Showing papers in "Annual Review of Astronomy and Astrophysics in 1992"

Smoothed particle hydrodynamics.

Abstract: In this review the theory and application of Smoothed particle hydrodynamics (SPH) since its inception in 1977 are discussed. Emphasis is placed on the strengths and weaknesses, the analogy with particle dynamics and the numerous areas where SPH has been successfully applied.

The Cosmological constant

Abstract: The cosmological constant problem is examined in the context of both astronomy and physics. Effects of a nonzero cosmological constant are discussed with reference to expansion dynamics, the age of the universe, distance measures, comoving density of objects, growth of linear perturbations, and gravitational lens probabilities. The observational status of the cosmological constant is reviewed, with attention given to the existence of high-redshift objects, age derivation from globular clusters and cosmic nuclear data, dynamical tests of Omega sub Lambda, quasar absorption line statistics, gravitational lensing, and astrophysics of distant objects. Finally, possible solutions to the physicist's cosmological constant problem are examined.

Dynamics of interacting galaxies

Abstract: The dynamics of interacting galaxies as observed in the present epoch is reviewed. Topics discussed include numerical methods for modeling galactic interactions, the signatures of interacting galaxies due to tidal forces, and events that add mass to a galaxy. The review also covers major mergers between systems of comparable mass, forms of activity triggered or induced by galactic interactions, and galaxies' return to normality and related cosmological issues. Finally, some questions that yet have to be answered are examined.

Solar Flares and Coronal Mass Ejections

Abstract: Attempts to clarify the nature of the terrestrial effects of solar activity and variability continue at an increasing pace. While mechanisms relating possible changes in terrestrial weather patterns to changes in solar lumin­ osity remain elusive, it has long been thought that intense geomagnetic storms and interplanetary disturbances can be traced directly to large solar flares. To describe the basic scenario in simple terms, a large release of energy first occurs in a region of strong magnetic field. The energy release results in a rapid heating of coronal and chromospheric material, which expands outward into the interplanetary medium. In the case of the most energetic events the expanding material produces an interplanetary shock wave. The most energetic aspect of the flare, the impulsive phase, is charac­ terized by the production of energetic (E > I MeV) electrons and protons, some of which can be observed as a solar energetic particle (SEP) event at I AU. Over the past half century attempts have been made to identify the solar flares and their particular properties that result in geomagnetic storms and SEP events. These extensive studies, of interest to both solar physicists and forecasters of effects on the terrestrial environment, seemed to lay a solid foundation for the idea that the flare itself was the cause of the subsequent activity observed in the interplanetary medium and at the Earth. About two decades ago large coronal eruptions, now known as coronal

Cosmological Applications of Gravitational Lensing

Abstract: The discovery by Walsh et al (1979) of the first bona fide gravitational lens, the doubly-imaged quasar, Q0957 + 561, happened at an opportune time, following several prescient theoretical papers, and just preceding the completion of radio and optical quasar surveys that have since yielded over a dozen examples of this phenomenon. Interest in gravitational lenses stretches back over more than seventy years (Eddington 1919, Lodge 1919). Zwicky (1937a,b) appears to have been the first to realize that gravitational lensing ought to have a major impact on cosmology, specifically by "weighing" nebulae and providing crude telescopes to magnify lensed sources. The discovery of quasi-stellar "point" sources added two more possible uses of lenses, for distance measurement (Klimov 1963, Liebes 1964, Refsdal 1964b) and as probes of the stellar composition of lenses (Chang & Refsdal 1979), both of which may be just coming to fruition. These four topics constitute the primary theme of this review.

Long-Baseline Optical and Infrared Stellar Interferometry

Abstract: The history and the current status of optical and infrared long-baseline interferometry are reviewed. In particular, attention is given to Michelson interferometry; the Mark III stellar interferometer and its applications to astrometry, measurement of stellar diameters, and observations of binary stars; and advanced techniques. The discussion then focuses on astrometry and imaging with space interferometers. Finally, the future of long-baseline interferometry is briefly discussed with particular reference to the interferometry of the moon.

Magnetic Fields of Degenerate Stars

Abstract: Magnetic fields are ubiquitous in the universe and were known over three decades ago to range from � 1 J.LG in interstellar space up to about 30,000 G in nondegenerate magnetic stars (Parker 1 979). The possible existence of even stronger magnetic fields in white dwarfs was suggested earlier by Blackett (1 947), who proposed that the magnetic moment, J.L = BR3, of a star or planet, with surface field B and radius R, is proportional to its angular momentum. He hence argued that if the angular momentum of a star is conserved during its evolution and collapse, white dwarfs with strong magnetic fields (�1 MG) may be formed since they have smaller radii than their main-sequence progenitors. However, Ginzburg ( 1 964) and W oltjer ( 1 964) proposed instead that the magnetic flux (�BR2) of a star is conserved during its evolution and collapse, so that strong magnetic fields would be generated in degenerate stars. Hence, a main-sequence star with R � 10 11 cm and B � 1 0-1 000 G would collapse to form a neutron star with R � 106 cm and B � 10 1 1_1013 G, or a white dwarf with R � 1 09 cm and B � 105_1 07 G. Shortly thereafter pulsars were discovered ser­ endipitously by Hewish et al ( 1 968) and identified as rotating neutron stars (Gold 1 968). Their magnetic fields were soon deduced to be � 10 1 1_10 1 3 G (see e.g. reviews by Ruderman 1 972, Michel 1 982, Taylor & Stine bring 1 986), apparently consistent with field amplification by flux conservation. These developments were followed by the identification of several X-ray sources as binaries containing accreting magnetized neutron stars (Schreier

Dust-gas interactions and the infrared emission from hot astrophysical plasmas

Abstract: Environments where the presence of dust is primarily inferred from its collisional interactions with the ambient gas are reviewed with emphasis on environments typically encountered behind fast (100 km/s or greater) shocks and in hot (few 10 exp 5 K) plasmas in galactic halos, some elliptical galaxies, or the intergalactic medium of galaxy clusters. The discussion covers interactions between dust grains and a hot gas, grain temperatures and infrared emission, supernova remnants and supershells, and dust and hot gas in, around, and between galaxies. Finally, future prospects in the field are briefly discussed.

The Pluto-Charon system

Abstract: Over sixty years after its discovery, Pluto remains an astronomer's planet: Unlike the other planets it has not been visited by a spacecraft, and therefore remains studied only through astronomical and astrophysical techniques. This review summarizes the present-day state of knowledge about Pluto and Charon, pointing out inconsistencies and critical obstacles to further progress. It is largely restricted to what I call the "modern era" of Pluto studies, beginning in 1 976. Therefore, Section 2 only briefly describes the chronology of progress from the time of Pluto's discovery through the mid1 970s; Section 3 briefly summarizes the present state of knowledge about Pluto's orbit and its orbit history; Section 4 reviews our current understanding of the Pluto-Charon system's bulk parameters; Sections 5, 6, and 7 then describe what is known about Pluto's surface, interior, and atmosphere, respectively; Section 8 summarizes the state of knowledge about Charon's physical properties; Section 9 discusses theories of the origin of the Pluto-Charon system; finally, Section 10 gives some per­ spective on where we have come and where we are going in the study of this unique, binary-planet system.

Evidence for Black Holes in Stellar Binary Systems

Abstract: The concept that black holes could actually be present in nature has fascinated both scientists and the general public for years. As early as 1916 Karl Schwarzschild proposed that black holes might exist, and Oppenheimer & Snyder (1939) showed that they could possibly form from massive stars. Although numerous black-hole candidates have been put forward, conclusive proof of their presence has been much harder to find. The fact that even the most likely examples are still referred to in the literature as "candidates" shows our hesitancy to accept their existence. In this review we summarize the observational evidence for black holes in a handful of stellar binary systems which, at present, appear to be the best documented cases. Although numerous observational characteristics have been cited as evidence that a black hole might be present (e.g. millisecond X-ray flicker­ ing, high/low states, two-component X-ray spectrum), the strongest case can be made for those systems in which radial velocity measurements provide a dynamical determination of the collapsed star's mass. Since theoretical work places the upper mass limit for a neutron star at about 3 solar masses (e.g. Chitre & Hartle 1976; see also the review by Baym & Pethick 1979), the most convincing black-hole binaries are those for which the mass measured for the compact star exceeds this limit. Necessary observational criteria include: 1. presence of strong X-ray emission, 2. firm identification of the optical candidate (from data such as correlated X-ray and optical variability), 3. a dynamical mass determination (from radial

X-ray astronomy missions!

Abstract: It has been 43 years since an experiment on board a 1 949 sounding rocket showed unambiguously with photon (Geiger) counters that the sun emits X rays (Friedman et al 1 95 1) and just 30 years since the first detection from another sounding rocket of a celestial X-ray source (Giacconi et al 1 962). The field of X-ray astronomy has since developed into a full fledged branch of astronomy that now contributes to many facets of astrophysics. The articles that have appeared in these volumes with primarily an X-ray astronomy focus i llustrate vividly the development of the field, e.g.

Observations of the isotropy of the cosmic microwave background radiation.

Abstract: Over the past decade the anisotropy of the microwave background radiation has emerged as a field of fundamental importance to astrophysics due to its significance in theories of galaxy formation and in the quest for the physical origin of fluctuations. The history of the development of this subject may be divided into three phases. In the first phase, lasting up until about 1982, searches for anisotropy were carried out on angular scales ranging from two arc minutes to 180°, and, apart from the dipole term due to the peculiar velocity of the Earth relative to the Hubble flow (e.g. Fixsen et al 1983, Lubin et al 1985, Halpern et al 1985, Strukov & Skulachev 1988, Meyer et al 1991b, Smoot et aI 1991b), no anisotropies were detected down to the level of AT/T ≈ 10^(-4)-10^(-3) expected from simple scenarios for galaxy formation. In the second phase, from ~1983 to ~1989, sensitivities were improved by a full order of magnitude, but still no intrinsic anisotropies were detected. Over the same period many theoretical models were eliminated and new models were explored which predicted fractional anisotropies approaching 10^(-6). We are presently in the third phase: Sensitivities on all angular scales have reached levels where confusing signals due to discrete sources or diffuse Galactic emission due to dust, free-free emission and synchrotron radiation-dominate the measurements and have to be removed before the intrinsic anisotropy can be studied.

From Steam to Stars to the Early Universe

Abstract: Much of what follows was first published in Les Prix Nobel en 1983 (Fowler 1984). I have updated the autobiographical material to the summer of 1991. I was born in 1911 in Pittsburgh, Pennsylvania, the son of John MacLeod Fowler and Jennie Summers Watson Fowler. My parents had two other children, my younger brother, Arthur Watson Fowler and my still younger sister, Nelda Fowler Wood. My paternal grandfather, William Fowler, was a coal miner in Slammannan, near Falkirk, Scotland who emigrated to Pittsburgh to find work as a coal miner around 1880. My maternal grandfather, Alfred Watson, was a grocer. He emigrated to Pittsburgh, also around 1880, from Taniokey, near Clare in County Armagh, Northern Ireland. His parents taught in the National School, the local grammar school for children, in Taniokey, for sixty years. The family lived in the central part of the school building; my great grandfather taught the boys in one wing of the building and my great grandmother taught the girls in the other wing. The school is still there and I have been to see it. I have also visited Slammannan.