Showing papers in "Bulletin of the American Astronomical Society in 1984"

Gravitational Lensing Effects of Vacuum Strings: Exact Solutions

Abstract: Exact interior and exterior solutions to Einstein's field equations are derived for vacuum strings. The exterior solution for a uniform density vacuum string corresponds to a conical space while the interior solution is that of a spherical cap. For Mu equals 0-1/4 the external metric is ds-squared = -dt-squared + dr-squared + (1-4 Mu)-squared r-squared dphi-squared + dz-squared, where Mu is the mass per unit length in the string in Planck masses per Planck length. A maximum mass per unit length for a string is 6.73 x 10 to the 27th g/cm. It is shown that strings cause temperature fluctuations in the cosmic microwave background and produce equal brightness double QSO images separated by up to several minutes of arc. Formulae for lensing probabilities, image splittings, and time delays are derived for strings in a realistic cosmological setting. String searches using ST, the VLA, and the COBE satellite are discussed.

Flicker of Extragalactic Radio Sources at Two Frequencies

Abstract: Dual-frequency observations of flat and steep-spectrum extragalactic radio sources made at Arecibo Observatory over a 20-day period are analyzed. As first reported by Heeschen (1982, 1984), flat-spectrum sources generally have larger intensity variations than steep-spectrum ones. A structure function analysis demonstrates a qualitative difference in the time series of the sources. The case against interstellar scintillation is examined, including a review of applicable scintillation theory. Relativistic source motion is treated as a solution to the brightness-temperature problems which arise if the variations are assumed intrinsic to the sources. 16 references.

A Search for Lithium-Rich Giant Stars

Abstract: Lithium abundances or upper limits have been determined for 644 bright G-K giant stars selected from the DDO photometric catalog. Two of these giants possess surface lithium abundances approaching the cosmic value of the interstellar medium and young main-sequence stars, and eight more giants have Li contents far in excess of standard predictions. At least some of these Li-rich giants are shown to be evolved to the stage of having convectively mixed envelopes, either from the direct evidence of low surface carbon isotope ratios, or from the indirect evidence of their H-R diagram positions. Suggestions are given for the unique conditions that might have allowed these stars to produce or accrete new lithium for their surface layers, or simply to preserve from destruction their initial lithium contents. The lithium abundance of the remaining stars demonstrates that giants only very rarely meet the expectations of standard first dredge-up theories; the average extra Li destruction required is about 1.5 dex. The evolutionary states of these giants and their average masses are discussed briefly, and the Li distribution of the giants is compared to predictions of Galactic chemical evolution. 110 refs.

Model for Artificial Night-Sky Illumination

Abstract: A model has been constructed to allow calculation of the night-sky brightness caused by a city at its center and outside the city, and at arbitrary zenith distances. A circular city of uniform brightness is assumed, with the total brightness proportional to the population. Molecular scattering and aerosol scattering are included, with the amount of aerosols being an adjustable parameter, and different scale heights being adopted for molecules and aerosols. The reflectivity of the ground and the fraction of light radiated above the horizontal are taken as parameters. Applications are given to several cities, to the general population-distance relations, to brightness-distance relations, and to the city center brightness-population relations.

Two Dimensional Compressible Convection Extending Over Multiple Scale Heights

Abstract: Two-dimensional simulations are used to study fully compressible thermal convection spanning multiple-density scale heights typical of a stellar envelope. The fluid is assumed to be a perfect gas with constant thermal conductivity and dynamic viscosity. The unstably stratified layer is thus represented by a polytropic index typically taken to be n = 1 (with n = 3/2 equivalent to an adiabatic stratification). The mean density ratio (bottom to top of the layer) ranges from about 1 to 21, with Rayleigh numbers up to about 1000 times critical. These highly nonlinear flows are studied with a two-dimensional numerical scheme based on a modified two-step Lax-Wendroff method. The convective motions in these simulations span the full height of the unstable layer, with no tendency to form a succession of rolls in the vertical as has been assumed in mixing length treatments of convection. Further, the flows remain subsonic because the center of the cell shifts toward the bottom of the layer as the density stratification is strengthened. The flows then display prominent downward-directed plumes surrounded by broader regions of upflow. The flow asymmetry leads to a kinetic energy flux which is directed downward, whereas the enthalpy or convective flux is upward. suchmore » asymmetry comes about because pressure fluctuations accentuate buoyancy driving in the downward plumes and can lead to buoyancy braking in the surrounding ascending flows. Compressional work is significant in the overall energetics.« less

The Evolution of Cataclysmic and Low-Mass X-Ray Binaries

Abstract: The observational data for the 124 cataclysmic and low-mass X-ray binaries of known orbital period are compiled. It is found that the eruption properties are very well correlated with the orbital periods, in a manner that suggests that the mass transfer is the all-important determinant of evolution and of the eruptive behavior. Transfer rates of 10 to the -11th to 10 to the -7th solar mass/year are found, which are well correlated with orbital period. It is concluded that the mechanism which drives these systems is the magnetic braking of the secondary's rotation by its own stellar wind, coupled with the enforcement of synchronous rotation by tidal friction. This permits CVs to lose sufficient angular momentum to begin mass transfer in a reasonable time and drives the mass transfer at a high rate once it begins. An account of the long-lived phases of CV evolution is given.

Improved Lower Limits on Lyman Alpha Forest Cloud Dimensions

Abstract: On a obtenu des spectres de 2345+007 A,B. Ces nouvelles observations soutiennent l'interpretation selon laquelle il s'agit d'une image gravitationnellement dedoublee. On a trouve plusieurs systemes de raies d'absorption Lyα en commun entre les deux images

Optical Polarimetry of BL Lacertae Objects and Violent Variable Quasars

Abstract: Recent optical polarimetric and photometric data on 24 BL Lacertae objects and optically violent variable quasars are presented. Out of 12 objects for which the polarimetric S/N is greater than 9, statistically significant wavelength dependence in the percent polarization was detected in five objects (OJ 287, Mrk 421, Ton 599, 1308 + 326, and 3C 345), with marginal evidence for such an effect in two more (3C 279 and BL Lac). Polarization in excess of 25 percent was detected at some time in eight objects out of 24. In BL Lac and 3C 345, there is evidence that the planes of optical polarization were fairly well aligned with the milliarsecond radio structures as determined by contemporaneous VLBI observations. 45 references.

On initial orbit determination.

Abstract: The classical methods of initial orbit determination are brought together within a larger viewpoint. This new synthesis stresses that all such techniques follow one of three approaches. Either they seek to compute the orbital element set, or its equivalent, by attacking the differential equations of motion (Laplace), the first integrals of the equations of motion (Taff), or the solution itself (Gauss). The particular technique pursued within a given type of approach should depend upon the nature of the observational data, the amount of a priori information one is willing to presume, and the object of the exercise. This might be a binary star system, a moon, a minor planet, or an artificial satellite. The efficacy of some algorithms for each approach is discussed briefly. Unfortunately, none of them work very well. Extensions of these techniques to radars or laser radars are trivial and have provided no new insights into the overall problem.

A Case for Submergence of Magnetic Flux in a Solar Active Region

Abstract: In NOAA Active Region 2372 (April 1980), 4 x 10 to the 20th maxwells of magnetic flux concentrated in an area 30 arcsec across disappeared overnight. Vector magnetograms show that all components of the magnetic field weakened together. If the field had weakened through diffusion or fluid flow, 90 percent of the original flux would still have been detected by the magnetograph within a suitably enlarged area. In fact there was a threefold decrease in detected flux. Evidently, magnetic field was removed from the photosphere. Since the disappearing flux was located in a region of low magnetic shear and low activity in H-alpha and Ly-alpha, it is unlikely that the field dissipated through reconnection. It is argued that the most likely possibility is that flux submerged. The observations suggest that even during the growth phase of active regions, submergence is a strong process comparable in magnitude to emergence.

A Two-Dimensional Spectrum of a Nonradiative Shock Filament in the Cygnus Loop

Abstract: A two-dimensional spectrum of a nonradiative filament in the Cygnus Loop covering the spectral region 4600-7500 A is presented and discussed. Emission lines of H, He II, and forbidden O III, as well as extremely faint N II and S II forbidden lines, were detected. Small spatial differences for some of the filament's emission lines and a possible velocity gradient across the filament's western edge were also observed. Measured relative line intensities are combined with previous nonradiative data and are compared to new nonradiative shock models which treat one, two, and three postshock temperature distributions. It is found that two- and three-temperature models provide a good fit to the observed relative line intensities. The observed weak N II and S II forbidden line emissions are interpreted as due to either radiative shock emissions produced by small density fluctuations in the shocked cloud (e.g., cloud cores), or faint dust grain reflection of the remnant's bright radiative filament emission.

A Partially Collisional Model of the Titan Hydrogen Torus

Abstract: Occasional collision effects are encompassed by the present numerical model of atomic hydrogen densities in Titan's hydrogen torus, which is noted to be azimuthally symmetric; the torus density peaks sharply at Titan's orbit, and rapidly decreases in the both radially outward and perpendicular directions. Collisions of Saturnian atoms with the torus population produce a corona. Without this coronal population, the torus model would not reproduce the Voyager 2 Lyman-alpha intensities. These observations may nevertheless be modelled by a torus-plus-corona model that also reconciles the Pioneer 11 torus observations with those of the two Voyager spacecraft.

Planispheric Astrolabes from the National Museum of American History

Abstract: Gibbs, Sharon, with George Sahba. Planispheric Astrolabes from the National Museum of American History. Smithsonian Studies in History and Technology, number 45, 231 pages, 130 figures, 23 tables, 1984.—This monograph describes via catalog entries and comparative analysis what has for many years been one of the five largest collections of planispheric astrolabes in the world. Until 1974, when seven instruments that had been on long term loan were returned to their owner, the National Museum of American History of the Smithsonian Institution preserved 48 examples of functioning astrolabes. This is the first detailed discussion of all 48, together referred to as the collection. The majority of the instruments, including the seven no longer on loan to the museum, once were part of the collection of Samuel Verplanck Hoffman of New York City. An introductory chapter, using words and drawings, describes the basic elements of a planispheric astrolabe, thereby introducing terms that appear frequently in later sections. The section \"Historical Perspective\" emphasizes the information conveyed by the makers' names and dates inscribed on instruments in the collection. It places this information in the larger context of the history of the development of the astrolabe. Each of the functional elements incorporated into the astrolabes in the collection is discussed in detail in a chapter devoted to comparative analysis. That section illuminates distinctions between European instruments and instruments made in India or in the Muslim world. In each section, the basic features of a functional element are described and any remarkable treatments of these features are noted. In addition, the traditional function of each element is specified, relying on instructions for its use prepared by Masha^'allah, al-Biruni, or Chaucer. Photographs illustrate each section. Complementing this comparative analysis is an illustrated catalog of the collection. It includes transcriptions and translations of inscriptions that appear on the instruments. Appended to the catalog are two sections that present and discuss the information conveyed by the gazetteers incorporated into many Muslim astrolabes and the star networks (or retes) included on all complete astrolabes in the collection. Finally, a third appendix describes the process used to prepare the ecliptic circle component of the astrolabe's star network. In doing so it conveys basic information about the construction of a planispheric astrolabe. Copyright © Smithsonian Institution 1984 OFFFCIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution's annual report, Smithsonian Year. SERIES COVER DESIGN: COVER: Front of astrolabe, CCA No. 39 (see page 73). Library of Congress Cataloging in Publication Data National Museum of American History (U.S.) Planispheric astrolabes from the National Museum of American History. (Smithsonian studies in history and technology ; no. 45) Bibliography: p. 1. Astrolabes. 2. Astrolabes—Catalogs. 3. National Museum of American History (U.S.) Catalogs. I. Gibbs, Sharon L. II. Saliba, George. III. Title. IV. Series. QB85.N37 1984 552'.4 83-600270