Annual Review of Astronomy and Astrophysics - Volume 33, 1995
Volume 33, 1995
- Preface
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- Review Articles
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The Diffuse Interstellar Bands
Vol. 33 (1995), pp. 19–73More LessThe diffuse interstellar bands are absorption features observed in the spectra of stars seen through significant column densities of interstellar material. Of the 127 confirmed DIBs in the optical region between 0.4 and about 1.3 /Lm, only 2 have (tentatively) been identified with a specific carrier. Because DIB strengths increase roughly in proportion to color excess, they were originally suspected of being produced on or in the interstellar grains, but current evidence favors some species of free polyatomic molecules, either neutral or ionized. DIBs are conspicuously broader than the atomic interstellar lines, having widths at half-depth ranging from 0.8 to about 30 Å. These widths are presumably due to unresolved rotational structure, possibly compounded by lifetime broadening of the upper state. Recent proposals that C60, some members of the PAH family, or polycarbon chains are responsible for the DIB spectrum are either not supported by observation or await better data.
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Stellar Pulsations Across the HR Diagram: Part 1
Vol. 33 (1995), pp. 75–113More LessStellar pulsations, either as radial or nonradial dynamical variabilities, are found in many phases of stellar evolution, and these variable stars occupy vastly differing regions on the Hertzsprung-Russell diagram. The diagnostic potential of such oscillations is considerable and its importance will grow in the future when smaller and smaller amplitude variabilities can be monitored as a matter of routine. This review of the theoretical aspects of stellar pulsations appears in two parts. The first part introduces the theoretical concepts and the physical mechanisms involved in stellar pulsations and reviews recent advances therein. Applications of the theory to model neutron-star oscillations and the pulsations of hydrogen-deficient stars are also reviewed.
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Explosive Nucleosynthesis Revisited: Yields
Vol. 33 (1995), pp. 115–132More LessThree new, independent projects that provide detailed predictions of nucleosynthesis yields (up to Z = 36) from massive stars are examined and compared. The general agreement is surprisingly good. Preliminary comparison with abundance data, especially for elemental abundances in extreme Population II stars, is promising. Implications of these comparisons and critical uncertainties are discussed.
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Blue Stragglers and Other Stellar Anomalies: Implications for the Dynamics of Globular Clusters
Vol. 33 (1995), pp. 133–162More LessThere are several observed phenomena in globular clusters that are thought to be the result of dynamical processes or binary star evolution. This review examines these manifestations of the interaction between globular cluster dynamics and stellar evolution. Blue stragglers may be formed by the evolution of primordial binaries or by collisions. Current evidence suggests that both processes are likely to occur, and that the observed blue straggler sequences can place dynamically interesting limits on rates. Color gradients in globular clusters are thought to be caused by the stripping of giants by collisions, although the creation of blue subdwarfs by the same process may also be required to explain the observations. The observed X-ray sources and radio pulsars are apparently also made by a variety of dynamical processes that are still not fully understood.
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Intraday Variability in Quasars and BL LAC Objects
S. J. Wagner, and A. WitzelVol. 33 (1995), pp. 163–197More LessActive galactic nuclei with flat radio spectra exhibit significant variations of continuum flux density on time scales of days or less throughout the entire wavelength range. These rapid variations tightly constrain the diameters of the emitting regions and imply extremely high photon densities if the variations are intrinsic. At radio frequencies the flux densities of compact objects may flicker due to interstellar scintillation, and in all frequency bands microlensing by stars in intervening galaxies may introduce variations that are not intrinsic to the source. We review the characteristics of these variations in the various electromagnetic bands. Extrinsic mechanisms may affect the light curves of compact extragalactic sources, but close correlations between flares recorded in different bands strongly support the assumption that intraday variability is an intrinsic phenomenon.
The apparent brightness temperatures in the radio regime exceed 1017 K and imply relativistic beaming with very high Doppler factors, coherent radiation mechanisms, or special geometric effects.
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Angular Momentum Evolution of Young Stars and Disks
Vol. 33 (1995), pp. 199–238More LessThe effects of angular momentum play an important role in star formation. However, when a young star emerges on the Hertzsprung-Russell diagram it is generally rotating slowly. This article reviews the observations regarding the angular momentum of molecular cloud cores, of proto stars, and of the disks, binary orbits, and spin of young stars. Various physical processes for transfer of angular momentum during the star formation process are discussed, including magnetic braking during the molecular cloud stage; fragmentation during the protostar collapse; viscosity, magnetic torques, and gravitational torques in disks; and magnetically driven mass loss in a star-disk system. Important problems for future study are highlighted.
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High-Energy Particles in Solar Flares
Vol. 33 (1995), pp. 239–282More LessAccelerated particles appear to coexist inseparably with most forms of energy release in solar flares and coronal mass ejections. We identify at least five different populations of high-energy electrons and ions. High-energy particles, accelerated efficiently in the flare in great numbers, transport a large fraction of the flare energy to other sites. This behavior makes them an integral part of the flare process. Much new data has come from two spacecraft launched in 1991: the Compton GammaRay Observatory and Yohkoh. This review concentrates on particles in flares, mainly using X-ray and gamma-ray data rather than measurements of “escaping” particles observed in interplanetary space.
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Magnetohydrodynamic Turbulence in the Solar Wind
Vol. 33 (1995), pp. 283–325More LessThe fluctuations in magnetic field and plasma velocity in the solar wind possess many features expected of fully developed magnetohydrodynamic turbulence. Understanding this nonlinear system is complicated by the dynamical effects of velocity shear between fast and slow solar wind streams, by the spherical expansion of the solar wind, and by its compressibility. Direct spacecraft observations from 0.3 to over 20 AU, radio scintillation observations of plasmas near the Sun, numerical simulations, and various models provide complementary methods that have shown convincingly that the fluctuations in the wind parameters undergo significant dynamical evolution independent of whatever turbulence might exist in the solar photosphere and corona. This rich area of study allows one to test theories of turbulence against direct observation, and to use observations to guide and motivate development of turbulence theories. The solar wind thus provides an excellent laboratory for studying many fundamental questions about turbulent plasmas.
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Completing the Copernican Revolution: The Search for Other Planetary Systems
Vol. 33 (1995), pp. 359–380More LessThe past few decades have witnessed significant advances in our understanding of how stars form, and there has been an associated increase in our knowledge of conditions and phenomena in the early solar system. These have led to the formulation of a paradigm for the origin of the solar system that is sufficiently complete that its basic elements can be tested directly through observations. A simple, but profound, consequence of the paradigm is that most if not all stars should be accompanied by planetary systems. The accuracy of instruments that can be used in such searches has improved to the point that Jupiter-like companions to a number of nearby stars could be detected. However, the results to date are that no other planetary systems have been detected, and the absence of detection is becoming statistically significant, particularly as it relates to the existence of brown dwarf companions to main-sequence stars.
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The Old Open Clusters of the Milky Way
Vol. 33 (1995), pp. 381–414More LessThe Galactic open clusters, in particular the oldest members, serve as excellent probes of the structure and evolution of the Galactic disk. Individual clusters provide excellent tests of stellar and dynamical evolution. Cluster spatial and age distributions provide insight into the processes of cluster formation and destruction that have allowed substantial numbers of old open clusters to survive. Spectroscopic and photometric data for the old clusters yield kinematic, abundance, and age information that clarifies the relationship between the old open cluster population and other Galactic populations. New samples of old open clusters, which span a large range in distance and age, are used to define disk abundance gradients and the cluster age-metallicity relationship, and they point to a complex history of chemical enrichment and mixing in the disk.
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Gamma-Ray Bursts
Vol. 33 (1995), pp. 415–458More LessThe present status of gamma-ray burst research is reviewed, with an emphasis on recent observations of their temporal, spectral, and global distribution properties. The observed sky distribution of weak gamma-ray bursts constrains the allowable geometrical models to sources in either a giant spherical galactic halo or to sources at cosmological distances. Observations of time dilation consistent with the latter have been reported. Extensive searches for a counterpart to gamma-ray bursts in other wavelength regions have thus far proved negative. In spite of the abundance of new observations of gamma-ray bursts, their energy source and emission mechanism remain highly speculative. New, rapid counterpart search efforts and several new space-borne experiments may provide the needed observations to make progress in the field
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The Solar Neutrino Problem
Vol. 33 (1995), pp. 459–503More LessThe solar neutrino problem has persisted for almost three decades. Recent results from Kamiokande, SAGE, and GALLEX indicate a pattern of neutrino fluxes that is very difficult to reconcile with plausible variations in standard solar models. This situation is reviewed, and suggested particle physics solutions are discussed. A summary is given of the important physics expected from SNO, SuperKamiokande, and other future experiments.
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Theory of Accretion Disks I: Angular Momentum Transport Processes
Vol. 33 (1995), pp. 505–540More LessAccretion disk flow is a common phenomenon in astrophysics. It provides the nursery for planetary system formation and the channel for mass transfer in interacting binary stars. Such flows are also associated with the central engine for active galactic nuclei.
Mass is redistributed in accretion disks as a consequence of angular momentum transfer. The identification of the dominant process involved is an important task in the development of accretion disk theory. Here, we review recent theoretical investigations on several important physical processes, including: 1. the removal of angular momentum from disks through hydromagnetic winds, 2. the amplification of local viscous stress through the onset of turbulence resulting from possible hydromagnetic, convective, or shear flow instabilities, 3. the transport of angular momentum carried by propagating waves, and 4. torque resulting from the presence of nonaxisymmetric unstable modes in self-gravitating and geometrically thick disks. Because of the technical nature of this subject, we present some of the mathematical formalisms in a pedagogical manner. We focus our attention on the physical discussion of the necessary conditions for each process to operate and the efficiency of angular momentum transfer to be expected. In Part II of this review, we shall present observational evidences and discuss applications of theoretical results in different astrophysical contexts.
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Previous Volumes
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Volume 62 (2024)
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Volume 61 (2023)
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Volume 60 (2022)
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Volume 59 (2021)
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Volume 58 (2020)
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Volume 57 (2019)
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Volume 56 (2018)
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Volume 55 (2017)
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Volume 54 (2016)
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Volume 53 (2015)
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Volume 52 (2014)
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Volume 51 (2013)
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Volume 50 (2012)
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Volume 49 (2011)
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Volume 48 (2010)
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Volume 47 (2009)
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Volume 46 (2008)
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Volume 45 (2007)
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Volume 44 (2006)
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Volume 43 (2005)
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Volume 42 (2004)
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Volume 41 (2003)
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Volume 40 (2002)
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Volume 39 (2001)
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Volume 38 (2000)
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Volume 37 (1999)
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Volume 36 (1998)
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Volume 35 (1997)
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Volume 34 (1996)
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Volume 33 (1995)
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Volume 32 (1994)
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Volume 31 (1993)
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Volume 30 (1992)
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Volume 29 (1991)
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Volume 28 (1990)
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Volume 27 (1989)
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Volume 26 (1988)
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Volume 25 (1987)
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Volume 24 (1986)
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Volume 23 (1985)
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Volume 22 (1984)
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Volume 21 (1983)
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Volume 20 (1982)
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Volume 19 (1981)
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Volume 18 (1980)
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Volume 17 (1979)
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Volume 16 (1978)
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Volume 15 (1977)
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Volume 14 (1976)
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Volume 13 (1975)
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Volume 12 (1974)
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Volume 11 (1973)
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Volume 10 (1972)
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Volume 9 (1971)
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Volume 8 (1970)
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Volume 7 (1969)
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Volume 6 (1968)
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Volume 5 (1967)
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Volume 4 (1966)
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Volume 3 (1965)
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Volume 2 (1964)
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Volume 1 (1963)
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Volume 0 (1932)