Showing papers in "Annual Review of Astronomy and Astrophysics in 2008"

Dark Energy and the Accelerating Universe

Abstract: Ten years ago, the discovery that the expansion of the universe is accelerating put in place the last major building block of the present cosmological model, in which the universe is composed of 4% baryons, 20% dark matter, and 76% dark energy. At the same time, it posed one of the most profound mysteries in all of science, with deep connections to both astrophysics and particle physics. Cosmic acceleration could arise from the repulsive gravity of dark energy—for example, the quantum energy of the vacuum—or it may signal that general relativity (GR) breaks down on cosmological scales and must be replaced. We review the present observational evidence for cosmic acceleration and what it has revealed about dark energy, discuss the various theoretical ideas that have been proposed to explain acceleration, and describe the key observational probes that will shed light on this enigma in the coming years.

Interstellar Polycyclic Aromatic Hydrocarbon Molecules

Abstract: Large polycyclic aromatic hydrocarbon (PAH) molecules carry the infrared (IR) emission features that dominate the spectra of most galactic and extragalactic sources. This review surveys the observed mid-IR characteristics of these emission features and summarizes laboratory and theoretical studies of the spectral characteristics of PAHs and the derived intrinsic properties of emitting interstellar PAHs. Dedicated experimental studies have provided critical input for detailed astronomical models that probe the origin and evolution of interstellar PAHs and their role in the universe. The physics and chemistry of PAHs are discussed, emphasizing the contribution of these species to the photoelectric heating and the ionization balance of the interstellar gas and to the formation of small hydrocarbon radicals and carbon chains. Together, these studies demonstrate that PAHs are abundant, ubiquitous, and a dominant force in the interstellar medium of galaxies.

Nuclear Activity in Nearby Galaxies

Abstract: A significant fraction of nearby galaxies show evidence of weak nuclear activity unrelated to normal stellar processes. Recent high-resolution, multiwavelength observations indicate that the bulk of this activity derives from black hole accretion with a wide range of accretion rates. The low accretion rates that typify most low-luminosity active galactic nuclei induce significant modifications to their central engine. The broad-line region and obscuring torus disappear in some of the faintest sources, and the optically thick accretion disk transforms into a three-component structure consisting of an inner radiatively inefficient accretion flow, a truncated outer thin disk, and a jet or outflow. The local census of nuclear activity supports the notion that most, perhaps all, bulges host a central supermassive black hole, although the existence of active nuclei in at least some late-type galaxies suggests that a classical bulge is not a prerequisite to seed a nuclear black hole.

Evolution of Debris Disks

Abstract: Circumstellar dust exists around several hundred main sequence stars. For the youngest stars, that dust could be a remnant of the protoplanetary disk. Mostly it is inferred to be continuously replenished through collisions between planetesimals in belts analogous to the Solar System’s asteroid and Kuiper belts, or in collisions between growing protoplanets. The evolution of a star’s debris disk is indicative of the evolution of its planetesimal belts and may be influenced by planet formation processes, which can continue throughout the first gigayear as the planetary system settles to a stable configuration and planets form at large radii. Evidence for that evolution comes from infrared photometry of large numbers of debris disks, providing snapshots of the dust present at different evolutionary phases, as well as from images of debris disk structure. This review describes the theoretical framework within which debris disk evolution takes place and shows how that framework has been constrained by observations.

Neutron-Capture Elements in the Early Galaxy

Abstract: The content of neutron-capture (trans-iron-peak) elements in the lowmetallicity Galactic halo varies widely from star to star. The differences are both in bulk amount of the neutron-capture elements with respect to lighter ones and in element-to-element ratios among themselves. Several well-defined abundance distributions have emerged that reveal characteristic rapid and slow neutron-capture nucleosynthesis patterns. In this review we summarize these observed metal-poor star’s abundances, contrasting them with the Solar-system values, comparing them to theoretical predictions, using them to assess the types of stars responsible for their specific anomalies, and speculating on the timing and nature of early Galactic nucleosynthesis.

The Growth Mechanisms of Macroscopic Bodies in Protoplanetary Disks

Abstract: The formation of planetesimals, the kilometer-sized planetary precursors, is still a puzzling process. Considerable progress has been made over the past years in the physical description of the first stages of planetesimal formation, owing to extensive laboratory work. This review examines the experimental achievements and puts them into the context of the dust processes in protoplanetary disks. It has become clear that planetesimal formation starts with the growth of fractal dust aggregates, followed by compaction processes. As the dust-aggregate sizes increase, the mean collision velocity also increases, leading to the stalling of the growth and possibly to fragmentation, once the dust aggregates have reached decimeter sizes. A multitude of hypotheses for the further growth have been proposed, such as very sticky materials, secondary collision processes, enhanced growth at the snow line, or cumulative dust effects with gravitational instability. We will also critically review these ideas.

Supernova Remnants at High Energy

Abstract: Many shell supernova remnants are now known to radiate synchrotron X-rays. Several objects have also been detected in TeV gamma rays. Nonthermal X-rays and gamma rays can be produced in shell remnants by extremely energetic ions and electrons due to decay of π0 mesons produced in inelastic collisions between ions and thermal gas, or by electron synchrotron, bremsstrahlung, or inverse-Compton radiation. Thus observations at X-ray and gamma-ray wavelengths constrain the process of particle acceleration to high energies in the shock waves of supernova remnants. This review examines the relevant characteristics of Type Ia and core-collapse supernovae, the dynamics of their evolution through the Sedov blast-wave phase, the basic physics of diffusive shock acceleration, and the physics of the relevant radiative processes. It also reviews the current status of observations of shell remnants from X-rays to TeV gamma rays, and summarizes what we can learn about particle acceleration.

The Crab Nebula: An Astrophysical Chimera

Abstract: The Crab Nebula, henceforth the Crab, the remnant of the historical super- nova of 1054 AD, has long been of intense interest. The pulsar at the center of the Crab has a spin-down luminosity ∼10 5 times that of the Sun. The outer nebula holds several solar masses of material ejected by the explosion. Between the two lies the trapped pulsar wind, visible as synchrotron radia- tion at radio wavelengths through X-ray wavelengths. Recent observations with the Hubble Space Telescope, the Chandra X-ray Observatory, and a host of other instruments have provided a wealth of information about the ex- traordinary structure and dynamics of the Crab. Understanding those data requires thinking of the Crab not in terms of its individual components, but instead as a single interconnected physical system formed as the axisymmet- rical wind from the pulsar pushes its way outward through a larger freely expanding supernova remnant.

Spectropolarimetry of Supernovae

Abstract: Overwhelming evidence has accumulated in recent years that supernova explosions are intrinsically three-dimensional phenomena with significant departures from spherical symmetry. We review the evidence derived from spectropolarimetry that has established several key results: Virtually all supernovae are significantly aspherical near maximum light; core-collapse supernovae behave differently than thermonuclear (Type Ia) supernovae; the asphericity of core-collapse supernovae is more pronounced in the inner layers, showing that the explosion process is strongly aspherical; core-collapse supernovae tend to establish a preferred direction of asymmetry; and the asphericity is stronger in the outer layers of thermonuclear supernovae, providing constraints on the burning process. We emphasize the utility of the Q/U plane as a diagnostic tool and revisit SN 1987A and SN 1993J in a contemporary context. An axially symmetric geometry can explain many basic features of core-collapse supernovae, but significant depar...

Pulsating White Dwarf Stars and Precision Asteroseismology

Abstract: Galactic history is written in the white dwarf stars. Their surface properties hint at interiors composed of matter under extreme conditions. In the forty years since their discovery, pulsating white dwarf stars have moved from side-show curiosities to center stage as important tools for unraveling the deep mysteries of the Universe. Innovative observational techniques and theoretical modeling tools have breathed life into precision asteroseismology. We are just learning to use this powerful tool, confronting theoretical models with observed frequencies and their time rate-of-change. With this tool, we calibrate white dwarf cosmochronology; we explore equations of state; we measure stellar masses, rotation rates, and nuclear reaction rates; we explore the physics of interior crystallization; we study the structure of the progenitors of Type Ia supernovae, and we test models of dark matter. The white dwarf pulsations are at once the heartbeat of galactic history and a window into unexplored and exotic physics.

The double pulsar.

Abstract: A new era in fundamental physics began with the discovery of pulsars 1967, the discovery of the first binary pulsar in 1974 and the first millisecond pulsar in 1982. Ever since, pulsars have been used as precise cosmic clocks, taking us beyond the weak-field regime of the solar-system in the study of theories of gravity. Their contribution is crucial as no test can be considered to be complete without probing the strong-field realm of gravitational physics by finding and timing pulsars. This is particularly highlighted by the discovery of the first double pulsar system which was discovered by our team in 2003. The double pulsar is unique in that both neutron stars are detectable as radio pulsars. This, combined with significantly higher mean orbital velocities and accelerations when compared to other binary pulsars, suggested that the system would become the best available testbed for general relativity and alternative theories of gravity in the strong-field regime. Indeed, this has been achieved only three years after its discovery with four independent strong-field tests of GR, more than has been obtained for any other system. Use of the theory-independent mass ratio of the two stars makes these tests uniquely different from all preceding studies. Our results confirm the validity of GR at the 0.05% level, which is by far the best precision yet achieved for the strong-field regime. Remarkably, the transverse velocity of the systems center of mass is extremely small, a result which is important for future GR tests and evolutionary studies.

The Spitzer View of the Extragalactic Universe

Abstract: The Spitzer Space Telescope was launched in August 2003. Scientists from around the world have applied its orders-of-magnitude gain in imaging and spectroscopic capability to a wide array of topics in extragalactic research. Spitzer studies have found massive galaxies at redshifts greater than 6, resolved the cosmic background at 200 μm > λ > 20 μm into the dusty infrared-luminous galaxies that comprise it, directly detected dust-enshrouded star formation, and measured the star formation history of the universe to z > 3. In this review we examine a small fraction of the extragalactic studies from Spitzer that have been conducted in its first three years of operations.

Water in the Solar System

Abstract: Water is ubiquitous in the Universe, and also in the Solar System. By setting the snow line at its condensation level in the protosolar disk, water was responsible for separating the planets into the terrestrial and the giant ones. Water ice is a major constituent of the comets and the small bodies of the outer Solar System, and water vapor is found in the giant planets, both in their interiors and in the stratospheres. Water is a trace element in the atmospheres of Venus and Mars today. It is very abundant on Earth, mostly in liquid form, but it was probably also abundant in the primitive atmospheres of Venus and Mars. Water is found in different states on the three planets, as vapor on Venus and ice (or permafrost) on Mars. Most likely, this difference has played a major role in the diverging destinies of the three planets.