Annual Review of Astronomy and Astrophysics - Volume 45, 2007

I came to the attention of astronomers through inventing the low temperature bolometer at Texas Instruments. I was quickly drawn into pioneering infrared (IR) astronomy. I soon transferred to the National Radio Astronomy Observatory and then to the University of Arizona to make astronomy my focus. Parallel programs were getting under way at the California Institute of Technology, Cornell, the Universities of Minnesota and of California, San Diego. Although our methods were crude, discoveries were easy and exciting. I was involved in many of them because I could supply good detectors and invented a number of new techniques. Eventually, I supplied detector systems and instruments through founding of a small company. By the early 1970s, systematic IR astronomy was under way on many important problems that are still active research topics. This rapid success led to investments in large new telescopes and in the IRAS survey satellite.

Use of infrared detector arrays in astronomy began roughly 20 years ago, and our detection capabilities in parts of this spectral range have doubled about every seven months since then. A variety of approaches are now used for detector arrays operating from 1 μm to 1 mm and beyond. They include direct hybrid arrays of InSb and HgCdTe photodiodes that operate from 0.6 μm to 5 μm, and of Si:As impurity band conduction detectors from 5 μm to 28 μm; a number of approaches to photoconductive detector arrays in the far-infrared; and bolometer arrays read out by transistors or superconducting devices in the far-infrared through millimeter-wave spectral range. The underlying principles behind these approaches are discussed. The application of these principles is illustrated through detailed discussion of a number of state-of-the-art detector arrays.

High resolution X-ray spectroscopy of the hot gas in galaxy clusters has shown that the gas is not cooling to low temperatures at the predicted rates of hundreds to thousands of solar masses per year. X-ray images have revealed giant cavities and shock fronts in the hot gas that provide a direct and relatively reliable means of measuring the energy injected into hot atmospheres by active galactic nuclei (AGN). Average radio jet powers are near those required to offset radiative losses and to suppress cooling in isolated giant elliptical galaxies, and in larger systems up to the richest galaxy clusters. This coincidence suggests that heating and cooling are coupled by feedback, which suppresses star formation and the growth of luminous galaxies. How jet energy is converted to heat and the degree to which other heating mechanisms are contributing, e.g., thermal conduction, are not well understood. Outburst energies require substantial late growth of supermassive black holes. Unless all of the ∼10⁶² erg required to suppress star formation is deposited in the cooling regions of clusters, AGN outbursts must alter large-scale properties of the intracluster medium.

The striking broad emission line spectroscopic appearance of Wolf-Rayet (WR) stars has long defied analysis, owing to the extreme physical conditions within their line- and continuum-forming regions. Recently, model atmosphere studies have advanced sufficiently to enable the determination of stellar temperatures, luminosities, abundances, ionizing fluxes, and wind properties. The observed distributions of nitrogen- (WN) and carbon (WC)-sequence WR stars in the Milky Way and in nearby star-forming galaxies are discussed; these imply lower limits to progenitor masses of ∼25, 40, and 75 M⊙ for hydrogen-depleted (He-burning) WN, WC, and H-rich (H-burning) WN stars, respectively. WR stars in massive star binaries permit studies of wind-wind interactions and dust formation in WC systems. They also show that WR stars have typical masses of 10–25 M⊙, extending up to 80 M⊙ for H-rich WN stars. Theoretical and observational evidence that WR winds depend on metallicity is presented, with implications for evolutionary models, ionizing fluxes, and the role of WR stars within the context of core-collapse supernovae and long-duration gamma-ray bursts.

At low redshift, only about one-tenth of the known baryons lie in galaxies or the hot gas seen in galaxy clusters and groups. Models posit that these “missing baryons” are in gaseous form in overdense filaments that connect the much denser virialized groups and clusters. About 30% is cool (<10⁵ K) and is detected in Lyα absorption studies, but about half is predicted to lie in the 10⁵–10⁷ K regime. Gas is detected in the 2–5 × 10⁵ K range through OVI absorption studies (7% of the baryons) and possibly near 10⁵ K from broad Lyα absorption (20% of the baryons). Hotter gas (0.5–2 × 10⁶ K) is detected at zero redshift by OVII and OVIII Kα X-ray absorption, and the OVII line strengths seem to correlate with the Galactic soft X-ray background, so it is probably produced by Galactic halo gas, rather than a Local Group medium. There are no compelling detections of the intergalactic hot gas (0.5–10 × 10⁶ K) either in absorption or emission and these upper limits are consistent with theoretical models. Claimed X-ray absorption lines are not confirmed, while most of the claims of soft emission are attributable to artifacts of background subtraction and field-flattening. The missing baryons should become detectable with moderate improvements in instrumental sensitivity.

All four giant planets in the Solar system possess irregular satellites, characterized by large, highly eccentric and/or inclined orbits that are distinct from the nearly circular, uninclined orbits of the regular satellites. This difference can be traced directly to different modes of formation. Whereas the regular satellites grew by accretion within circumplanetary disks the irregular satellites were captured from initially heliocentric orbits at an early epoch. Recently, powerful survey observations have greatly increased the number of known irregular satellites, permitting a fresh look at the group properties of these objects and motivating a re-examination of the mechanisms of capture. None of the suggested mechanisms, including gas-drag, pull-down, and three-body capture, convincingly fit the group characteristics of the irregular satellites. The sources of the satellites also remain unidentified.

The structure of the global heliospheric field reflects the physical properties of the low corona. The implied connections between solar and heliospheric evolution have been analyzed by a powerful multisatellite solar and heliospheric observatory that includes the Ulysses and the Solar and Heliospheric Observatory (SOHO) spacecraft. This coordinated investigation yielded unprecedented observational opportunities. Some of these results confirmed predictions by models based on time-stationary conditions of the corona at its lowest energy state. However, there are results that exhibit important contradictions to these models, and limit their applicability, pointing to the importance of photospheric dynamics and small-scale field emergence for the structure of the heliospheric field. The interpretation of these data provides challenges to our understanding of the physics of the energy transport from the convective layers of the Sun into the heliosphere and motivates new theoretical approaches to the evolution of the heliospheric field.

Cold dark clouds are nearby members of the densest and coldest phase in the Galactic interstellar medium, and represent the most accessible sites where stars like our Sun are currently being born. In this review we discuss recent progress in their study, including the newly discovered IR dark clouds that are likely precursors to stellar clusters. At large scales, dark clouds present filamentary mass distributions with motions dominated by supersonic turbulence. At small, subparsec scales, a population of subsonic starless cores provides a unique glimpse of the conditions prior to stellar birth. Recent studies of starless cores reveal a combination of simple physical properties together with a complex chemical structure dominated by the freeze-out of molecules onto cold dust grains. Elucidating this combined structure is both an observational and theoretical challenge whose solution will bring us closer to understanding how molecular gas condenses to form stars.

Since the detection a decade ago of the planetary companion of 51 Peg, more than 200 extrasolar planets have been unveiled, mostly by radial-velocity measurements but also in a few cases by photometric transit observations or in microlensing experiments. They present a wide variety of characteristics such as large masses with small orbital separations, high eccentricities, period resonances in multiplanet systems, etc. Meaningful features of the statistical distributions of orbital parameters for giant planets or of parent stellar properties have emerged. We discuss them in the context of the constraints they provide for planet-formation models, and in comparison to Neptune-mass planets in short-period orbits recently detected by radial-velocity surveys. Finally, the role of radial-velocity follow-up measurements of transit candidates is emphasized. Planetary physical parameters are determined, bringing important constraints for inner planet structure models.

Relativistic X-ray emission lines from the inner accretion disks around black holes are reviewed. Recent observations with the Chandra X-ray Observatory, X-ray Multi-Mirror Mission-Newton, and Suzaku are revealing these lines to be good probes of strong gravitational effects. A number of important observational and theoretical developments are highlighted, including evidence of black hole spin and effects such as gravitational light bending, the detection of relativistic lines in stellar-mass black holes, and evidence of orbital-timescale line flux variability. In addition, the robustness of the relativistic disk lines against absorption, scattering, and continuum effects is discussed. Finally, prospects for improved measures of black hole spin and understanding the spin history of supermassive black holes in the context of black hole–galaxy coevolution are presented. The best data and most rigorous results strongly suggest that relativistic X-ray disk lines can drive new explorations of General Relativity and disk physics in the future.

Although fundamental for astrophysics, the processes that produce massive stars are not well understood. Large distances, high extinction, and short timescales of critical evolutionary phases make observations of these processes challenging. Lacking good observational guidance, theoretical models have remained controversial. This review offers a basic description of the collapse of a massive molecular core and a critical discussion of the three competing concepts of massive star formation:

• ▪   monolithic collapse in isolated cores
• ▪   competitive accretion in a protocluster environment
• ▪   stellar collisions and mergers in very dense systems

We also review the observed outflows, multiplicity, and clustering properties of massive stars, the upper initial mass function and the upper mass limit. We conclude that high-mass star formation is not merely a scaled-up version of low-mass star formation with higher accretion rates, but partly a mechanism of its own, primarily owing to the role of stellar mass and radiation pressure in controlling the dynamics.

We review current understanding of star formation, outlining an overall theoretical framework and the observations that motivate it. A conception of star formation has emerged in which turbulence plays a dual role, both creating overdensities to initiate gravitational contraction or collapse, and countering the effects of gravity in these overdense regions. The key dynamical processes involved in star formation—turbulence, magnetic fields, and self-gravity—are highly nonlinear and multidimensional. Physical arguments are used to identify and explain the features and scalings involved in star formation, and results from numerical simulations are used to quantify these effects. We divide star formation into large-scale and small-scale regimes and review each in turn. Large scales range from galaxies to giant molecular clouds (GMCs) and their substructures. Important problems include how GMCs form and evolve, what determines the star formation rate (SFR), and what determines the initial mass function (IMF). Small scales range from dense cores to the protostellar systems they beget. We discuss formation of both low- and high-mass stars, including ongoing accretion. The development of winds and outflows is increasingly well understood, as are the mechanisms governing angular momentum transport in disks. Although outstanding questions remain, the framework is now in place to build a comprehensive theory of star formation that will be tested by the next generation of telescopes.