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Showing papers in "Annual Review of Astronomy and Astrophysics in 2003"


Journal ArticleDOI
TL;DR: The first extensive catalog of galactic embedded clusters is compiled, finding that the embedded cluster birthrate exceeds that of visible open clusters by an order of magnitude or more indicating a high infant mortality rate for protocluster systems.
Abstract: ▪ Abstract Stellar clusters are born embedded within giant molecular clouds (GMCs) and during their formation and early evolution are often only visible at infrared wavelengths, being heavily obscured by dust. Over the past 15 years advances in infrared detection capabilities have enabled the first systematic studies of embedded clusters in galactic molecular clouds. In this article we review the current state of empirical knowledge concerning these extremely young protocluster systems. From a survey of the literature we compile the first extensive catalog of galactic embedded clusters. We use the catalog to construct the mass function and estimate the birthrate for embedded clusters within ∼2 kpc of the sun. We find that the embedded cluster birthrate exceeds that of visible open clusters by an order of magnitude or more indicating a high infant mortality rate for protocluster systems. Less than 4–7% of embedded clusters survive emergence from molecular clouds to become bound clusters of Pleiades age. Th...

2,949 citations


Journal ArticleDOI
TL;DR: In this article, the authors survey the observed properties of interstellar dust grains: the wavelength-dependent extinction of starlight, including absorption features, from UV to infrared; optical luminescence; and optical luminance.
Abstract: ▪ Abstract This review surveys the observed properties of interstellar dust grains: the wavelength-dependent extinction of starlight, including absorption features, from UV to infrared; optical lum...

2,288 citations


Journal ArticleDOI
TL;DR: In this paper, the intrinsic UV and X-ray absorbers of active galaxies were studied with the assistance of monitoring observations and photoionization models to provide constraints on their kinematics] physical conditions.
Abstract: Blueshifted absorption lines in the UV and X-ray spectra of active galaxies reveal the presence of massive outflows of ionized gas from their nuclei. The intrinsic UV and X-ray absorbers show large global covering factors of the central continuum source, and the inferred mass loss rates are comparable to the mass accretion rates. Many absorbers show variable ionic column densities which are attributed to a combination of variable ionizing flux and motion of gas into and out of the line of sight . Detailed studies of the intrinsic absorbers. with the assistance of monitoring observations and photoionization models. provide constraints on their kinematics] physical conditions. and locations relative to the central continuum source. which range from the inner nucleus (approx.0.01 pc) to the galactic disk or halo (approx.10 kpc) . Dynamical models that make use of thermal winds. radiation pressure. and/or hydromagnetic flows have reached a level of sophistication that permits comparisons with the observational constraints .

586 citations


Journal ArticleDOI
TL;DR: Weak gravitational lensing provides a unique method to map directly the distribution of dark matter in the universe and to measure cosmological parameters as mentioned in this paper, which is a cosmic-shear technique.
Abstract: ▪ Abstract Weak gravitational lensing provides a unique method to map directly the distribution of dark matter in the universe and to measure cosmological parameters. This cosmic-shear technique is...

538 citations


Journal ArticleDOI
TL;DR: In this paper, a detailed observational picture has been built up of the internal rotation of our nearest star, showing that the radiative interior is found to rotate roughly uniformly, unlike the predictions of stellar evolution models, which had been that the rotation rate would depend primarily on the distance from the rotation axis.
Abstract: ▪ Abstract Helioseismology has transformed our knowledge of the Sun's rotation. Earlier studies revealed the Sun's surface rotation, but now a detailed observational picture has been built up of the internal rotation of our nearest star. Unlike the predictions of stellar-evolution models, the radiative interior is found to rotate roughly uniformly. The rotation within the convection zone is also very different from prior expectations, which had been that the rotation rate would depend primarily on the distance from the rotation axis. Layers of rotational shear have been discovered at the base of the convection zone and in the subphotospheric layers. Studies of the time variation of rotation have uncovered zonal-flow bands, extending through a substantial fraction of the convection zone, which migrate over the course of the solar cycle, and there are hints of other temporal variations and of a jet-like structure. At the same time, building on earlier work with mean-field models, researchers have made great...

479 citations


Journal ArticleDOI
TL;DR: In this article, it is shown that Earth's climate has remained conducive to life for the past 3.5 billion years or more, despite a large increase in solar luminosity, probably because of previous higher concentrations of CO2 and/or CH4.
Abstract: ■ Abstract Giant planets have now been discovered around other stars, and it is only a matter of time until Earth-sized planets are detected. Whether any of these planets are suitable for life depends on their volatile abundances, especially water, and on their climates. Only planets within the liquid-water habitable zone (HZ) can support life on their surfaces and, thus, can be analyzed remotely to determine whether they are inhabited. Fortunately, current models predict that HZs are relatively wide around main-sequence stars not too different from our sun. This conclusion is based on studies of how our own planet has evolved over time. Earth’s climate has remained conducive to life for the past 3.5 billion years or more, despite a large increase in solar luminosity, probably because of previous higher concentrations of CO2 and/or CH4. Both these gases are involved in negative feedback loops that help to stabilize the climate. In addition to these topics, we also briefly discuss the rise of atmospheric O 2 and O3, along with their possible significance as indicators of life on other planets.

465 citations


Journal ArticleDOI
TL;DR: In this paper, a review is presented on the ample data obtained on post-AGB stars, both on the central stars and their circumstellar material, and a surprisingly wide variety of chemical signatures observed.
Abstract: ▪ Abstract In this contribution, a review is presented on the ample data obtained on post-AGB stars, both on the central stars and their circumstellar material. The fast evolutionary phase is characterized by a rapid change in the properties of the objects, but the variety is so large that there is yet no clear consensus on how the detailed studies of individual objects are linked together by evolutionary channels. The absence of strong molecular veiling in the photospheres of the central stars, together with a spread in intrinsic metallicity make post-AGB stars very useful in constraining AGB chemical evolutionary models. We discuss the surprisingly wide variety of chemical signatures observed. The onset in the creation process of the panoply of structures and shapes observed in planetary nebulae occurs during the short post-AGB evolution, but the physical nature of the processes involved is still badly understood. In the rapidly growing field of circumstellar mineralogy, post-AGB stars have their story ...

458 citations


Journal ArticleDOI
TL;DR: In this paper, the status of angular momentum transport in accretion disks is reviewed and the last decade has seen a dramatic increase both in the recognition of key physical properties and in the understanding of the transport.
Abstract: ▪ Abstract The status of our current understanding of angular momentum transport in accretion disks is reviewed. The last decade has seen a dramatic increase both in the recognition of key physical...

457 citations


Journal ArticleDOI
TL;DR: In this article, the origin, evolution, and physical nature of hot gas in elliptical galaxies and associated galaxy groups are reviewed, including the role of heating by active galactic nuclei, the influence of radio lobes on the hot gas, details of the cooling process, possible relation between the hot and colder gas, and the complexities of stellar enrichment of the hot gases.
Abstract: ▪ Abstract We review the origin, evolution, and physical nature of hot gas in elliptical galaxies and associated galaxy groups. Unanticipated recent X-ray observations with Chandra and XMM indicate much less cooling than previously expected. Consequently, many long-held assumptions must be reexamined or discarded and new approaches must be explored. Chief among these are the role of heating by active galactic nuclei, the influence of radio lobes on the hot gas, details of the cooling process, possible relation between the hot and colder gas in elliptical galaxies, and the complexities of stellar enrichment of the hot gas.

400 citations


Journal ArticleDOI
Philip Massey1
TL;DR: In this article, the authors discuss what is involved in identifying various kinds of massive stars in nearby galaxies: the hydrogen-burning O-type stars and their evolved He-burning evolutionary descendants, the luminous blue variables, red supergiants, and Wolf-Rayet stars.
Abstract: ▪ Abstract The galaxies of the Local Group serve as important laboratories for understanding the physics of massive stars. Here I discuss what is involved in identifying various kinds of massive stars in nearby galaxies: the hydrogen-burning O-type stars and their evolved He-burning evolutionary descendants, the luminous blue variables, red supergiants, and Wolf-Rayet stars. Primarily I review what our knowledge of the massive star population in nearby galaxies has taught us about stellar evolution and star formation. I show that the current generation of stellar evolutionary models do well at matching some of the observed features and provide a look at the sort of new observational data that will provide a benchmark against which new models can be evaluated.

345 citations


Journal ArticleDOI
TL;DR: In this paper, the current status of the numerical simulations of emitting gas, with particular emphasis on photoionized clouds and the underlying simplicity that governs these nebulae; the types of questions that can be addressed by today's codes; and the big questions that remain unanswered.
Abstract: ▪ Abstract Photoionized clouds are ubiquitous. They define the endpoints of stellar evolution (H II regions and planetary nebulae), constitute the interstellar and intergalactic media, and are found in high redshift quasars and star-forming galaxies. The spectra of these objects are dominated by emission lines that are sensitive to details of the emitting gas. These details include the microscopic atomic processes that cause the gas to glow; the density, composition, and temperature of the gas; and the radiation field of the central continuum source. Large-scale numerical codes that incorporate all the needed physics and predict the observed spectrum have become essential tools in understanding these objects. This article reviews the current status of the numerical simulations of emitting gas, with particular emphasis on photoionized clouds and the underlying simplicity that governs these nebulae; the types of questions that can be addressed by today's codes; and the big questions that remain unanswered.

Journal ArticleDOI
TL;DR: In this article, the authors discuss the types of data produced and illustrate how the laboratory measurements support such missions as ASCA, EUVE, Chandra, XMM, and ASTRO-E2.
Abstract: ▪ Abstract Targeted laboratory astrophysics measurements are being conducted to address the needs of X-ray astronomy. The measurements are producing large sets of reliable atomic data, which include ionization and recombination cross sections for charge balance calculations, as well as line lists, excitation cross sections, and dielectronic recombination rates for interpreting X-ray line formation. Additional experiments focus on resolving specific puzzles posed by astrophysical observations, as well as on calibrating existing and developing new X-ray line diagnostics. We discuss the types of data produced and illustrate how the laboratory measurements support such missions as ASCA, EUVE, Chandra, XMM, and ASTRO-E2.

Journal ArticleDOI
TL;DR: The XMM-Newton and Chandra X-ray spectrometers as mentioned in this paper provide high-resolution spectra of most classes of astrophysical sources, yielding a wealth of discrete detail, yielding sensitive constraints on physical conditions in the emitting plasmas.
Abstract: ▪ Abstract The launches of the Chandra X-Ray Observatory in June 1999 and the XMM-Newton Observatory in December 1999 opened a new era in X-ray astronomy. Both of these missions incorporate novel diffraction grating spectrometers that are providing the first high-resolution X-ray spectra of most classes of astrophysical sources. The spectra obtained to date exhibit a wealth of discrete detail, yielding sensitive constraints on physical conditions in the emitting plasmas. We review the essential characteristics of these instruments, the basics of X-ray spectral formation in cosmic sources, and the exciting new results that have emerged from Chandra and XMM-Newton grating observations of a wide variety of astrophysical systems.

Journal ArticleDOI
TL;DR: In this paper, the authors review the current understanding of the physics of cool white dwarfs, including their mass distribution, chemical evolution, magnetism, and cooling, and examine the role of white dwarFS as tracers of various stellar populations, both in terms of observational searches and theoretical models.
Abstract: ▪ Abstract Old, cool white dwarfs convey valuable information about the early history of our Galaxy. They have been used to determine the age of the galactic disk, several open clusters, and a globular cluster. We review the current understanding of the physics of cool white dwarfs, including their mass distribution, chemical evolution, magnetism, and cooling. We also examine the role of white dwarfs as tracers of various stellar populations, both in terms of observational searches and theoretical models.

Journal ArticleDOI
TL;DR: A review of the development of electrodynamics and gravitation as direct particle theories can be found in this article, where the authors discuss the work by Hoyle, the author, and others on the development.
Abstract: ▪ Abstract The first law of theoretical physics, the Newtonian law of gravitation, relies on the concept of action at a distance. The success of this law led to the concept being applied to electricity and magnetism, which were next to be explored in depth. Here the action at a distance had a limited success and ultimately had to be abandoned in favor of the increasingly more popular field theory. Nevertheless, in the 1940s, an attempt was made to revive the concept of action at a distance in a relativistically invariant way by Wheeler & Feynman (1945, 1949). It inspired a series of investigations in both electrodynamics and gravity in which the field concept was not used but the interaction was described as taking place directly between particles. As it impinged very intimately on cosmology, Hoyle was keenly interested in it. This review discusses the work by Hoyle, the author, and others on the development of electrodynamics and gravitation as direct particle theories. In this review, the author discuss...

Journal ArticleDOI
TL;DR: Astrophysics has been an important part of my personal and scientific life three times: in 1938, when I did work on stellar energy production, and nearly 30 years later when that work was rewarded with the Nobel Prize in physics.
Abstract: ▪ Abstract Astrophysics has been an important part of my personal and scientific life three times. The first was in 1938 when I did work on stellar energy production. The second was a joyful period nearly 30 years later when that work was rewarded with the Nobel Prize in physics. And the third has lasted over the time since my retirement in 1975 during which Gerry Brown and I have had a very satisfactory collaboration exploring various aspects of supernovae and, more recently, binary pairs.