Annual Review of Astronomy and Astrophysics - Volume 36, 1998

The focus of this review is the work that has been done during the 1990s on using Type Ia supernovae (SNe Ia) to measure the Hubble constant (H0). SNe Ia are well suited for measuring H0. A straightforward maximum-light color criterion can weed out the minority of observed events that are either intrinsically subluminous or substantially extinguished by dust, leaving a majority subsample that has observational absolute-magnitude dispersions of less than σobs (MB) ≃ σobs (MV) ≃ 0.3 mag. Correlations between absolute magnitude and one or more distance-independent SN Ia or parent-galaxy observables can be used to further standardize the absolute magnitudes to better than 0.2 mag. The absolute magnitudes can be calibrated in two independent ways: empirically, using Cepheid-based distances to parent galaxies of SNe Ia, and physically, by light curve and spectrum fitting. At present the empirical and physical calibrations are in agreement at MB ≃ MV ≃ −19.4 or −19.5. Various ways that have been used to match Cepheid-calibrated SNe Ia or physical models to SNe Ia that have been observed out in the Hubble flow have given values of H0 distributed throughout the range of 54–67 km s⁻¹ Mpc⁻¹. Astronomers who want a consensus value of H0 from SNe Ia with conservative errors could, for now, use 60 ± 10 km s⁻¹ Mpc⁻¹.

Eight extrasolar planet candidates have now been identified, all revealed by Keplerian Doppler shifts in their host stars. The masses (m sin i) lie between 0.5 and 7 MJUP, and the semimajor axes are less than 2.1 astronomical units (AU). Doppler detectability favors high masses and small orbits, and improvements will render Saturn masses detectable within a few years. The substellar mass function (dN/dM) for companions is roughly flat from 70 down to 10 MJUP, but it exhibits a sharp increase for masses below 5 MJUP. For three of these companions (47 UMa, ρ Crb, and 55 Cnc), their circular orbits must be primordial (not tidally induced), indicating formation in a disk, as presumed for Solar System planets. Eccentric orbits may be explained by gravitational perturbations, either by companion stars, other planets, or disk resonances. The detections imply that ∼6% of solar-type stars have giant planets within 2 AU. The small orbits (a < 2 AU) imply that the planets formed either in situ, without the benefit of ice grains, or suffered inward migration. Orbital decay within 1 Myr in disks appears inevitable and may shape the planet mass distribution. The observed stability of spectral line shapes suggests that nonradial stellar oscillations do not affect the planet detections.

After recalling the principle of the Hipparcos mission and of the data reduction, the main statistical features of the final Hipparcos and Tycho catalogues are presented. The results, characterized by accuracies in annual proper motions and parallaxes of the order of one milliarcsecond have been already widely used for astrophysical investigations described in this review. In the field of galactic kinematics, new results were obtained in the analysis of the motions of nearby stars, the evolution of OB associations, and the modeling of the Gould Belt. An important result is that, on average, stars are farther apart than previously thought. Among the consequences is a reconciliation between the ages of the Universe and globular clusters. In particular, the evolutionary sequences of metal-poor stars need to be reassessed. The HR diagram is now extended to the giant horizontal branch and A stars in the main sequence. The Hyades are seen for the first time in three dimensions, allowing a detailed description of their dynamics. Some unexplained inconsistencies between the HR diagrams of open clusters were found. Other results in galactic dynamics, open clusters, variable stars, unseen companions of stars, and the post-Newtonian parameters γ are also presented.

Radio emission from solar flares offers a number of unique diagnostic tools to address long-standing questions about energy release, plasma heating, particle acceleration, and particle transport in magnetized plasmas. At millimeter and centimeter wavelengths, incoherent gyrosynchrotron emission from electrons with energies of tens of kilo electron volts to several mega electron volts plays a dominant role. These electrons carry a significant fraction of the energy released during the impulsive phase of flares. At decimeter and meter wavelengths, coherent plasma radiation can play a dominant role. Particularly important are type III and type III–like radio bursts, which are due to upward- and downward-directed beams of nonthermal electrons, presumed to originate in the energy release site. With the launch of Yohkoh and the Compton Gamma-Ray Observatory, the relationship between radio emission and energetic photon emissions has been clarified. In this review, recent progress on our Dunderstanding of radio emission from impulsive flares and its relation to X-ray emission is discussed, as well as energy release in flare-like phenomena (microflares, nanoflares) and their bearing on coronal heating.

Observations of star formation rates (SFRs) in galaxies provide vital clues to the physical nature of the Hubble sequence and are key probes of the evolutionary histories of galaxies. The focus of this review is on the broad patterns in the star formation properties of galaxies along the Hubble sequence and their implications for understanding galaxy evolution and the physical processes that drive the evolution. Star formation in the disks and nuclear regions of galaxies are reviewed separately, then discussed within a common interpretive framework. The diagnostic methods used to measure SFRs are also reviewed, and a self-consistent set of SFR calibrations is presented as an aid to workers in the field.

We review the wide range of observed properties of Herbig Ae/Be stars and try to combine this rich data set into a consistent picture of their circumstellar environment and evolutionary status. We discuss in some detail the geometry of the circumstellar environment. The presence of disks in Herbig Ae/Be stars is inferred from direct and indirect observational evidence. Envelopes can dominate the spectral energy distributions on large angular scales. In some stars, a fairly massive disk persists until the star has reached the main sequence. The evidence for an evolutionary link between isolated Herbig Ae/Be stars and β Pictoris is summarized.

Observations of redshifted Lyman α (Lyα) forest absorption in the spectra of quasistellar objects (QSOs) provide a highly sensitive probe of the distribution of gaseous matter in the universe. Over the past two decades, optical spectroscopy with large ground-based telescopes, and more recently ultraviolet (UV) spectroscopy from space, have yielded a wealth of information on what appears to be a gaseous, photoionized intergalactic medium (IGM), partly enriched by the products of stellar nucleosynthesis, residing in coherent structures over many hundreds of kiloparsecs.

Recent progress with cosmological hydro-simulations based on hierarchical structure formation models has led to important insights into the physical structures giving rise to the forest. If these ideas are correct, a truly inter- and protogalactic medium [at high redshift (z ∼ 3), the main repository of baryons] collapses under the influence of dark matter gravity into flattened or filamentary structures, which are seen in absorption against background QSOs. With decreasing redshift, galaxies forming in the denser regions may contribute an increasing part of the Lyα absorption cross section. Comparisons between large data samples from the new generation of telescopes and artificial Lyα forest spectra from cosmological simulations promise to become a useful cosmological tool.

Recent advances in the understanding of the chemical processes that occur during all stages of the formation of stars, from the collapse of molecular clouds to the assemblage of icy planetesimals in protoplanetary accretion disks, are reviewed. Observational studies of the circumstellar material within 100–10,000 AU of the young star with (sub)millimeter single-dish telescopes, millimeter interferometers, and ground-based as well as space-borne infrared observatories have only become possible within the past few years. Results are compared with detailed chemical models that emphasize the coupling of gas-phase and grain-surface chemistry. Molecules that are particularly sensitive to different routes of formation and that may be useful in distinguishing between a variety of environments and histories are outlined. In the cold, low-density prestellar cores, radicals and long unsaturated carbon chains are enhanced. During the cold collapse phase, most species freeze out onto the grains in the high-density inner region. Once young stars ignite, their surroundings are heated through radiation and/or shocks, whereupon new chemical characteristics appear. Evaporation of ices drives a “hot core” chemistry rich in organic molecules, whereas shocks propagating through the dense envelope release both refractory and volatile grain material, resulting in prominent SiO, OH, and H2O emission. The role of future instrumentation in further developing these chemical and temporal diagnostics is discussed.

Absolute magnitudes are estimated for carbon stars of various subtypes in the Hipparcos catalogue and as found in the Magellanic Clouds. Stellar radii fall within the limits of 2.4–4.7 AU. The chemical composition of carbon stars indicates that the C-N stars show nearly solar C/H, N/H, and ¹²C/¹³C ratios. This indicates that much of the C and N in our Galaxy came from mass-losing carbon stars. Special carbon stars such as the C-R, C-H, and dC stars are described.

Mass loss from asymptotic giant branch carbon stars, at rates up to several × 10⁻⁵M year⁻¹, contributes about half of the total mass return to the interstellar medium. R stars do not lose mass and may be carbon-rich red giants. The mass loss rates for Miras are about 10 times higher than for SRb and Lb stars, whose properties are similar enough to show that they are likely to belong to the same population. The distribution of carbon star mass loss rates peaks at about 10⁻⁷M year⁻¹, close to the rate of growth of the core mass and demonstrative of the close relationship between mass loss and evolution. Infrared spectroscopy shows that dust mixtures can occur. Detached shells are seen around some stars; they appear to form on the time scales of the helium shell flashes and to be a normal occurrence in carbon star evolution.

The Local Group dwarf galaxies offer a unique window to the detailed properties of the most common type of galaxy in the Universe. In this review, I update the census of Local Group dwarfs based on the most recent distance and radial velocity determinations. I then discuss the detailed properties of this sample, including (a) the integrated photometric parameters and optical structures of these galaxies, (b) the content, nature, and distribution of their interstellar medium (ISM), (c) their heavy-element abundances derived from both stars and nebulae, (d) the complex and varied star-formation histories of these dwarfs, (e) their internal kinematics, stressing the relevance of these galaxies to the “dark matter problem” and to alternative interpretations, and (f) evidence for past, ongoing, and future interactions of these dwarfs with other galaxies in the Local Group and beyond. To complement the discussion and to serve as a foundation for future work, I present an extensive set of basic observational data in tables that summarize much of what we know and do not know about these nearby dwarfs. Our understanding of these galaxies has grown impressively in the past decade, but fundamental puzzles remain that will keep the Local Group at the forefront of galaxy evolution studies for some time.

If Earth-like planets orbit nearby stars, they could be detectable with specially designed telescopes. Direct observations would be very revealing, particularly low resolution infrared spectra, which could establish habitability on the basis of temperature and atmospheric water. Abundant, primitive life based on organized molecular structure might reveal itself, as on Earth, by an atmospheric composition modified in ways unlikely to be from inorganic processes. The technical challenge is to detect and obtain spectra of an object with Mbol ∼ 28 that is very close to a star and some 5 × 10⁹ times less luminous. Indirect methods, used to detect Jupiter-mass planets, do not seem to offer an easy intermediate step to finding Earth-like planets. However, the direct detection techniques needed for spectroscopy also offer a viable method for discovery by imaging. Thermal infrared wavelengths, in which a planet emits most energy, are the most favorable. A robust search for planets of ∼100 nearby solar-type stars, with spectroscopic follow-up of Earth-like candidates, could be made with an interferometer ∼75 m in length. In visible light, the Next Generation Space Telescope (NGST) could, with the addition of a high resolution correction instrument, see Earth-like planets around a dozen or so of the nearest stars. Both infrared and optical instruments are possible within the range of current space agency plans.

Extragalactic jets were discovered and initially studied by radio astronomers in connection with extended radio sources. At present, the combination of jets and disks is considered the crucial element in unification models for all active galactic nuclei (AGNs). The acceleration and propagation conditions of jets, together with the aspect ratio of the disk/jet geometry with respect to the observer, shape the morphologies of AGNs. However, these phenomenological models are very complex from the physical and mathematical point of view, as they involve different elements of the theories of gravitation, fluid dynamics, and electrodynamics in a highly nonlinear combination and in conditions not easily reproducible in laboratory plasma or fluid experiments. In the last ten years, theorists have attacked the subject with advanced analytical and numerical methods, and some important results have already been established that confirm the global scenario, although we are still far from a complete physical interpretation. This review summarizes the main results on the art of jet modeling, emphasizing the limitations of the available models and the possibility of new developments.

Cosmic structure has formed as a result of gravitational amplification of primordial density fluctuations together with the action of other physical processes (adiabatic gas dynamics, radiative cooling, photoionization and recombination, radiative transfer). These complex nonlinear processes, acting over a wide range of length scales (from kiloparsecs to tens of megaparsecs), make this a difficult problem for computation. During the last two decades, significant progress has been made in developing numerical methods and statistical tools for analyzing simulations and data. Combined with observational advances, numerical simulations have led to the demise of several formerly popular models and to an improved understanding of galaxy clusters, quasistellar object (QSO) absorption line systems, and other phenomena. This review summarizes these advances.