surface at a rate of order 10⁻⁷ year⁻¹. The required high accretion rates can be supplied by mass transfer on a thermal time scale (10⁶–10⁷ years) from close companion stars that are more massive than the white dwarf accretor, typically 1.3–2.5 . Steady burning can also occur in a post-nova stage, but for shorter time scales, and it has been observed in a few classical novae and symbiotic novae. A few supersoft sources have been found to be recurrent transients. They are possibly connected with very massive white dwarfs accreting at high rates. Luminous supersoft sources may make a considerable contribution to the Type Ia supernova rate in spiral and irregular galaxies.

The influence of Malmquist bias on the studies of extragalactic distances is reviewed, with brief glimpses of the history from Kapteyn to Scott. Special attention is paid to two kinds of biases, for which the names Malmquist biases of the first and second kind are proposed. The essence of these biases and the situations where they occur are discussed.

The bias of the first kind is related to the classical Malmquist bias (involving the “volume effect”), while the bias of the second kind appears when standard candles are observed at different (true) distances, whereby magnitude limit cuts away a part of the luminosity function. In particular, study of the latter bias in distance indicators such as Tully Fisher, available for large fundamental samples of galaxies, allows construction of an unbiased absolute distance scale in the local galaxy universe where approximate kinematic relative distances can be derived. Such investigations, using the method of normalized distances or of the Spaenhauer diagram, support the linearity of the Hubble law and make it possible to derive an unbiased value of the Hubble constant.

As progressively cooler stellar and substellar objects are discovered, the presence first of molecules and then of condensed particulates greatly complicates the understanding of their physical properties. Accurate model atmospheres that include these processes are the key to establishing their atmospheric parameters. They play a crucial role in determining structural characteristics by setting the surface conditions of model interiors and providing transformations to the various observational planes. They can reveal the spectroscopic properties of brown dwarfs and help establish their detectability. In this paper, we review the current state-of-the-art theory and modeling of the atmospheres of very low mass stars, including the coolest known M dwarfs, M subdwarfs, and brown dwarfs, i.e. Teff ≤ 4,000 K and −4.0 ≤ [M/H] ≤ +0.0. We discuss ongoing efforts to incorporate molecular and grain opacities in cool stellar spectra, as well as the latest progress in (a) deriving the effective temperature scale of M dwarfs, (b) reproducing the lower main sequences of metal-poor subdwarfs in the halo and globular clusters, and (c) results of the models related to the search for brown dwarfs.

All neutral atomic hydrogen gas and a large fraction of the molecular gas in the Milky Way Galaxy and external galaxies lie in PDRs, and PDRs are the origin of most of the nonstellar infrared (IR) and the millimeter CO emission from a galaxy. On the surfaces (Av < 1−3) of interstellar clouds, the absorption of far ultraviolet (FUV) photons (hν < 13.6 eV) by gas and dust grains leads to intense emission of [C II] 158 μm, [O I] 63, 146 μm, and H2 rovibrational transitions, as well as IR dust continuum and polycyclic aromatic hydrocarbon (PAH) emission features. Deeper in PDRs, CO rotational and [C I] 370, 609 μm lines originate. The transition of H to H2 and C⁺ to CO occurs within PDRs. Theoretical models compared with observations diagnose such physical parameters as the density and temperature structure, the elemental abundances, and the FUV radiation field in PDRs. Applications include clouds next to H II regions, reflection nebulae, planetary nebulae, red giant outflows, circumstellar gas around young stars, diffuse clouds, the warm neutral medium (WNM), and molecular clouds in the interstellar radiation field: in summary, much of the interstellar medium in galaxies. This review focuses on dense PDRs in the Milky Way Galaxy. Theoretical PDR models help explain the observed correlation of the CO J = 1–0 luminosity with the molecular mass and also suggest FUV-induced feedback mechanisms that may regulate star formation rates and the column density through giant molecular clouds.

High-velocity clouds (HVCs) consist of neutral hydrogen (HI) at velocities incompatible with a simple model of differential galactic rotation; in practice one uses vLSR90 km/s to define HVCs. This review describes the main features of the sky and velocity distributions, as well as the available information on cloud properties, small-scale structure, velocity structure, and observations other than in 21-cm emission. We show that HVCs contain heavy elements and that the more prominent ones are more than 2 kpc from the Galactic plane. We evaluate the hypotheses proposed for their origin and reject those that account for only one or a few HVCs. At least three different hypotheses are needed: one for the Magellanic Stream and possibly related clouds, one for the Outer Arm Extension, and one (or more) for the other HVCs. We discuss the evidence for the accretion and the fountain model but cannot rule out either one.

The properties of galaxies that are lower in surface brightness than the dark night sky are reviewed. There are substantial selection effects against the discovery of galaxies that are unevolved or diffuse; these systems are missing from most wide field catalogs. Low surface brightness galaxies make up a significant amount of the luminosity density of the local universe. They contribute substantial but poorly determined amounts to the census of baryons and dark matter. Low surface brightness galaxies are also relevant to the interpretation of quasar absorption lines and to the understanding of rapidly evolving galaxy populations in the more distant universe. Theories of galaxy formation and evolution must accomodate the properties of these diffuse stellar systems.

The temporal evolution of the optical spectra of various types of supernovae (SNe) is illustrated, in part to aid observers classifying supernova candidates. Type II SNe are defined by the presence of hydrogen, and they exhibit a very wide variety of photometric and spectroscopic properties. Among hydrogen-deficient SNe (Type I), three subclasses are now known: those whose early-time spectra show strong Si II (Ia), prominent He I (Ib), or neither Si II nor He I (Ic). The late-time spectra of SNe Ia consist of a multitude of blended emission lines of iron-group elements; in sharp contrast, those of SNe Ib and SNe Ic (which are similar to each other) are dominated by several relatively unblended lines of intermediate-mass elements. Although SNe Ia, which result from the thermonuclear runaway of white dwarfs, constitute a rather homogeneous subclass, important variations in their photometric and spectroscopic properties are undeniably present. SNe Ib/Ic probably result from core collapse in massive stars largely stripped of their hydrogen (Ib) and helium (Ic) envelopes, and hence they are physically related to SNe II. Indeed, the progenitors of some SNe II seem to have only a low-mass skin of hydrogen; their spectra gradually evolve to resemble those of SNe Ib. In addition to the two well-known photometric subclasses (linear and plateau) of SNe II, which may exhibit minor spectroscopic differences, there is a new subclass (SNe IIn) distinguished by relatively narrow emission lines with little or no P Cygni absorption component and slowly declining light curves. These objects probably have unusually dense circumstellar gas with which the ejecta interact.

Compact groups of galaxies have posed a number of challenging questions. Intensive observational and theoretical studies are now providing answers to many of these and, at the same time, are revealing unexpected new clues about the nature and role of these systems. Most compact groups contain a high fraction of galaxies having morphological or kinematical peculiarities, nuclear radio and infrared emission, and starburst or active galactic nuclei (AGN) activity. They contain large quantities of diffuse gas and are dynamically dominated by dark matter. They most likely form as subsystems within looser associations and evolve by gravitational processes. Strong galaxy interactions result and merging is expected to lead to the ultimate demise of the group. Compact groups are surprisingly numerous and may play a significant role in galaxy evolution.

The physical properties of the faint blue galaxy population are reviewed in the context of observational progress made via deep spectroscopic surveys and Hubble Space Telescope imaging of field galaxies at various limits and theoretical models for the integrated star formation history of the universe. Notwithstanding uncertainties in the properties of the local population of galaxies, convincing evidence has emerged from several independent studies for a rapid decline in the volume-averaged star-formation rate of field galaxies since a redshift z ≈ 1. Together with the small angular sizes and modest mean redshift of the faintest detectable sources, these results can be understood in hierarchical models where the bulk of the star formation occurred at redshifts between z ≈ 1−2. The physical processes responsible for the subsequent demise of the faint blue galaxy population remain unclear. Considerable progress will be possible when the evolutionary trends can be monitored in the context of independent physical parameters such as the underlying galactic mass.

A large collective effort to study the variability of active galactic nuclei (AGN) over the past decade has led to a number of fundamental results on radio-quiet AGN and blazars. In radio-quiet AGN, the ultraviolet (UV) bump in low-luminosity objects is thermal emission from a dense medium, very probably an accretion disk, irradiated by the variable X-ray source. The validity of this model for high-luminosity radio-quiet AGN is unclear because the relevant UV and X-ray observations are lacking. The broad-line gas kinematics appears to be dominated by virialized motions in the gravity field of a black hole, whose mass can be derived from the observed motions. The “accretion disk plus wind” model explains most of the variability (and other) data and appears to be the most appropriate model at present. Future investigations are outlined.

In blazars, rapid variability at the highest energies (gamma-rays) implies that the whole continuum is relativistically boosted along the line of sight. The general correlation found between variations in TeV gamma rays and in X rays for Mrk 421, and between variations in GeV gamma rays and in the IR–optical–UV bands for 3C 279, two prototype objects, supports models in which the same population of relativistic electrons radiates the low-frequency continuum via synchrotron and the high frequency continuum via inverse Compton scattering of soft photons. Identifying the dominant source of soft photons, which is at present unclear, will strongly constrain the jet physics.

The metallicity of stars in the Galaxy ranges from [Fe/H] = −4 to +0.5 dex, and the solar iron abundance is ϵ(Fe) = 7.51 ± 0.01 dex. The average values of [Fe/H] in the solar neighborhood, the halo, and Galactic bulge are −0.2, −1.6, and −0.2 dex respectively.

Detailed abundance analysis reveals that the Galactic disk, halo, and bulge exhibit unique abundance patterns of O, Mg, Si, Ca, and Ti and neutron-capture elements. These signatures show that environment plays an important role in chemical evolution and that supernovae come in many flavors with a range of element yields.

The 300-fold dispersion in heavy element abundances of the most metal-poor stars suggests incomplete mixing of ejecta from individual supernova, with vastly different yields, in clouds of ∼10⁶ M.

The composition of Orion association stars indicates that star-forming regions are significantly self-enriched on time scales of 80 million years. The rapid self-enrichment and inhomogeneous chemical evolution models are required to match observed abundance trends and the dispersion in the age-metallicity relation.

Three cases for mixing not present in standard stellar models are presented: Light element depletion in low mass main-sequence stars, deep mixing in massive stars, and deep mixing in low mass giants. The review begins with the mixing indicators and the predictions of standard models. The observational evidence for anomalous mixing is then presented, followed by the physics of mixing outside the standard model. The status of theoretical models that include extra mixing is then examined.

Observations of parsec-scale radio jets associated with active galactic nuclei are reviewed, with a particular emphasis on high-luminosity core-dominated sources where the most detailed information on individual objects exists. Extensive imaging surveys with very long baseline interferometry (VLBI) have made possible a morphologic classification of compact radio sources and systematic studies of the statistics of apparent (often faster-than-light) motions found in an increasing number of sources. VLBI monitoring studies at centimeter and millimeter wavelengths, enhanced by spectral and polarization imaging, of representative types of AGN can discriminate detailed physical models. The observations are especially discussed in light of the variant of the relativistic beaming model that explains kinematic, spectral, and polarization properties of parsec-scale jets through shocks in an underlying continuous jet flow.

We discuss the present observational and theoretical understanding of the stellar populations of bulges and their implications for galaxy formation and evolution. The place of bulges as key to the Hubble Sequence remains secure, but some old paradigms are giving way to new ones as observations develop. Detailed studies of Local Group bulges and haloes provide a basis on which we consider higher redshift data. We present the evidence for and against the currently common preconceptions that bulges are old, above solar metallicity in the mean, and simply scaled-down versions of ellipticals. We conclude life is not so simple: Bulges are diverse and heterogeneous, and although their properties vary systematically, sometimes they are reminiscent of disks, sometimes of ellipticals. The extant observational data are, however, limited. New and future surveys will rectify this, and we discuss the questions those data will address.