Annual Review of Astronomy and Astrophysics - Volume 50, 2012

The modest science of cosmology I encountered a half century ago has grown into big science. I comment on steps in this development I think I understand because I was there or, in some cases, wish I had been. Wonderful insights—or lucky guesses—and elegant deductions from measurements were accompanied by the usual mix of unlucky guesses and disregard of unwelcome evidence. I say “usual” because I suspect the course of development of any other natural science is similarly erratic. An example in cosmology is Einstein's homogeneous Universe, which was largely accepted as a working hypothesis when there was scant evidence and seriously challenged after we had a reasonable case for homogeneity. Similar mixes of insight and inattention led to the eventual identification of the 3K microwave background, the demonstration that large-scale structure grew by the gravitational instability of the expanding Universe, and the completion of a tight network of cosmological tests. A half century ago, we had little idea what would become of what we were doing in cosmology. We have a better picture now, but I expect there to be more surprises.

This review examines observations of magnetic fields in molecular clouds and what those observations tell us about the theory of molecular cloud evolution and star formation. First, the review briefly summarizes classes of theoretical models of molecular clouds and specific predictions of the models that can be tested by observation. Then, the review describes the techniques for observing and mapping magnetic fields in molecular clouds, followed by discussion of important examples of observational studies using each technique. A synthesis of results from all observational techniques summarizes the current state, which is that though magnetic fields generally dominate turbulence, there is no definitive evidence for magnetic fields dominating gravity in molecular clouds or for ambipolar-diffusion-driven star formation. Finally, the review discusses prospects for advances in our observational capabilities with telescopes and instruments now beginning operation or under construction.

The discovery of large numbers of young low-mass stars and brown dwarfs over the past decade has made it possible to investigate star formation and early evolution in a previously unexplored mass regime. In this review, we begin by describing surveys for low-mass members of nearby associations, open clusters, star-forming regions, and the methods used to characterize their stellar properties. We then use observations of these populations to test theories of star formation and evolution at low masses. For comparison to the formation models, we consider the initial mass function, stellar multiplicity, circumstellar disks, protostellar characteristics, and kinematic and spatial distributions at birth for low-mass stars and brown dwarfs. To test the evolutionary models, we focus on measurements of dynamical masses and empirical Hertzsprung-Russell diagrams for young brown dwarfs and planetary companions.

Understanding massive stars is essential for a variety of branches of astronomy including galaxy and star cluster evolution, nucleosynthesis and supernovae, pulsars, and black holes. It has become evident that massive star evolution is very diverse, being sensitive to metallicity, binarity, rotation, and possibly magnetic fields. Although the problem to obtain a good statistical observational database is alleviated by current large spectroscopic surveys, it remains a challenge to model these diverse paths of massive stars toward their violent end stage.

I show that the main sequence stage offers the best opportunity to gauge the relevance of the various possible evolutionary scenarios. This also allows sketching the post-main-sequence evolution of massive stars, for which observations of Wolf-Rayet stars give essential clues. Recent supernova discoveries owing to the current boost in transient searches allow tentative mappings of progenitor models with supernova types, including pair-instability supernovae and gamma-ray bursts.

Nuclear reaction rates play a critical role in the understanding of stellar evolution and explosions. However, in many cases nuclear reaction rates still carry large uncertainties due to the paucity of experimental data and incomplete theoretical understanding of the underlying reaction mechanisms. New experimental methods and techniques, combined with the development of new theoretical tools, have exposed fresh avenues to pursue nuclear reactions of significance for nucleosynthesis at, or near, the actual temperatures of stellar burning. This review provides an overview of the most critical nuclear reactions for a number of nucleosynthesis environments. It also presents the current status of these reactions and provides insight into the specific uncertainties associated with the reaction rates. We identify existing shortcomings in the data and highlight the needs and opportunities for additional future experiments.

As planets form and grow within gaseous protoplanetary disks, the mutual gravitational interaction between the disk and planet leads to the exchange of angular momentum and migration of the planet. We review current understanding of disk-planet interactions, focusing in particular on physical processes that determine the speed and direction of migration. We describe the evolution of low-mass planets embedded in protoplanetary disks and examine the influence of Lindblad and corotation torques as a function of the disk properties. The role of the disk in causing the evolution of eccentricities and inclinations is also discussed. We describe the rapid migration of intermediate-mass planets that may occur as a runaway process and examine the transition to gap formation and slower migration driven by the viscous evolution of the disk for massive planets. The roles and influence of disk self-gravity and magnetohydrodynamic turbulence are discussed in detail, as a function of the planet mass, as is the evolution of multiple planet systems. Finally, we address the question of how well global models of planetary formation that include migration are able to match observations of extrasolar planets.

Studies of stellar populations, understood to mean collections of stars with common spatial, kinematic, chemical, and/or age distributions, have been reinvigorated during the past decade by the advent of large-area sky surveys such as the Sloan Digital Sky Survey, the Two-Micron All Sky Survey, the Radial Velocity Experiment, and others. We review recent analyses of these data that, together with theoretical and modeling advances, are revolutionizing our understanding of the nature of the Milky Way and galaxy formation and evolution in general. The formation of galaxies like the Milky Way was long thought to be a steady process leading to a smooth distribution of stars. However, the abundance of substructure in the multidimensional space of various observables, such as position, kinematics, and metallicity, is now proven beyond doubt and demonstrates the importance of mergers in the growth of galaxies. Unlike smooth models that involve simple components, the new data reviewed here clearly exhibit many irregular structures, such as the Sagittarius dwarf tidal stream and the Virgo and Pisces overdensities in the halo and the Monoceros stream closer to the Galactic plane. These recent developments have made it clear that the Milky Way is a complex and dynamic structure, one that is still being shaped by the merging of neighboring smaller galaxies. We also briefly discuss the next generation of wide-field sky surveys, such as SkyMapper, Panoramic Survey Telescope & Rapid Response System, Global Astrometric Interferometer for Astrophysics, and the Large Synoptic Survey Telescope, which will improve measurement precision manyfold and include billions of individual stars. The ultimate goal, development of a coherent and detailed story of the assembly and evolutionary history of the Milky Way and other large spirals like it, now appears well within reach.

Adaptive optics is a prime example of how progress in observational astronomy can be driven by technological developments. At many observatories it is now considered to be part of a standard instrumentation suite, enabling ground-based telescopes to reach the diffraction limit and, thus, providing spatial resolution superior to that achievable from space with current or planned satellites. In this review, we consider adaptive optics from the astrophysical perspective. We show that adaptive optics has led to important advances in our understanding of a multitude of astrophysical processes and describe how the requirements from science applications are now driving the development of the next generation of novel adaptive optics techniques.

Formation of galaxy clusters corresponds to the collapse of the largest gravitationally bound overdensities in the initial density field and is accompanied by the most energetic phenomena since the Big Bang and by the complex interplay between gravity-induced dynamics of collapse and baryonic processes associated with galaxy formation. Galaxy clusters are, thus, at the cross-roads of cosmology and astrophysics and are unique laboratories for testing models of gravitational structure formation, galaxy evolution, thermodynamics of the intergalactic medium, and plasma physics. At the same time, their large masses make them a useful probe of growth of structure over cosmological time, thus providing cosmological constraints that are complementary to other probes. In this review, we describe our current understanding of cluster formation: from the general picture of collapse from initial density fluctuations in an expanding Universe to detailed simulations of cluster formation including the effects of galaxy formation. We outline both the areas in which highly accurate predictions of theoretical models can be obtained and areas where predictions are uncertain due to uncertain physics of galaxy formation and feedback. The former includes the description of the structural properties of the dark matter halos hosting clusters, their mass function, and clustering properties. Their study provides a foundation for cosmological applications of clusters and for testing the fundamental assumptions of the standard model of structure formation. The latter includes the description of the total gas and stellar fractions and the thermodynamical and nonthermal processes in the intracluster plasma. Their study serves as a testing ground for galaxy formation models and plasma physics. In this context, we identify a suitable radial range where the observed thermal properties of the intracluster plasma exhibit the most regular behavior and, thus, can be used to define robust observational proxies for the total cluster mass. Finally, we discuss the formation of clusters in nonstandard cosmological models, such as non-Gaussian models for the initial density field and models with modified gravity, along with prospects for testing these alternative scenarios with large cluster surveys in the near future.

Unlike most other planet-detection techniques, gravitational microlensing does not rely on detection of photons from either the host or the planet. Rather, planets are discovered by their gravitational perturbation of light from a more distant source. I review the fundamental concepts of microlensing planet searches and discuss their practical application. I show how the strengths and peculiarities of the method flow from the basic manner in which planets are discovered. In particular, microlensing is sensitive to very low-mass planets on wide orbits and free-floating planets, and can be used to search for planets orbiting host stars with a broad range of masses and Galactocentric distances. However, microlensing events are rare and cannot be predicted in advance, the majority of the host stars are extremely faint, and the planetary signals typically last less than a day. These strengths motivate microlensing searches as powerful, complementary probes of unexplored parameter space that have already provided important constraints on the demographics of planets beyond the “snow line.” However, the real-world challenges associated with the practical application of the method have driven the organization and evolution of the microlensing field and will continue to drive future developments from next-generation ground-based experiments through possible future space-based missions.

Radiation, winds, and jets from the active nucleus of a massive galaxy can interact with its interstellar medium, and this can lead to ejection or heating of the gas. This terminates star formation in the galaxy and stifles accretion onto the black hole. Such active galactic nuclei (AGN) feedback can account for the observed proportionality between the central black hole and the host galaxy mass. Direct observational evidence for the radiative or quasar mode of feedback, which occurs when AGN are very luminous, has been difficult to obtain but is accumulating from a few exceptional objects. Feedback from the kinetic or radio mode, which uses the mechanical energy of radio-emitting jets often seen when AGN are operating at a lower level, is common in massive elliptical galaxies. This mode is well observed directly through X-ray observations of the central galaxies of cool core clusters in the form of bubbles in the hot surrounding medium. The energy flow, which is roughly continuous, heats the hot intracluster gas and reduces radiative cooling and subsequent star formation by an order of magnitude. Feedback appears to maintain a long-lived heating/cooling balance. Powerful, jetted radio outbursts may represent a further mode of energy feedback that affects the cores of groups and subclusters. New telescopes and instruments from the radio to X-ray bands will come into operation over the next several years and lead to a rapid expansion in observational data on all modes of AGN feedback.

Galactic halo gas traces inflowing star-formation fuel and feedback from a galaxy's disk and is therefore crucial to our understanding of galaxy evolution. In this review, we summarize the multiwavelength observational properties and origin models of Galactic and low-redshift spiral galaxy halo gas. Galactic halos contain multiphase gas flows that are dominated in mass by the ionized component and extend to large radii. The densest, coldest halo gas observed in neutral hydrogen (Hi) is generally closest to the disk (<20 kpc), and absorption line results indicate warm and warm-hot diffuse halo gas is present throughout a galaxy's halo. The hot halo gas detected is not a significant fraction of a galaxy's baryons. The disk-halo interface is where the multiphase flows are integrated into the star-forming disk, and there is evidence for both feedback and fueling at this interface from its temperature and kinematic gradients, and Hi structures.

The origin and fate of halo gas are considered in the context of cosmological and idealized local simulations. Accretion along cosmic filaments occurs in both hot (>10^5.5 K) and cold modes in simulations, with the compressed material close to the disk being the coldest and densest, in agreement with observations. There is evidence in halo gas observations for radiative and mechanical feedback mechanisms, including escaping photons from the disk, supernova-driven winds, and a galactic fountain. Satellite accretion also leaves behind abundant halo gas. This satellite gas interacts with the existing halo medium, and much of this gas will become part of the diffuse halo before it reaches the disk. The accretion rate from cold and warm halo gas is generally below a galaxy disk's star-formation rate, but gas at the disk-halo interface and stellar feedback may be important additional fuel sources.

We review progress over the past decade in observations of large-scale star formation, with a focus on the interface between extragalactic and Galactic studies. Methods of measuring gas contents and star-formation rates are discussed, and updated prescriptions for calculating star-formation rates are provided. We review relations between star formation and gas on scales ranging from entire galaxies to individual molecular clouds.

Burst oscillations, a phenomenon observed in a significant fraction of Type I (thermonuclear) X-ray bursts, involve the development of highly asymmetric brightness patches in the burning surface layers of accreting neutron stars. Intrinsically interesting as nuclear phenomena, they are also important as probes of dense matter physics and the strong gravity, high magnetic field environment of the neutron star surface. Burst oscillation frequency is also used to measure stellar spin, and doubles the sample of rapidly rotating (above 10 Hz) accreting neutron stars with known spins. Although the mechanism remains mysterious, burst oscillation models must take into account thermonuclear flame spread, nuclear processes, rapid rotation, and the dynamical role of the magnetic field. This review provides a comprehensive summary of the observational properties of burst oscillations, an assessment of the status of the theoretical models that are being developed to explain them, and an overview of how they can be used to constrain neutron star properties such as spin, mass, and radius.