Annual Review of Astronomy and Astrophysics - Volume 48, 2010

Is space-time only brought into being by its energy content? The jury is still out, but other questions that have been with me for much of my life—giant black holes in galactic nuclei, the formation of the Galaxy, the connection between first-order phase transitions and negative specific heats, the cause of the large-scale flow of galaxies relative to the cosmic microwave background—have all received reasonable answers. I have found great fun in understanding the dynamical mechanisms underlying such phenomena as magnetohydrodynamic jets, relativistic disks, and the bars, spirals, and chemical evolution of galaxies. The great challenges for future astronomers will be the exploration of the 96% of the Universe now believed to be neither atomic nor baryonic but perhaps partially leptonic. However, most advances do not come via frontal attack but from “bread-and-butter” investigations in related areas where observation is possible today!

Silicate dust particles are an important player in the cosmic life cycle of matter. They have been detected in a wide variety of environments, ranging from nearby protoplanetary disks to distant quasars. This review summarizes the fundamental properties of silicates relevant to astronomical observations and processes. It provides a review of our knowledge about cosmic silicates, mostly based on results from IR spectroscopy.

This review examines our current understanding of the possible birth environments of our Solar System. Because most stars form within groups and clusters, the question becomes one of determining the nature of the birth aggregate of the Sun. This discussion starts by reviewing Solar System properties that provide constraints on our environmental history. We then outline the range of star-forming environments that are available in the Galaxy and discuss how they affect star and planet formation. The nature of the solar birth cluster is constrained by many physical considerations, including radiation fields provided by the background environment, dynamical scattering interactions, and by the necessity of producing the short-lived radioactive nuclear species inferred from meteoritic measurements. Working scenarios for the solar birth aggregate can be constructed, as discussed herein, although significant uncertainties remain.

Strong lensing is a powerful tool to address three major astrophysical issues: understanding the spatial distribution of mass at kiloparsec and subkiloparsec scale, where baryons and dark matter interact to shape galaxies as we see them; determining the overall geometry, content, and kinematics of the Universe; and studying distant galaxies, black holes, and active nuclei that are too small or too faint to be resolved or detected with current instrumentation. After summarizing strong gravitational lensing fundamentals, I present a selection of recent important results. I conclude by discussing the exciting prospects of strong gravitational lensing in the next decade.

Measurement of the spatial distribution of neutral hydrogen via the redshifted 21-cm line promises to revolutionize our knowledge of the epoch of reionization and the first galaxies, and may provide a powerful new tool for observational cosmology from redshifts 1<z<4. In this review we discuss recent advances in our theoretical understanding of the epoch of reionization (EoR), the application of 21-cm tomography to cosmology and measurements of the dark energy equation of state after reionization, and the instrumentation and observational techniques shared by 21-cm EoR and postreionization cosmology machines. We place particular emphasis on the expected signal and observational capabilities of first generation 21-cm fluctuation instruments.

A fraction of the dust in the interstellar medium (ISM) enters the heliosphere and is measured in situ from spacecraft. This review surveys the in situ measurements and discusses a hence derived model of dust in the local interstellar cloud (LIC). The LIC dust model bears similarities to pristine cometary dust and is characteristic of the warm ISM clouds that fill a part of the ISM in the vicinity of the Sun. Recent and future dust in situ measurements provide a basis for closely studying physical processes in the ISM surrounding the Solar System. The LIC dust is the only dust component measurable in the Solar System that was not previously incorporated in larger Solar System objects. Issues for future LIC dust studies are measuring dust fluxes at the outer heliosphere, measuring the mass distribution with meteor observations and observations from spacecraft, and measuring the LIC dust composition in situ in space.

To understand how planetary systems form in the dusty disks around pre-main-sequence stars, a detailed knowledge of the structure and evolution of these disks is required. Although this is reasonably well understood for the regions of the disk beyond about 1 AU, the structure of these disks inward of 1 AU remains a puzzle. This is partly because it is very difficult to spatially resolve these regions with current telescopes. But it is also because the physics of this region, where the disk becomes so hot that the dust starts to evaporate, is poorly understood. With infrared interferometry it has become possible in recent years to directly spatially resolve the inner 1 AU of protoplanetary disks, albeit in a somewhat limited way. These observations have partly confirmed current models of these regions, but also posed new questions and puzzles. Moreover, it has turned out that the numerical modeling of these regions is extremely challenging. In this review, we give a rough overview of the history and recent developments in this exciting field of astrophysics.

The first flare on the Sun was observed exactly 150 years ago. During most of the long history, only secondary effects have been noticed, so flares remained a riddle. Now the primary flare products, high-energy electrons and ions, can be spatially resolved in hard X-rays (HXRs) and gamma rays on the Sun. Soft X-rays (SXRs) are observed from most stars, including young stellar objects. Structure and bulk motions of the corona are imaged on the Sun in high temperature lines and are inferred from line shifts in stellar coronae. Magnetic reconnection is the trigger for reorganization of the magnetic field into a lower energy configuration. A large fraction of the energy is converted into nonthermal particles that transport the energy to higher density gas, heating it to SXR-emitting temperatures. Flares on young stars are several orders of magnitude more luminous and more frequent; they significantly ionize protoplanetary disks and planetary ionospheres.

The Sun supports a rich spectrum of internal waves that are continuously excited by turbulent convection. The Global Oscillation Network Group (GONG) network and the SOHO/MDI (Solar and Heliospheric Observatory/Michelson Doppler Imager) space instrument provide an exceptional database of spatially resolved observations of solar oscillations, covering more than an entire sunspot cycle (11 years). Local helioseismology is a set of tools for probing the solar interior in three dimensions using measurements of wave travel times and local mode frequencies. Local helioseismology has discovered (a) near-surface vector flows associated with convection, (b) 250 m s⁻¹ subsurface horizontal outflows around sunspots, (c) ∼50 m s⁻¹ extended horizontal flows around active regions (converging near the surface and diverging below), (d) the effect of the Coriolis force on convective flows and active region flows, (e) the subsurface signature of the 15 m s⁻¹ poleward meridional flow, (f) a ±5 m s⁻¹ time-varying depth-dependent component of the meridional circulation around the mean latitude of activity, and (g) magnetic activity on the farside of the Sun.

Whether the stellar initial mass function (IMF) is universal or is instead sensitive to environmental conditions is of critical importance: The IMF influences most observable properties of stellar populations and thus galaxies, and detecting variations in the IMF could provide deep insights into the star formation process. This review critically examines reports of IMF variations, with a view toward whether other explanations are sufficient given the evidence. Studies of the field, young clusters and associations, and old globular clusters suggest that the vast majority were drawn from a universal system IMF: a power law of Salpeter index (Γ = 1.35) above a few solar masses, and a log normal or shallower power law (Γ ∼ 0–0.25) for lower mass stars. The shape and universality of the substellar IMF is still under investigation. Observations of resolved stellar populations and the integrated properties of most galaxies are also consistent with a universal IMF, suggesting no gross variations over much of cosmic time. Indications of “nonstandard” IMFs in specific local and extragalactic environments clearly warrant further study. However, there is no clear evidence that the IMF varies strongly and systematically as a function of initial conditions after the first few generations of stars.

This review discusses smoothed particle hydrodynamics (SPH) in the astrophysical context, with a focus on inviscid gas dynamics. The particle-based SPH technique allows an intuitive and simple formulation of hydrodynamics that has excellent conservation properties and can be coupled to self-gravity with high accuracy. The Lagrangian character of SPH allows it to automatically adjust its resolution to the clumping of matter, a property that makes the scheme ideal for many application areas in astrophysics, where often a large dynamic range in density is encountered. We discuss the derivation of the basic SPH equations in their modern formulation, and give an overview about extensions of SPH developed to treat physics such as radiative transfer, thermal conduction, relativistic dynamics, or magnetic fields. We also briefly describe some of the most important applications areas of SPH in astrophysical research. Finally, we provide a critical discussion of the accuracy of SPH for different hydrodynamical problems, including measurements of its convergence rate for important classes of problems.

Young massive clusters (YMCs) are dense aggregates of young stars that form the fundamental building blocks of galaxies. Several examples exist in the Milky Way Galaxy and the Local Group, but they are particularly abundant in starburst and interacting galaxies. The few YMCs that are close enough to resolve are of prime interest for studying the stellar mass function and the ecological interplay between stellar evolution and stellar dynamics. The distant unresolved clusters may be effectively used to study the star-cluster mass function, and they provide excellent constraints on the formation mechanisms of young cluster populations. YMCs are expected to be the nurseries for many unusual objects, including a wide range of exotic stars and binaries. So far only a few such objects have been found in YMCs, although their older cousins, the globular clusters, are unusually rich in stellar exotica. In this review, we focus on star clusters younger than ∼100 Myr, more than a few current crossing times old, and more massive than ∼10⁴M⊙; the size of the cluster and its environment are considered less relevant as distinguishing parameters. We describe the global properties of the currently known young massive star clusters in the Local Group and beyond, and discuss the state of the art in observations and dynamical modeling of these systems. In order to make this review readable by observers, theorists, and computational astrophysicists, we also review the cross-disciplinary terminology.

The identity of dark matter is a question of central importance in both astrophysics and particle physics. In the past, the leading particle candidates were cold and collisionless, and typically predicted missing energy signals at particle colliders. However, recent progress has greatly expanded the list of well-motivated candidates and the possible signatures of dark matter. This review begins with a brief summary of the standard model of particle physics and its outstanding problems. I then discuss several dark matter candidates motivated by these problems, including weakly interacting massive particles (WIMPs), superWIMPs, light gravitinos, hidden dark matter, sterile neutrinos, and axions. For each of these, I critically examine the particle physics motivations and present their expected production mechanisms, basic properties, and implications for direct and indirect detection, particle colliders, and astrophysical observations. Upcoming experiments will discover or exclude many of these candidates, and progress may open up an era of unprecedented synergy between studies of the largest and smallest observable length scales.

We present a review of spatially resolved giant molecular clouds (GMCs) in nearby galaxies, aiming at providing a template of GMC properties, which may be extrapolated to distant galaxies. We focus on the Magellanic system including the Large and Small Magellanic Clouds (LMC, SMC), M33, and a few dwarfs as observed in the J = 1–0 ¹²CO transition at 2.6-mm wavelength. The X factor, a conversion factor of the ¹²CO intensity to total molecular column density, and the GMC mass distribution, dN/dM, are similar among these galaxies, suggesting that GMCs share similar properties in the Local Group. The GMCs are classified into three types according to their level of star-formation activity and the types are interpreted in terms of evolution in 20–30 Myr rather than as three different generic types. A three-dimensional comparison including the velocity axis has revealed that GMCs in the LMC are associated with Hi envelopes. The Hi envelopes are probably gravitationally bound and may be infalling to increase the GMC mass via Hi-H2 conversion. Recent submillimeter observations are revealing dense and warm clumps in GMCs, suggesting that the interior of a GMC also follows contraction leading to star formation on a similar timescale. Finally, we present an attempt to place these GMC properties among more distant galaxies and discuss future observational prospects.

The age of an individual star cannot be measured, only estimated through mostly model-dependent or empirical methods, and no single method works well for a broad range of stellar types or for a full range in age. This review presents a summary of the available techniques for age-dating stars and ensembles of stars, their realms of applicability, and their strengths and weaknesses. My emphasis is on low-mass stars because they are present from all epochs of star formation in the Galaxy and because they present both special opportunities and problems. The ages of open clusters are important for understanding the limitations of stellar models and for calibrating empirical age indicators. For individual stars, a hierarchy of quality for the available age-dating methods is described. Although our present ability to determine the ages of even the nearest stars is mediocre, the next few years hold great promise as asteroseismology probes beyond stellar surfaces and starts to provide precise interior properties of stars and as models continue to improve when stressed by better observations.

At the dawn of the first discovery of exoplanets orbiting Sun-like stars in the mid-1990s, few believed that observations of exoplanet atmospheres would ever be possible. After the 2002 Hubble Space Telescope detection of a transiting exoplanet atmosphere, many skeptics discounted it as a one-object, one-method success. Nevertheless, the field is now firmly established, with over two dozen exoplanet atmospheres observed today. Hot Jupiters are the type of exoplanet currently most amenable to study. Highlights include: detection of molecular spectral features, observation of day-night temperature gradients, and constraints on vertical atmospheric structure. Atmospheres of giant planets far from their host stars are also being studied with direct imaging. The ultimate exoplanet goal is to answer the enigmatic and ancient question, “Are we alone?” via detection of atmospheric biosignatures. Two exciting prospects are the immediate focus on transiting super Earths orbiting in the habitable zone of M-dwarfs, and ultimately the spaceborne direct imaging of true Earth analogs.

Considerable progress has been made in determining the Hubble constant over the past two decades. We discuss the cosmological context and importance of an accurate measurement of the Hubble constant, focusing on six high-precision distance-determination methods: Cepheids, tip of the red giant branch, maser galaxies, surface brightness fluctuations, the Tully-Fisher relation, and Type Ia supernovae. We discuss in detail known systematic errors in the measurement of galaxy distances and how to minimize them. Our best current estimate of the Hubble constant is 73±2 (random) ± 4 (systematic) km s⁻¹ Mpc⁻¹. The importance of improved accuracy in the Hubble constant will increase over the next decade with new missions and experiments designed to increase the precision in other cosmological parameters. We outline the steps that will be required to deliver a value of the Hubble constant to 2% systematic uncertainty and discuss the constraints on other cosmological parameters that will then be possible with such accuracy.