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  • ASTROPHYSICS  (1,236)
  • ASTRONOMY
  • 1990-1994  (1,530)
  • 1980-1984
  • 1925-1929
  • 1991  (1,530)
  • 1
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    The best-fit universe (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Inflation provides very strong motivation for a flat Universe, Harrison-Zel'dovich (constant-curvature) perturbations, and cold dark matter. However, there are a number of cosmological observations that conflict with the predictions of the simplest such model: one with zero cosmological constant. They include the age of the Universe, dynamical determinations of Omega, galaxy-number counts, and the apparent abundance of large-scale structure in the Universe. While the discrepancies are not yet serious enough to rule out the simplest and most well motivated model, the current data point to a best-fit model with the following parameters: Omega(sub B) approximately equal to 0.03, Omega(sub CDM) approximately equal to 0.17, Omega(sub Lambda) approximately equal to 0.8, and H(sub 0) approximately equal to 70 km/(sec x Mpc) which improves significantly the concordance with observations. While there is no good reason to expect such a value for the cosmological constant, there is no physical principle that would rule out such.
    Keywords: ASTROPHYSICS
    Type: In: Primordial nucleosynthesis and evolution of early universe; Proceedings of the International Conference, Tokyo, Japan, Sept. 4-8, 1990 (A93-17626 05-90); p. 337-350.
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  • 2
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    Pulsar polarization measurements and the nonsymmetric gravitational theory (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Because of the breakdown of the Einstein equivalence principle in the nonsymmetric gravitational theory (NGT) of Moffat, orthogonally polarized electromagnetic waves can propagate at different velocities in a gravitational field. Moffat has proposed that galactic dark matter, in the form of cosmions, may act as a significant source of gravity in the NGT. We discuss how observations of the highly polarized radiation from distant pulsars could provide significant limits on the strength of the coupling of cosmions in the NGT.
    Keywords: ASTROPHYSICS
    Type: Physical Review D - Particles and Fields, 3rd Series (ISSN 0556-2821); 44; 8; p. R2211-R2213.
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  • 3
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    Need for active structures in future large IR and sub-mm telescopes (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The potential of active structures technology for future IR and sub-mm telescopes deployed in space is discussed. A space infrared telescope with an aperture of at least 8 m, operating at about 50 k with surface precision better that 0.5 microns is required for future work in space IR and sub-mm astronomy. Telescope parameters, noise, and sensitivity are discussed. The effects of large scale figure errors are examined and conceptual designs of future space IR and sub-mm telescopes are presented. Possible uses of active structures technology and feedback control in future IR and sub-mm space observatories are suggested.
    Keywords: ASTRONOMY
    Type: In: Active and adaptive optical systems; Proceedings of the Meeting, San Diego, CA, July 22-24, 1991 (A93-39451 15-74); p. 328-358.
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  • 4
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    Cosmic evolution of Quasar radio structure (1991)
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    Publication Date: 2011-08-24
    Description: We discuss the results of a survey of Quasar radio structures over redshifts from 0.6 to 3.7. There are clear evolutionary trends in size and luminosity, which suggest that the duty cycle of individual Quasars has increased over cosmic time. This affects source count statistics and gives clues on the evolution of Quasar environments.
    Keywords: ASTROPHYSICS
    Type: In: The space distribution of quasars; Proceedings of the Workshop, Victoria, Canada, June 3-5, 1991 (A93-28776 10-90); p. 331, 332; Discuss
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  • 5
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    A search for primeval galaxies (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: We are conducting a search for emission-line objects at large redshifts, ostensibly young and forming galaxies. Our method is to search for strong line emission, in particular Ly-alpha, using two techniques: a direct, narrow-band imaging search using a Fabry-Perot interferometer; and a serendipitous long-slit spectroscopic search. Assuming that none of the faint candidates found so far are actually primeval galaxies, we can set the strongest limits to date on the surface density of Ly-alpha-luminous primeval galaxies on the sky. Some of the faint emission line galaxies detected in our survey at z roughly 0.5 - 1 may be representative of the faint blue galaxy population detected in deep continuum imaging surveys.
    Keywords: ASTRONOMY
    Type: In: The space distribution of quasars; Proceedings of the Workshop, Victoria, Canada, June 3-5, 1991 (A93-28776 10-90); p. 354-357.
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  • 6
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    Are there two disk star cluster systems in the LMC? (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The surface distribution of star clusters in the LMC has revealed the existence of two elliptical systems superimposed. A small surface density outer ellipse outlining the large system and a higher density elliptical inner system. The major axis of the two subsystems form an angle of 60 deg. The central subsystem contains all youngest populous globular star clusters from both the stellar and dynamical point of view. The large outer system seems to contain all old globular clusters, showing a dumpy distribution and outlining two arms at the northeast and southwest. The small LMC clusters were found to have masses less than 1000 solar masses and are distributed all over the large elliptical system.
    Keywords: ASTROPHYSICS
    Type: In: The formation and evolution of star clusters (A93-48676 20-90); p. 407-409.
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  • 7
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    Early phases of LMC star clusters? (1991)
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    Publication Date: 2011-08-24
    Description: On the basis of optical and IUE observations we have investigated the possibility that some of the very young stellar systems of the LMC, such as SL360, may be regarded as globular clusters at the very early phases of their dynamical evolution after the gas cloud expulsion.
    Keywords: ASTROPHYSICS
    Type: In: The formation and evolution of star clusters (A93-48676 20-90); p. 404-406.
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  • 8
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    Space Infrared Telescope Facility (SIRTF) telescope overview (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The Space Infrared Telescope Facility (SIRTF) will be the first true infrared observatory in space, building upon the technical and scientific experience gained through its two NASA survey-oriented predecessors: the Infrared Astronomical Satellite and the Cosmic Background Explorer. During its minimum five year lifetime, the SIRTF will perform pointed scientific observations at wavelengths from 1.8 to 1200 microns with an increase in sensitivity over previous missions of several orders of magnitude. This paper discusses a candidate design for the SIRTF telescope, encompassing optics, cryostat, and instrument accommodation, which has been undertaken to provide a fulcrum for the development of functional requirements, interface definition, risk assessment and cost. The telescope optics employ a baffled Ritchey-Chretien Cassegrain system with a 1-m class primary mirror, an active secondary mirror, and a stationary facetted tertiary mirror. The optics are embedded in a large superfluid He cryostat designed to maintain the entire telescope-instrument system at temperatures below 3 K.
    Keywords: ASTRONOMY
    Type: In: Infrared technology XVII; Proceedings of the Meeting, San Diego, CA, July 22-26, 1991 (A93-38376 15-35); p. 51-62.
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  • 9
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    Imaging the sun in hard X-rays - Spatial and rotating modulation collimators (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Several approaches to imaging hard X-rays emitted from solar flares have been proposed or are planned for the nineties including the spatial modulation collimator (SMC) and the rotating modulation collimator (RMC). A survey of current solar flare theoretical literature indicates the desirability of spatial resolutions down to 1 arcsecond, field of views greater than the full solar disk (i.e., 32 arcminutes), and temporal resolutions down to 1 second. Although the sun typically provides relatively high flux levels, the requirement for 1 second temporal resolution raises the question as to the viability of Fourier telescopes subject to the aforementioned constraints. A basic photon counting, Monte Carlo 'end-to-end' model telescope was employed using the Astronomical Image Processing System (AIPS) for image reconstruction. The resulting solar flare hard X-ray images compared against typical observations indicated that both telescopes show promise for the future.
    Keywords: ASTRONOMY
    Type: In: EUV, X-ray, and gamma-ray instrumentation for astronomy II; Proceedings of the Meeting, San Diego, CA, July 24-26, 1991 (A93-34376 13-35); p. 155-179.
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  • 10
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    Space Infrared Telescope Facility cryogenic and optical technology (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: SIRTF will require new liquid helium cryogenics and optical technology at liquid helium temperatures to meet the scientific requirements. In particular, it will require a helium cryogenic system operating at 1.25 K with a lifetime of five years. The optical system will require a 1-m mirror operating at 2 K, which is diffraction limited at 3 microns. This paper describes the advances which will be needed and the approaches to be taken.
    Keywords: ASTRONOMY
    Type: In: Infrared technology XVII; Proceedings of the Meeting, San Diego, CA, July 22-26, 1991 (A93-38376 15-35); p. 88-96.
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  • 11
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    Objectives for the Space Infrared Telescope Facility (1991)
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    Publication Date: 2011-08-24
    Description: The Space Infrared Telescope Facility (SIRFT) is a one-meter-class, liquid-helium-cooled, earth-orbiting astronomical observatory that will be the infrared component of NASA's family of Great Observatories. SIRTF will investigate numerous scientific areas including formation and evolution of galaxies, stars, and other solar systems; supernovae; phenomena in our own solar system; and, undoubtedly, topics that are outside today's scientific domain. SIRTF's three instruments will permit imaging at all infrared wavelengths from 1.8 to 1200 microns and spectroscopy from 2.5 to 200 microns. The observatory will operate at an altitude of 100,000 km where it will achieve a five-year lifetime and operate with better than 80 percent on-target efficiency. The scientific importance and technical and programmatic readiness of SIRTF has been recognized by the 1991 report of the National Research Council's Astronomy and Astrophysics Survey Committee which recently identified SIRTF as the highest priority major new initiative in all of astronomy for the coming decade.
    Keywords: ASTRONOMY
    Type: In: Infrared technology XVII; Proceedings of the Meeting, San Diego, CA, July 22-26, 1991 (A93-38376 15-35); p. 2-14.
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  • 12
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    Why did Halley hiccup? (1991)
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    Publication Date: 2011-08-24
    Description: The outburst of Halley comet that occurred in February 1991 when the comet was over 14 AU from the sun, between the orbits of Saturn and Uranus, is described. This region of the solar system was known to be so cold that a comet nucleus would be totally inert there. On February 12th, 1991 Halley brightened by a factor of about 300, and a dust coma was observed around the nucleus. Different explanations of the comet's strange behavior are suggested.
    Keywords: ASTROPHYSICS
    Type: Nature (ISSN 0028-0836); p. 793, 794.
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  • 13
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    Horizontal-branch evolution with time-dependent convective overshooting (1991)
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    Publication Date: 2011-08-24
    Description: Horizontal-branch (HB) sequences have been computed for different rates of convective overshooting at the edge of the convective core. We find that the convective core undergoes a series of 'breathing pulses' during most of the HB phase. The average behavior of these sequences closely mimics the behavior of sequences computed with canonical semiconvection.
    Keywords: ASTROPHYSICS
    Type: In: The formation and evolution of star clusters (A93-48676 20-90); p. 299-301.
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  • 14
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    Infrared and the search for extrasolar planets (1991)
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    Publication Date: 2011-08-24
    Description: Search for evidence concerning the existence of extrasolar planets will involve both indirect detection as well as direct (imaging). Indirect detection may be possible using ground based instrumentation on the Keck telescope, Imaging probably will require an orbiting system. Characterizing other planets for complex molecules will require a large orbiting or lunar-based telescope or inteferometer. Cryogenic infrared techniques appear to be necessary. Planning for a NASA ground and space-based program, Toward Other Planet Systems (TOPS), is proceeding.
    Keywords: ASTRONOMY
    Type: In: Infrared technology XVII; Proceedings of the Meeting, San Diego, CA, July 22-26, 1991 (A93-38376 15-35); p. 196-201.
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  • 15
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    Molecular hydrogen emission in L1448 associated with a highly collimated molecular outflow (1991)
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    Publication Date: 2011-08-24
    Description: A near-infrared camera was used to search for jets around low-luminosity embedded infrared sources in nearby molecular clouds. The near-infrared offers the advantage that the extinction is very low compared with the optical. A jet is detected in molecular hydrogen at 2.12 microns toward a source in L1448. Given the sample size this indicates a detection rate of no more than a couple percent. The average visual extinction in L1448 is roughly 5 mag. The properties of the molecular hydrogen emission are similar to those measured for known Herbig-Haro objects, suggesting the jet is a buried Herbig-Haro object/jet that would be visible in the optical if the extinction were lower. The L1448 jet coincides with the unusual CO outflow that is highly collimated and contains high-velocity CO 'bullets'. The properties of the L1448 source suggest it defines a transition case between molecular outflows and Herbig-Haro jets, combining the characteristics of both.
    Keywords: ASTROPHYSICS
    Type: Societa Astronomica Italiana, Memorie (ISSN 0037-8720); 62; 4, 19
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  • 16
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    Radar and spacecraft ranging to Mercury between 1966 and 1988 (1991)
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    Publication Date: 2011-08-24
    Description: Improved solutions have been obtained for the orbit and equatorial cross-section of Mercury using radar ranging data spanning 22 years. These data have yielded new results on the precession of Mercury's perihelion and better limits on a possible time variation in the gravitational constant G.
    Keywords: ASTRONOMY
    Type: Astronomical Society of Australia, Proceedings (ISSN 0066-9997); 9; 2, 19
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  • 17
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    Ultraviolet flares from RS CVn-type stars and related systems (1991)
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    Publication Date: 2011-08-24
    Description: Properties and parameters of UV flares have been observed from RS CVn systems approximately once per few days observing time. The peak flare luminosities are five or six orders of magnitude more intense than the most energetic solar flares. The nonradiative energy losses, e.g., turbulence and flows, are likely to be at least comparable to the radiative loss.
    Keywords: ASTROPHYSICS
    Type: Societa Astronomica Italiana, Memorie (ISSN 0037-8720); 62; 291-305
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  • 18
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    Encke, the comet (1991)
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    Publication Date: 2011-08-24
    Description: The observations, physical studies, and orbital investigations of Encke's comet are reviewed and its significance in the quest for understanding the evolution and nature of comets is discussed. Attention is given to Encke's study on the comet's anomalous orbital motion and the hypothesis of a resisting interplanetary medium. Consideration is given to recent major changes in both dynamic and physical investigations of the comet, including studies of its precession and rotation, the nature of the nongravitational anomaly, and the evolution of discrete active regions on the nucleus.
    Keywords: ASTRONOMY
    Type: Royal Astronomical Society of Canada, Journal (ISSN 0035-872X); 85; 324-376
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  • 19
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    The cosmic neutrino background (1991)
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    Publication Date: 2011-08-24
    Description: The cosmic neutrino background is expected to consist of relic neutrinos from the big bang, of neutrinos produced during nuclear burning in stars, of neutrinos released by gravitational stellar collapse, and of neutrinos produced by cosmic ray interactions with matter and radiation in the interstellar and intergalactic medium. Formation of baryonic dark matter in the early universe, matter-antimatter annihilation in a baryonic symmetric universe, and dark matter annihilation could have also contributed significantly to the cosmic neutrino background. The purpose of this paper is to review the properties of these cosmic neutrino backgrounds, the indirect evidence for their existence, and the prospects for their detection.
    Keywords: ASTROPHYSICS
    Type: In: After the first three minutes; Proceedings of the 1st Astrophysics Workshop, Univ. of Maryland, College Park, Oct. 15-17, 1990 (A93-23605 07-90); p. 497-511.
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  • 20
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    The X-ray flux dipole of AGNs as an indicator of the local gravitational field (1991)
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    Publication Date: 2011-08-24
    Description: We have investigated the dipole anisotropy of the sky X-ray flux from nearby AGNs to compare with the peculiar motion of the Local Group (LG) using a more extensive sample than that of Miyaji and Boldt (1990). We have sampled 56 low redshift emission-line AGNs (z less than 0.06) from the HEAO 1 A-2 experiment with a flux limit lower than that of Piccinotti et al. (1982) and with a lower galactic latitude cutoff. Our sample shows a significant dipole anisotropy whose apex is only about 30 deg away from the direction of the Local Group's peculiar motion for the objects with z less than 0.015. The dipole growth vs. redshift shows a sharp rise between z = 0.006 and z = 0.015; the amplitude of the dipole is 40 +/- 10 percent of the corresponding monopole at z = 0.015. The outer redshift cutoff is consistent with the results obtained from analyses using optical and IRAS galaxies. The present sample strengthens our previous conclusion that X-ray emission from AGNs traces the underlying mass distribution as strongly as optical and IR emission from galaxies.
    Keywords: ASTROPHYSICS
    Type: In: After the first three minutes; Proceedings of the 1st Astrophysics Workshop, Univ. of Maryland, College Park, Oct. 15-17, 1990 (A93-23605 07-90); p. 431-434.
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  • 21
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    The missing mass in clusters of galaxies and elliptical galaxies (1991)
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    Publication Date: 2011-08-24
    Description: We review the available data for the existence of dark matter in clusters of galaxies and elliptical galaxies. While the amount of dark matter in clusters is not well determined, both the X-ray and optical data show that more than 50 percent of the total mass must be dark. There is in general fair agreement in the binding mass estimates between the X-ray and optical techniques, but there is not detailed agreement on the form of the potential or the distribution of dark matter. The X-ray spectral and spatial observations of elliptical galaxies demonstrate that dark matter is also required in these objects and that it must be considerably more extended than the stellar distribution.
    Keywords: ASTROPHYSICS
    Type: In: After the first three minutes; Proceedings of the 1st Astrophysics Workshop, Univ. of Maryland, College Park, Oct. 15-17, 1990 (A93-23605 07-90); p. 394-404.
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  • 22
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    Evidence for evolution of the luminosity function of clusters of galaxies (1991)
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    Publication Date: 2011-08-24
    Description: From an all sky, X-ray flux limited sample of clusters of galaxies evidence for a significant deficit in the number of high luminosity clusters is found in the redshift range z approximately 0.1 to 0.2 compared with numbers of nearby clusters. This indicates that the X-ray luminous clusters are undergoing strong evolution. The strength of the effect is consistent with hierarchical merging models. The implications of such strong evolution for clusters are discussed.
    Keywords: ASTROPHYSICS
    Type: In: After the first three minutes; Proceedings of the 1st Astrophysics Workshop, Univ. of Maryland, College Park, Oct. 15-17, 1990 (A93-23605 07-90); p. 343-346.
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  • 23
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    Spectral-luminosity evolution of active galactic nuclei and the cosmic X-ray background (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Black hole accretion disk dynamo processes are generally regarded as the central power source for AGN (Rees, 1984). If the precursor active galaxies for such AGN are formed at redshift z greater than about 4 and contain initial central seed black holes about 10 exp 6 solar masses, then the Eddington limited X-ray radiation emitted during their lifetime will undergo the phenomenon of 'spectral-luminosity evolution'. When accretion disks are first formed at the onset of galaxy formation, the accretion rate occurs at high values of luminosity/size compactness parameter L/R greater than 10 exp 30 erg/cm-sec. Such high values of L/R generate dynamic constraints which suppress nonthermal black hole accretion disk dynamo processes in favor of thermal processes. This causes the spectrum of X-radiation emitted by early AGN to be predominantly thermal. A superposition of such thermal, comptonized PAG sources can account for the residual cosmic X-ray background and can act as a source of X-ray heating of the intergalactic medium for z greater than about 4.
    Keywords: ASTROPHYSICS
    Type: In: After the first three minutes; Proceedings of the 1st Astrophysics Workshop, Univ. of Maryland, College Park, Oct. 15-17, 1990 (A93-23605 07-90); p. 339-342.
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  • 24
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    Regular variability of the shape of the primary minimum of the orbital light curve of SS 433 with the phase of the precessional period (1991)
    In:  CASI
    Details
    Publication Date: 2009-11-16
    Description: The regular variations of the shape of the primary minimum of the orbital 13.082-day light curve of SS 433 with the phase of the 162.5-day processional period were discovered by analysis of the photometrical databank. The regular variability of the shape of the primary minimum of the discovered light curve of SS 433 reflects displacement with the 26-day double orbital period of at least two hot bright spots on the surface of the processional accretional disk and their eclipse by a normal star. Other aspects of the investigation are further discussed.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 65-67
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  • 25
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    Hydrodynamic study of Supernova 1987A: The phase of a wave of cooling and recombination (1991)
    In:  CASI
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    Publication Date: 2006-06-13
    Description: The dependence of the bolometric light curve and of the effective temperature on the density distribution in a progenitor, its chemical composition, mass, radius, and explosion energy is studied. It is shown that, just before the supernova 1987 A outburst, the outside layers of the blue supergiant Sk-69.202 deg had a density distribution similar to that of the polytropic model with an index of n equals 3, a chemical composition with a mass fraction of hydrogen of the order of 0.1, and a relative helium abundance of about 0.9.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 403-409
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  • 26
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    Basic physics and cosmology from pulsar timing data (1991)
    In:  CASI
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    Publication Date: 2006-06-13
    Description: Radio pulsars provide unparalleled opportunities for making measurements of astrophysically interesting phenomena. The author concentrates on two particular applications of high precision timing observations of pulsars: tests of relativistic gravitation theory using the binary pulsar 1913+16, and tests of cosmological models using timing data from millisecond pulsars. New upper limits are presented for the energy density of a cosmic background of low frequency gravitational radiation.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 385-393
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  • 27
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    Gas flow and generation of x ray emission in WR+OB binaries (1991)
    In:  CASI
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    Publication Date: 2006-06-13
    Description: The supersonic flow of the ionized gas in WR+OB binaries and X-ray generation are considered. X-ray emission is caused by gas heating up to temperatures of 10(exp 7) to 10(exp 8) K behind the front of shock waves. These are found in the collision of gas flowing out from the WR star with either the OB star's surface or the gas of the OB star's wind. The distribution of temperature and concentration behind the shock front are obtained. Using these distributions, the spectral power of bremsstrahlung X-ray emission of hot gas is calculated. Possible reasons that lead to a considerable difference between the observed parameters of X-ray emission of the WR binary of V 444 Cygni and the theoretically expected are discussed.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 394-402
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  • 28
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    X ray radiation from Supernova 1987A. The results of the Kvant module in 1987-1989 (1991)
    In:  CASI
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    Publication Date: 2006-06-13
    Description: The results of two years of SN1987A hard X-ray observations by the HEXE instrument aboard the Kvant module are summarized. By May to June 1989, the hard x-ray flux had declined more than 8.5 times in comparison with the maximum of the x-ray light curve. The upper limit of the ratio of Co-57/Co-56 abundances at the level of ratio of Fe-57/Fe-56 abundances at the Earth is a factor of 1.5.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 368-384
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  • 29
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    The peculiar velocity field predicted from the distribution of IRAS galaxies (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: It is noted, in the wake of the recent completion of a redshift survey of about 2300 IRAS galaxies with a characteristic depth of about 4000 km/sec, that the redshift distribution obtained is entirely consistent with that of the observed optical galaxies in the direction of the Great Attractor. The IRAS velocity field also qualitatively reproduces recent observations of the spiral and elliptical galaxies. Although it is not possible to rule out the existence of excess mass in the galactic plane in the direction of the Great Attractor, this is not needed to explain the observed peculiar velocities.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 356-367
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    Extragalactic x ray source counts (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: Extragalactic x-ray source counts carry information about the luminosity function and cosmic evolution of galaxies, BL Lac objects, Seyfort galaxies, and quasars. We discuss two available x-ray source samples with complete optical identifications and redshifts. We find evidence for instrumental bias in the detection of clusters for cosmic evolution of quasars, and of absorption effects in low luminosity Seyfert galaxies. Modest spectral and density evolution of Seyfort galaxies would allow the soft x-ray background to be made up entirely of discrete sources. We present a source count prognosis for the Advanced X-ray Astrophysics Facility (AXAF) energy range 0.5 to 10 keV.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 336-343
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    Ultraluminous infrared galaxies (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: One of the major discoveries of the the IRAS all-sky survey was a population of sources having the bolometric luminosities of quasars, but where more than 90 percent of the luminosity emerges in the infrared. These objects, more numerous than quasars, are found exclusivly in interacting/merging galaxies that are extremely rich in interstellar gas. We have accumulated evidence that suggests that these systems are indeed quasars, obscured by many tens of magnitudes of extinction. We suggest that these Ultraluminous Infrared Galaxies are the formation stage of quasars, and that colliding galaxies, ultraluminous infrared galaxies, and quasars might be linked through an evolutionary sequence where the infrared bright phase is one in which the quasar is formed in the nucleus of a merger system, and is enshrouded in gas and dust, while the UV excess quasars are at the end state of quasar evolution where most of the enveloping dust cloud has been dissipated, and the quasar is visible directly.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 344-355
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    The PSR 2127+12 as an indicator of a massive blac k hole in the core of globular cluster M 15 (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: The 110-millisecond pulsar PSR 2127+12 in the core of the globular cluster M15 is distinguished by having a negative period derivative P approximately equals -2 x 10(exp -17). This value cannot be provided by acceleration in the mean gravitational potential of the core. A flyby of a star approximately equal to 300 AU away could explain the P observed, but the probability of such an event is small, approximately equal to 10(exp -13). We suggest that the pulsar motion is governed by the presence of a moderately massive (approximately equals 2 divided by 3 x 10(exp 4) solar mass) black hole in the clusters center. The idea is further supported by an observed post-collapse morphology of the M15 core.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 316-321
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    On the observational appearances of a freely precessing neutron star in Hercules X-1 (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: Evidence for neutron star free precession is discussed, which is often presumed to be responsible for the observed 35-day cycle in Hercules X-1. The precise formula for the period derivative due to free precession is obtained under assumption that the precession period is much longer than that of the neutron star rotation. The optical light curves to be seen from the binary, with freely precessing accretion neutron star are simulated numerically. This simulation takes into account the reflection effect on the surface of the secondary component and on the accretion disk itself for different diagrams of x-ray emission.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 307-315
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  • 34
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    The large-scale surface brightness distribution of the x ray background (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: The x-ray background (XRB) and the microwave background are the dominant isotropic radiation fields available for measurement. There was extensive work on trying to determine the physical origin of the background. That is, whether it is due to a superposition of numerous faint well-known sources, such as active galaxies, an early unidentified population of AGN at high redshift, a new population of objects, or to truly diffuse processes or to a superposition of these. However, while of great intrinsic interest, these studies were not aimed at using the XRB to provide the cosmological information that was gleaned from the microwave background. An alternate approach is presented, which uses the available information on the large, greater than 5 deg, scale distribution of the sky flux to see if the XRB can provide such constraints.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 285-296
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  • 35
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    The evolution of the gravitational radiation from stellar components of galaxies (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: The evolution is discussed of gravitational waves spectra produced by binary stars, supernova explosions, and coalescences of binary compact stars in outer galaxies. These spectra are integrated over a simple model of the universe to give an estimate of the stochastic gravitational wave background due to astrophysical sources.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 261-269
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  • 36
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    Quasi-periodic oscillations in low-mass x ray binaries (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: Variability on short time scales in the x-ray flux is a very general property of binary x-ray sources. Not until after the discovery of intensity-dependent quasi-periodic oscillations (QPO) and associated red noise form luminous low-mass x-ray binaries were systematic studies of the shape of the power-spectral components made. A brief account is given of the main developments since this discovery which have led to a new picture of the properties of low-mass x-ray binaries.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 251-260
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    Observing SN 1987A with the International Ultraviolet Explorer (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: The International Ultraviolet Explorer (IUE) satellite played a leading role in elucidating the nature of SN 1987A, providing a unique ultraviolet perspective on the brightest supernova since 1604. IUE observations of SN 1987A began promptly after discovery and were frequent through 1988 and 1989, using the FES (Fine Error Sensor) for photometry, low dispersion spectra for the supernova spectrum, high dispersion observations for the interstellar medium when the supernova was bright, and for circumstellar gas surrounding the supernova as the initial event faded. The UV data were especially useful in determining which star exploded, assessing the ionizing pulse produced as the shock hit the surface of the star, and in constraining the stellar evolution that preceded the explosion through observation of a circumstellar shell.
    Keywords: ASTRONOMY
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 237-250
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    Georgian space research program (1991)
    In:  CASI
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    Publication Date: 2006-06-13
    Description: Considerations are presented of telescopes and spectrometers planned to be designed and made by the Abastumani Astrophysical Observatory of the Georgian SSR Academy of Science. The purpose of this topic is to clarify actual scientific problems which are to be solved by using telescopes. The experiments will begin in 1995.
    Keywords: ASTRONOMY
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 218-224
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    Gravitational lenses: The current sample, recent results, and continuing searches (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: Gravitational lensing is one of the topics in astrophysics that was quite extensively discussed over time before it was actually discovered. Ten years after the discovery of the first one, it is interesting to note how the field has developed. After an initial slow rate of discovery, the last few years have seen an explosion in the number of reported cases. Attention was drawn to the first few cases because quasars at the same red shift, with similar optical spectra, were observed with angular separations of only a few arc seconds. Most observational effort has been devoted to searching for new candidate lens systems and carefully measuring their properties, both to test whether they are indeed lensed and to provide constraints for modeling. A classification of the lenses is into rings, arcs, multiples, and doubles, where the progression is from sources close to the optical axis to far from the optical axis. The known candidate systems are listed. The searches for gravitational lenses are proving to be successful, and more lenses continue to be discovered serendipitously. Many searches are under way, along with instruments that will routinely increase the resolution of astronomical imaging.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 192-203
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  • 40
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    Cluster research with x ray observations (1991)
    In:  CASI
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    Publication Date: 2006-06-13
    Description: Past x ray surveys have shown that clusters of galaxies contain hot gas. Observations of this hot gas yield measurements of the fundamental properties of clusters. Results from a recent study of the x ray luminosity function of local Abell clusters are described. Future surveys are discussed and the potential for studying the evolution of clusters is analyzed.
    Keywords: ASTRONOMY
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 112-133
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  • 41
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    On two-dimensional relativistic stellar winds (1991)
    In:  CASI
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    Publication Date: 2006-06-13
    Description: Stellar wind is of great interest, because many of the astrophysical systems possess wind like structures. For a long time only nonrelativistic winds have been studied. However, recently it was proposed by Kennel et al. that the relativistic pulsar wind with the plasma, consisting of electrons and positrons, can be responsible for the observed features of the Crab nebula. The study of Kennel et al. revealed the inconsistency of the assumption of the wind zero temperature with the observational data. It has been shown that only for high relativistic temperatures can high Mach numbers be reached, which allows the possibility of a shock formation. The analysis of Kennel et al. was extended into the 2-D case by using conventional magnetohydrodynamic equations for a relativistic plasma with an isotropic relativistic temperature. The state equation is assumed polytropic.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 108-111
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  • 42
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    Optical observations of active galactic nuclei (1991)
    In:  CASI
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    Publication Date: 2006-06-13
    Description: The most important observed characteristics are described of active galactic nuclei, concentrating on their optical spectra. Connections with the standard model of such objects are emphasized. Some of the new observational developments that are leading to modifications of the standard model are outlined; these include spatial variations of the ionization parameters in the broad line region, the possible existence of a very dense component in the broad line region, and the presence of an intermediate density zone between the broad line and narrow line regions. Evidence for hidden Seyfert 1 nuclei and anisotropic ionizing radiation is also reviewed. The search for, and properties of, intrinsically weak Seyfert 1 nuclei are summarized, with emphasis given to the very low luminosity object in the late type dwarf galaxy NGC 4395. A discussion is also presented of the possibility that low level activity in some galactic nuclei might actually be produced by bursts of star formation and their associated supernovae.
    Keywords: ASTRONOMY
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 91-107
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  • 43
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    Radio telescopes as the detectors of super-high-energy neutrinos (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: The registration of super high energy neutrinos is a very difficult and also very important problem that requires construction of detectors with large effective target masses. Askaryan pointed out the possibility of registering cascades in dense media by the Cherenkov radio emission of an excess of negative charges in the cascades which arose in interaction between high energy particles and the atoms of medium. The telescopes for cosmic high energy neutrino detection by radioemission of cascades induced underground, but whose development continues in the atmosphere were proposed by others. The effective target masses of such detectors could be approx. 10(exp 9) tons and more. The properties of Cherenkov radio emission of cascades and the properties of ice in the Antarctic Region make it possible to propose Radio Antarctic Muon and Neutrino Detection (RAMAND): antennas should be placed on the ice surface of approx. 10 sq km to search for radio signals for neutrino (muon) cascades of energy. It is evident from data given that the largest radio telescopes gives the opportunity for registration of the cascades induced by neutrinos with the energies E is greater than or = 10(exp 20) eV.
    Keywords: ASTRONOMY
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 87-90
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    The cyclotron absorption line and eclipse transition phenomena of 4U 1538-52 (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: Observations of the eclipsing binary X ray pulsar, 4U 1538-52 have revealed a cyclotron absorption feature at 20 keV in the x ray spectrum. The pulse phase dependence of the intensity and spectrum can be mimicked by a model of x ray emission from thin accretion heated slabs at the magnetic poles of a rotating neutron star with its magnetic dipole axis inclined at 45 deg from the rotation axis. The observations also yielded data on the eclipse transitions which show that the radial density function at the base of the supersonic wind of the O-type supergiant primary has the form of an exponential like that which characterizes the density run in the similar region of the O-type supergiant primary of Cen X-3. As in the Cen X-3 system, the scale height of the exponential implies a temperature in the base region much greater than that of the supersonic wind.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 77-86
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  • 45
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    Chaotic inflationary universe and the anisotropy of the large-scale structure (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: It has been realized that the inflationary universe is in fact chaotic, that globally it is strongly inhomogeneous, and that the inflation in the universe as a whole is eternal. In such a picture the region available to modern observations is just a tiny part of the universe, in which inflation finished about 10(exp 10) years ago. In spite of the great popularity of the chaotic inflationary universe models, it is usually taken for granted that their specific features (such as strong global inhomogeneity of the universe) can hardly lead to any observable consequences. The argument is that all that is seen is just a tiny part of the universe, a region about 10(exp 28) cm, and the typical scales of considerable inhomogeneities are much greater than this size. In contrast to this opinion, an attempt is made to show that such observable consequences can really exist. The phenomenon closely connected with the origin of structure (galaxies, clusters, etc.) in the observable region is discussed. The main idea considered is the vacuum fluctuations evolution on the inhomogeneous background.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 68-76
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  • 46
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    Turbulization of shear flows in astrophysics (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: The problem of instability in shear flows is treated in a number of works which present a global analysis of irrotational (potential) modes in a 2-D compressible shear flow, implying that, in the presence of reflecting boundaries, those modes can increase and that the characteristic time of the increase is several orders as high as the dynamic time in the medium. Due to the slowness of the increase of irrotational modes and to the problematic character of the theory's applicability for thin accretion disks, an attempt was made to develop an alternative theory of turbulization shear flows. It is quite contrary to the above theory. Instead of the global analysis, a local one is made far from the flow boundaries. Instead of irrotational perturbations, vortical ones are examined. Lastly, the medium is considered to be incompressible. The flow in the accretion disk can be modeled by a plane Couette flow, without regard for boundary conditions: it can be modeled by a plane shear flow in infinite incompressible fluid. According to this scenario, there is a critical perturbation level in the free shear flow. If the level of initial perturbations exceeds the critical one, the flow is turbulized.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 55-64
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  • 47
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    Manifestations of dynamo driven large-scale magnetic field in accretion disks of compact objects (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: A turbulent dynamo nonlinear theory of turbulence was developed that shows that in the compact objects of accretion disks, the generated large-scale magnetic field (when the generation takes place) has a practically toroidal configuration. Its energy density can be much higher than turbulent pulsations energy density, and it becomes comparable with the thermal energy density of the medium. On this basis, the manifestations to which the large-scale magnetic field can lead at the accretion onto black holes and gravimagnetic rotators, respectively, are presented.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 46-54
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  • 48
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    The shock breakout in SN1987A modelled with the time-dependent radiative transfer (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: A fully implicit high-order scheme was developed for the independent multi-group radiative transfer coupled with implicit hydrodynamics. The application of this scheme to the SN1987A explosion shows that shortly after the shock breakout a dense shell forms. A gasdynamic code describing the time dependent radiation transport in the multi-group approximation with variable Eddington factors was developed. Results and other aspects of the project are presented.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 39-45
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    Axially symmetrical supernova remnants (1991)
    In:  CASI
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    Publication Date: 2006-06-13
    Description: The origin of cylindrically symmetric supernova remnants (SNR's) is discussed. The results of numerical simulations of two most distinguished barrel-like SNR's SN1006 and G296.5+10.0 are presented.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 19-27
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    On the evolution of pulsars (1991)
    In:  CASI
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    Publication Date: 2006-06-13
    Description: Data from a previous investigation on the angle chi between the axis of rotation and the magnetic dipole axis, determined from polarization observations, provides a complete catalog which makes it possible to carry out a detailed comparison of the theoretical results of this present investigation with the observed distribution of radio pulsars over the angel chi. Before such a comparison is made, the main features of a theory for pulsar evolution is described.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 9-13
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    Instabilities in SN1987A and other supernovae (1991)
    In:  CASI
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    Publication Date: 2006-06-13
    Description: While giving a remarkably good description of many aspects of SN1987A, calculations assuming spherical symmetry have a number of flaws. Many of these problems naturally disappear in the development of a 2-D calculation, as nonspherical instabilities grow. This nonspherical behavior, made evident in SN1987A, has implications for other types of supernovae.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Societ Perspectives; p 1-8
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    The spin down of the radio pulsars: Braking index (1991)
    In:  CASI
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    Publication Date: 2006-06-13
    Description: Presently, the value of the retardation dP/dt is well known for most radio pulsars. It is negative for all cases except one and is of the order of 10(exp -15). That single case is when the pulsar, which is located in the star globular system, can have a considerable acceleration leading to the opposite sign of P'= dP/dt due to the Doppler effect. Careful measurements of the period, P, also allow one to determine the variation of this retardation with the course of time- P'' = d(exp 2)P/dt(exp 2). The results of these measurements are usually represented in the form of the dimensionless retardation index n = omega'' omega/omega(exp 2)= 2 - P''P/P(exp 2) (omega is the angular velocity). The data for 21 pulsars are given. The parameter, n, is strongly undetermined both in value and sign in all cases except for four pulsars. Changes of the rotation period, P, and the inclination angle, chi, the angle between the axes of rotation and the magnetic moment are caused by two processes: the regular retardation and nutation due to deviation from the strict spherical shape of the neutron star. Losses which are caused by the currents flowing in the magnetosphere of the neutron star and by being closed on the star surface are considered. Such losses are critical for the neutron star magnetosphere which is full of dense plasma. Since the radio emission is generated in the dense plasma of the polar magnetosphere, then practically all radio pulsars are retarded by the current mechanism. The formula for the braking index is presented along with other aspects of the investigation.
    Keywords: ASTROPHYSICS
    Type: NAS-NRC, High-Energy Astrophysics. American and Soviet Perspectives; p 14-18
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    The 10 January 1986 disconnection event in Comet Halley (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The disconnection event (DE) in the plasma tail of Comet Halley on January 9-12, 1986 is examined. The distances between the comet head and the disconnected tail are measured for a series of images for that time period and then extrapolated to the nucleus to determine the disconnection time: January 9.60 +/- 0.2 days. The approximate solar-wind conditions at the time of the DE were obtained by corotation of IMP-8 satellite data in earth orbit to Comet Halley. At the time of the DE, Comet Halley is inferred to have been close to a magnetic-sector boundary and a high-speed stream-compression region. However, a heliographic latitude separation of 22 degrees (between the comet and IMP-8) and gaps in the IMP-8 data render a more definitive statement about the linkage of the DE to external conditions quite difficult. It is not possible to resolve the effects of magnetic changes associated with the sector boundary and plasma pressure in the compression region.
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    The future of cometary plasma research (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Advances in the acquisition and analysis of cometary data are reviewed with attention given to current research and projects under development. The need for supplementing the present cometary data is underscored by discussing observational missions from the ground and earth orbit such as the Giotto Extended Mission and the Comet Rendezvous Asteroid Flyby. The interpretation of Comet Halley data is characterized as advanced with respect to both complete observational data and sophisticated modeling.
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    A flat array large telescope concept for use on the moon, earth, and in space (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: An astronomical optical telescope concept is described which can provide very large collecting areas, of order 1000 sq m. This is an order of magnitude larger than the new generation of telescopes now being designed and built. Multiple gimballed flat mirrors direct the beams from a celestial source into a single telescope of the same aperture as each flat mirror. Multiple images of the same source are formed at the telescope focal plane. A beam combiner collects these images and superimposes them into a single image, onto a detector or spectrograph aperture. This telescope could be used on the earth, the moon, or in space.
    Keywords: ASTRONOMY
    Type: Review of Scientific Instruments (ISSN 0034-6748); 62; 2931-293
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    A nonradial pulsation model for the rapidly rotating Delta Scuti star Kappa(2) Bootis (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: A sectorial nonradial pulsation model is used to construct theoretical line profiles which mimic the variations for Kappa(2) Boo. Synthetic spectra generated with the appropriate Teff and log g are used as input. It is found that the data can be reproduced by the combination of a high-degree l is approximately equal to 12 mode with P(osc) aproximately equal to 0.071 d, and a low-degree mode, l is approximately equal to 0-2 with P(osc) approximately equal to 0.071-0.079 d. The projected rotational velocity (v sin i - 115 +/-5 km/s) was determined by fitting synthetic line profiles to the observed spectra. The velocity amplitude of the high-degree oscillations is estimated to be about 3.5 km/s. It is found that the ratio of the horizontal and radial pulsation amplitudes is small (about 0.02) and consistent with p-mode oscillations. Comparisons are made with models invoking starspots, and it is impossible to fit the observations of Kappa(2) Boo by a starspot model without assuming unrealistic values of radius or equatorial velocity.
    Keywords: ASTROPHYSICS
    Type: Astronomical Society of the Pacific, Publications (ISSN 0004-6280); 103; 1250-125
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    New insights from a statistical analysis of IUE spectra of dwarf novae and nova-like stars. I - Inclination effects in lines and continua (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: IUE observations of dwarf novae at maximum at quiescence and novalike objects at the high brightness state are analyzed for effects of the inclination angle on the emitted continuum and line radiation. A clear pattern in the continuum flux distribution is exhibited only by dwarf novae at maximum where some 80 percent of the non-double-eclipsing systems show essentially identical distributions. This result is not in disagreement with theoretical expectations. All classes of objects exhibit a clear, but in each case different, dependence of the line radiation on the inclination angle.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 252; 1, De
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    Inhomogeneous structure in the chromospheres of dwarf M stars (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Linear combinations of observed spectra of the H-alpha and Ca-II resonance and IR lines from the chromospheres of a quiet (Gl 1) and an active (Gl 735) dwarf-M star are compared with the corresponding spectra from a star of intermediate activity (Gl 887). It is shown that the intermediate spectra cannot be explained as a simple juxtaposition of the extreme chromospheric states. It is concluded that the range of observed strengths of chromospheric activity indicators in dwarf-M stars is due, at least in part, to changes in the radial structure of the chromospheric heating function and not to changes in the area filling factor.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 253; 575-578
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    Cometary apparitions - 1990-2010 (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Osculating orbital elements are listed (chronologically by perihelion passage time) for all periodic comets expected to arrive at perihelion during the 1990-2010 interval. Plots which make it possible to readily determine the earth-based viewing conditions of a particular cometary apparition are presented. These plots are drawn in a rotating coordinate system so that the sun-earth line is fixed for each apparition.
    Keywords: ASTROPHYSICS
    Type: In: Comets in the post-Halley era. Vol. 2 (A93-13551 02-90); p. 1281-1308.
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    Cometary plasma waves and instabilities (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Various plasma waves and instabilities that have been observed near comets are discussed. Observational results are ordered by plasma parameters and compared with specific instabilities. The variation in the cometary ion and electron beam densities and velocities are studied by examining regions far from and near the comet nucleus. Spacecraft observations relevant to nonlinear wave evolution, wave cascading, and the development of turbulence are reviewed. The implications of the results for wave-particle interactions - linear, nonlinear, resonant, and nonresonant - and their effects on stochastic particle acceleration are addressed. Higher-frequency ELF/VLF wave observations are also reviewed, and comparisons between the various measurements are made.
    Keywords: ASTROPHYSICS
    Type: In: Comets in the post-Halley era. Vol. 2 (A93-13551 02-90); p. 1171-1210.
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    Cometary activity, discrete outgassing areas, and dust-jet formation (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Conceptual models for various types of features observed in cometary comae (jets, spirals, halos, fans, etc.), their computer simulation, and the hydrodynamic models for jet formation are critically reviewed, and evidence for anisotropic, strongly collimated flows of ejecta emanating from discrete active regions (vents) on the rotating cometary nuclei is presented. Techniques employed to generate synthetic comet images that simulate the features observed are described, and their relevance to the primary objects of coma-morphology studies is discussed. Modeling of temporal variations in the water emission from discrete active regions suggests that production curves asymmetric with respect to perihelion should be commonplace. Critical comparisons with the activity profiles of Enke's comet and with light curves of disappearing comets and comets that undergo outbursts are presented. Recent developments in the understanding of the processes that cause the nongravitational perturbations of cometary motions are reviewed, and the observed discontinuities are identified with the birth of new sources and/or deactivation of old vents.
    Keywords: ASTROPHYSICS
    Type: In: Comets in the post-Halley era. Vol. 2 (A93-13551 02-90); p. 769-823.
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    Dynamical history of the Oort cloud (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: An overview of recent dynamical studies on the Oort cloud of comets surrounding the solar system is presented. Cometary orbits in the cloud evolve under the complex interaction of stellar, galactic, and giant molecular cloud perturbations, as well as planetary and nongravitational perturbations when the orbits reenter the planetary region. There is mounting evidence for a dense, inner Oort cloud of comets which acts as a reservoir to replenish the outer cloud as comets there are stripped away. A ring of comets beyond the orbit of Neptune, which may be the source of the short-period comets, is also likely. Temporal variations in the flux of comets from the Oort cloud into the planetary region by a factor of 50 percent are typical, and by factors of 20 to 200 are possible. Comets in the Oort cloud are processed by galactic cosmic rays, heated by nearby supernovae, eroded by interstellar dust impacts, and disrupted by mutual collisions.
    Keywords: ASTROPHYSICS
    Type: In: Comets in the post-Halley era. Vol. 1 (A93-13551 02-90); p. 463-486.
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    The temperature of the circumstellar envelope of Alpha Orionis (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The processes that heat and cool the circumstellar envelope of Alpha Ori are analyzed and the temperature calculated as a function of distance from the star. Cooling by fine structure lines is included for the first time in a general way in the theory of the thermal balance of a circumstellar envelope. Adiabatic cooling, dust-drag heating, and various line coolants contribute in the outer envelope, whereas the fine structure lines of O I and Si II and adiabatic cooling are dominant in the inner envelope. The rotational levels of CO cool the gas only at very small and at very large distances; elsewhere they provide a small amount of heating. The temperature declines very rapidly at first and then varies roughly as r exp -1/2 in the outer envelope. At very large distances, the temperature distribution flattens out due to photoelectron heating and reaches a minimum of about 4 K at about 250 arcsec.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 382; 606-616
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    10 micron spectra of protostars and the solid methanol abundance (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: A search has been made for the strong C-O stretching absorption of solid methanol near 9.8 microns toward the heavily obscured protostars AFGL 961, AFGL 2591, the BN object and Mon R2 IRS 3. There is no clear evidence for this feature in the spectra, resulting in very conservative upper limits to the methanol abundance of 6 percent to 17 percent relative to solid H2O toward these objects. This is well below previous estimates of 50-80 percent obtained toward W33 A, NGC 7538 IRS 9, AFGL 2136, and W3 IRS 5, which were based on the assignment of the interstellar 6.85 microns absorption feature to the methanol C-H bending mode. This study shows that such high methanol abundances are not a characteristic of all interstellar ices.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 382; 523-529
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    On the evolution of gravitationally unstable protostellar disks (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: 2D, N-body simulations of the nonlinear development of long-wavelength instabilities predicted to occur early in the formation stage of protostellar disks forming single stars are presented under conditions of controlled energy loss. It is found that the thermodynamics and long-term evolution of gravitationally unstable disks are intimately coupled: the global evolution rate is linearly proportional to the rate at which dissipated energy reaches and escapes from the surfaces of the disk. An upper limit is obtained to the rate at which energy is lost if it is assumed that the disk maintains vertical isothermality.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 382; 530-543
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    Wind-tunnel study of an observatory dome with a circular aperture (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Results are presented from wind tunnel tests of the novel, 'fixed shutter' observatory dome concept, which is intended to minimize wind shake effects. In order to enclose larger telescopic apertures, this design allows an upper spherical segment to rotate under a shell of slightly larger diameter which is fixed to a lower rotatable segment. A side-by-side wind tunnel test comparison of this and a conventional dome shows that the mean and fluctuating velocity through the aperture and in the center of the new dome are lower than those of conventional domes, thereby lowering the likelihood of flow-induced telescope vibration.
    Keywords: ASTRONOMY
    Type: Astronomical Society of the Pacific, Publications (ISSN 0004-6280); 103; 1211-121
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    The accumulation and structure of comets (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The paper reviews evidence for the accumulation of the terrestrial planets and comets from solid grains, with emphasis on the various proposals for the formation of cometary nuclei. With three exceptions, all hypotheses conclude or imply that a single compact object forms. Several hypotheses start with Goldreich-Ward-type gravitational instabilities. The collapse for this case also occurs at low velocities in the cm/s to m/s range. Experiment and theory show that under these conditions, low-density, filamentary clusters form that are fractal aggregates with a fractal dimension approximately equal to 2. In order to form cometary nuclei, the initial temperature must be about 50 K and not undergo a significant temperature rise during the accumulation process. The calculations show that accumulation will occur at low temperatures. Models of cometary nuclei are reviewed, and a simple model of the structure that results fom the accumulation of fluffy aggregates is described.
    Keywords: ASTROPHYSICS
    Type: In: Comets in the post-Halley era. Vol. 1 (A93-13551 02-90); p. 335-359.
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    The modern role of visual observations of comets (1991)
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    Publication Date: 2011-08-24
    Description: This paper reviews the role of past and future visual observations in cometary research. The strengths and limitations of visual observations are explored for the benefit of both investigators who might have to use them and observers who wish to make real contributions to the field. We consider the characteristics of the eye-brain combination as a detector and compare them with those of modern detectors. We specifically evaluate visual discoveries, magnitude estimates, and drawings.
    Keywords: ASTRONOMY
    Type: In: Comets in the post-Halley era. Vol. 1 (A93-13551 02-90); p. 209-223.
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    Cometary orbital dynamics and astrometry (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: An overview of research on cometary orbital dynamics and astrometry is presented, with emphasis on the nongravitational acceleration model which provides accurate orbits and ephemerides, and on new asymmetric models that represent the cometary outgassing phenomena more closely. Using a combination of long-focus telescopes, CCD detectors, microdensitometer reductions, and modern star catalogs, cometary astrometric data that are accurate to the subarcsecond level can be generated.
    Keywords: ASTROPHYSICS
    Type: In: Comets in the post-Halley era. Vol. 1 (A93-13551 02-90); p. 3-17.
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    Synchrotron emission as a tracer of the outflow in M82 (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The discovery of radio synchrotron emission from the outflow in M82 is reported. It is found that the outflow in M82 is visible in synchrotron radiation. IC losses are evident in the steepening of the radio spectrum, though this process is not a major contributor to the observed halo X-ray emission. Particle acceleration and/or diffusion in the wind may be important. Outflow speeds of 1000-3000 km/s are inferred from the spectral index profiles, which significantly exceeds the optically determined values. The fraction of the wind energy in the form of relativistic particles is consistent with a supernova origin. A termination shock produced by the intergalactic medium may be shaping the observed halo of M82.
    Keywords: ASTROPHYSICS
    Type: In: The interstellar disk-halo connection in galaxies (A93-18998 05-90); p. 281-285.
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    The effect of the changing polarity and neutral sheet of the IMF on the cosmic ray diurnal anisotropy at neutron monitor energies (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: A drift with a simulated wavy neutral sheet have been used to study the effects of the reversal of the solar magnetic field every 11 years and the changes in the waviness of the heliospheric neutral sheet, corresponding to changes in solar activity, on the diurnal anisotropy at an energy of 20 GeV. The results indicate that the long-term behavior of the diurnal anisotropy, especially the phase shift from one solar minimum period to another, which seems to depend on the polarity of the IMF, has a theoretical explanation in the drift picture of the modulation of cosmic rays in the heliosphere.
    Keywords: ASTROPHYSICS
    Type: Planetary and Space Science (ISSN 0032-0633); 39; 1233-123
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    Energetic water-group ions at Comet Giacobini-Zinner - An overview of observations by the EPAS instrument (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: An overview is presented of the observations of energetic (45-480 keV) cometary water group ions made by the EPAS instrument on the ICE spacecraft during the fly-by of Comet Giacobini-Zinner in September 1985. The discussion covers ion observations in the large-scale ion pick-up region surrounding the comet on about 10 exp 6 km spatial scales, observations of the cometary shock at about 100,000 km and the region of slowed mass-loaded flows which occur downstream, and the finite gyroradius effects which occur near the closest approach.
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    Noise-driven radiative shocks - A new model for the optical quasi-periodic oscillations of the AM Herculis objects (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A model for the 0.3-1.2 Hz optical quasi-periodic oscillations (QPOs) observed in a number of AM Her-type binary systems has been developed. It is suggested that the observed optical modulation is the result of shock oscillations induced by nonsteady accretion flows. It is shown that time-dependent models of radiative shock waves in nonsteady accretion flows onto magnetic white dwarfs with mass 0.6 solar mass and magnetic field strength of 30 MG can produce optical QPOs similar to those observed in the AM Her objects. Theoretical calculations have shown that oscillations cannot be sustained for these white dwarf parameters when the accretion rate is constant.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 375; L31-L34
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    GRIS observations of positron annihilation radiation from the Galactic center (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: In the present Gamma-Ray Imaging Spectrometer (GRIS) observations, the 511 keV line has been spectrally resolved and the Galactic plane and center components have been independently measured. These data, in combination with those from other narrow-field observations in the 1980s, support the two-component model of the positron annihilation source. A strong hard-X-ray continuum was detected in the Galactic plane observation; this 'diffuse' continuum component solves the mystery as to why wide-field instruments have detected such high continuum emission from the Galactic center, in virtue of its source contributions.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 375; L13-L16
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    High-luminosity single carbon stars in stellar and galactic evolution (1991)
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    Publication Date: 2011-08-19
    Description: In the solar neighborhood, approximately half of all intermediate mass main sequence stars with initially between 1 solar mass and about 5 solar masses become carbon stars with luminosities near 10,000 lunar luminosities for typically less than 1 million years. These high luminosity carbon stars lose mass at rates nearly always in excess of 10 to the -7th solar mass/yr and sometimes in excess of 0.00001 solar mass/yr. Locally, close to half of the mass returned into the interstellar medium by intermediate mass stars before they become white dwarfs is during the carbon star phase. A much greater fraction of lower metallicity stars become carbon-rich before they evolve into planetary nebulae, than do higher-metallicity stars; therefore, carbon stars are much more important in the outer than in the inner Galaxy.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics Review (ISSN 0935-4956); 2; 227-247
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    The chromospheres of late-type stars. I - Eridani as a test case of multiline modelling (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A new model of the lower chromosphere of the dwarf K2 star Epsilon Eridani is derived by matching flux profiles of the Ca IR triplet lines 8498 and 8542 A H-alpha and H-beta lines and the Na D lines (all observed simultaneously at the AAT), and the Ca II K line. The coupled non-LTE equations of statistical equilibrium and radiative transfer are solved under the constraint of hydrostatic equilibrium using the Carlsson (1986) code. Within the framework of the model, the Na D lines are an important photospheric diagnostic, and the Ca IR triplet lines can be used to locate the temperature minimum. The computed H-alpha and H-beta depths are highly sensitive constraints on the transition zone gradients and base pressures allowing us to derive a pressure at the base of the transition zone of 0.9 dyn/cm.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 250; 14-23
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    A sensitive upper limit on the methane abundance in Comet Levy (1990c) (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Results are presented on a ground-based search for fluorescent emission from CH4 at 3.3 microns in a newly discovered long-period comet, Comet Levy (1990c). It was found that the upper limit to the CH4 abundance in Comet Levy is similar to the Comet Halley CH4 abundance estimated from lower resolution IR spectra by Kawara et al. (1988). The Comet Levy CH4 abundance is significantly lower than the abundance derived by Larson et al. (1989) from a possible detection of CH4 in Comet Wilson.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 372; L113-L11
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    Velocity structure of the ring nebula around supernova 1987A (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A series of high-resolution spectra of the pc-sized nebula around SN1987A are presented. The hollow, 1.7 arcsec-wide nebula has the velocity field of a ring expanding at 10.3 km/s, not that of a limb-brightened spheroid. Fainter nebulosity within 3 arcsec is expanding slightly faster, as indicated by redshifted emission from behind the SN. The geometry of the emission can be used to infer structure, provided that light travel time and recombination delays are accounted for. The analysis sets an upper limit to the velocity of the wind from the progenitor during its red supergiant (RGS) phase, and implies an RSG lifetime of less than 400,000 yr and an interval of about 20,000 yr between the end of the RSG phase and the SN explosion.
    Keywords: ASTROPHYSICS
    Type: Nature (ISSN 0028-0836); 350; 683-685
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    Simulations of space VLBI (1991)
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    Publication Date: 2011-08-19
    Description: The paper concentrates on the results from space VLBI simulations undertaken during the past four years, beginning with a study of the imaging potential of the proposed medium-term mission QUASAT, and continuing with the Japanese VSOP and Soviet Radioastron missions. The purpose of the study is to determine what quality of images can be expected from space VLBI, given realistic data errors and spacecraft orbits yielding incomplete UV coverage. Much of the paper is devoted to the problem of spurious symmetrization, and a method of overcoming this problem by using a set of UV-constraints is presented. Constraints limiting the imaging ability of the VSOP and Radioastron projects are discussed, and it is concluded that the application of constraints in either sky- or UV-plane will make it possible to overcome spurious symmetrization.
    Keywords: ASTRONOMY
    Type: Advances in Space Research (ISSN 0273-1177); 11; 2, 19; 415-420
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    Cosmological investigations from SIRTF (1991)
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    Publication Date: 2011-08-19
    Description: An overview is presented of the Space Infrared Telescope Facility (SIRTF), a one-meter-class cryogenically-cooled observatory for IR astronomy from space that will be the IR component of NASA's family of Great Observatories. SIRTF will operate at an altitude of 100,000 km and achieve its five year lifetime without the complication of on-orbit cryogenic replenishment and have twice the on-target efficiency that could be realized in LEO. The observations will permit the development of a direct empirical picture of the time evolution of cosmic phenomena, such as evolution of the chemical and stellar content of galaxies and the growth and decay of the quasar population.
    Keywords: ASTRONOMY
    Type: Advances in Space Research (ISSN 0273-1177); 11; 2, 19
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    Evolving starburst galaxies, faint number counts, and the 2 keV background (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Blue and 60 micron extragalactic number counts and redshift distributions are modeled using evolutionary models in which galaxies periodically undergo transient starburst events. Support is found for the hypothesis that the same starburst phenomenon is responsible for the excess source counts seen at both wavelengths, though observed at lower mean redshift in the far infrared than in the blue. The soft X-ray emission from these evolving starburst events is not likely to exceed the 2 keV background unless the evolutionary rate is greater than (1 + z)-cubed (luminosity or density evolution) and the redshift of galaxy formation is greater than about five.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 11; 2, 19; 333-336
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    VLBI astrometry and the Hipparcos link to the extragalactic reference frame (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Intermediate results are reported from a program of VLBI radio observations designed to establish a link between the rotating reference frame of the ESA Hipparcos astrometric satellite and the extragalactic VLBI frame being developed by the International Earth Rotation Service. A group of 12 link stars have been observed at various epochs since 1982, and more observations are being undertaken during the 3-yr Hipparcos mission (1989-1992). Analysis of data on Algol indicates that phase-reference VLBI can determine an expected sky displacement of 4 marcsec with an uncertainty of 0.5 marcsec, even when the activity is only a few mJy.
    Keywords: ASTRONOMY
    Type: Advances in Space Research (ISSN 0273-1177); 11; 2, 19
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    Early results from the Cosmic Background Explorer (COBE) (1991)
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    Publication Date: 2011-08-19
    Description: Data obtained with the FIR Absolute Spectrophotometer, Differential Microwave Radiometers, and Diffuse IR Background Experiment (DIRBE) on the COBE satellite since its launch in November 1989 are briefly characterized. The COBE spacecraft and its 900-km 99-deg orbit are described; the scientific goals and capabilities of the instruments are reviewed; and sample DIRBE data are presented in a map and graph. Upper limits on the Comptonization parameter (y less than 0.001) and the chemical potential (mu less than 0.01 at the 3sigma level) are determined, and the spectrum of the dipole anisotropy is shown to be that of a Doppler-shifted blackbody. The DIRBE 100-micron sky brightness values at the ecliptic poles are found to be significantly lower than those measured by IRAS.
    Keywords: ASTRONOMY
    Type: Advances in Space Research (ISSN 0273-1177); 11; 2, 19; 181-191
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    Dynamical capture of field stars by interstellar clouds (1991)
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    Publication Date: 2011-08-19
    Description: Evidence suggesting the existence of old stars in young galactic clusters, has motivated an investigation of a model in which such stars are captured from the field during the gas cloud collapse phase of star formation. A hyperbolic field star cannot be gravitationally captured by a static cloud since the star's total energy is conserved. However, if the cloud's potential is an explicit function of time, the energy of a penetrating star can be decreased. As a first step in modeling the dynamical capture of field stars, capture by freely falling, homogeneous spherical cloud is considered. This simplified model is solved, and a prediction is obtained for number of captured field stars in terms of the cloud parameters and dispersion velocities of the field stars.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 245; 1, Ma; 75-78
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    The effects of X-ray illumination on accretion disk interiors (1991)
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    Publication Date: 2011-08-19
    Description: The structure of accretion disks illuminated by X-ray from a central compact object. At radii larger than a certain value, X-ray heating can dominate the viscous heating and determine the disk structure in a standard alpha-disk model. The disk structure resulting from X-ray illumination is calculated both analytically and numerically for low-mass X-ray binaries as well as for active galactic nuclei. X-ray illumination does have important effects on the disk structures, under a variety of circumstances; instabilities may arise under significant X-ray heating.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 374; 721-731
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  • 86
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    Astro - Science in the fast lane (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The Space Shuttle mission devoted to astronomy (Astro) is described and data obtained during the nine-day flight are briefly reviewed. The Wisconsin Ultraviolet Photo-Polarimeter Experiment reveals a trough at 2200 A in the spectrum of Alpha Camelopardalis. It is suggested that the 2200-A feature is caused by absorption by graphite-rich interstellar dust. The Broad Band X-ray Telescope made it possible to study the NGC 1399 X-ray spectrum, the spectrum of the BL Lacertae object PKS 2155-304, and a cooling flow in a cluster of galaxies. The Astro mission's search for the missing mass in the form of neutrinos is discussed. Attention is also given to the effect of delays on the Astro activity, and to equipment problems and how they were solved.
    Keywords: ASTRONOMY
    Type: Sky and Telescope (ISSN 0037-6604); 81; 591-596
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  • 87
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    A rapid energetic X-ray flare in the quasar PKS0558 - 504 (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A recent observation from the Ginga satellite of the quasar PKS0558 - 504 is reported during which the X-ray flux increased by 67 percent in the space of only three minutes. There was no significant change in the spectrum. Comprehensive analysis of the data strongly indicates that this was a genuine X-ray flare originating in the quasar. The implied rate of change in luminosity in the 2-10 keV range, assuming a Hubble constant of 70 km/s/Mpc, and a cosmological deceleration parameter q0 = 0.5, is 3.2 x 10 to the 42nd erg/sq s, the highest value measured for a quasar. When photon scattering is considered, this is about 16 times greater than could be produced, with a three-minute rise time, in an isotropically emitting plasma. It is argued that the apparent luminosity must be enhanced by relativistic beaming. This is the first indication of beaming in an 'ordinary' unpolarized quasar.
    Keywords: ASTROPHYSICS
    Type: Nature (ISSN 0028-0836); 350; 589-592
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  • 88
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    Observations of the peculiar object MWC 560 in outburst (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The results of ultraviolet spectroscopy, photoelectric photometry, and supplemental high-resolution H(alpha) spectroscopy of a photometric outburst of MWC 560 are discussed. Ultraviolet spectra are shown to be consistent with the ejection of an optically thick shell that produced strong absorption blends of Fe II and Cr II. The velocities reported exceed by far those previously found in symbiotic stars or recurrent novas. In addition to the variable high-velocity system of broad absorption features, a relatively stable system of Mg II, Mg I, Fe II, Cr II, and other ionic absorptions is observed. It is pointed out that the spectroscopic phenomena in MWC 560 resemble those found in XX Ophiuchi, but the velocities in the MWC 560 are an order of magnitude higher than those found in XX Oph.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 371; 761-771
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  • 89
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    Accretion in wind-driven X-ray sources (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The hydrodynamics of axisymmetric accretion flow in stellar wind-fed X-ray sources is investigated, including momentum deposition by radiation, as well as radiative heating and cooling by Compton and bremsstrahlung processes. The results of two-dimensional numerical simulations for mass accretion rates ranging from 0.02 to 0.4 of the Eddington value reveal a variety of behaviors. At low rates radiative effects are unimportant, and the accretion flow is nearly adiabatic. In this case the flow is steady. For intermediate accretion rates, radiative heating and cooling effects become important. The flow remains steady, but the accretion rate is significantly reduced from the Hoyle-Lyttleton estimate. At the highest mass accretion rates, radiative momentum transfer and energy exchange are important and lead to nonsteady behavior. For the parameters relevant to massive X-ray binary systems accreting at high rates, the results reveal that the accretion flows can be complex and time dependent.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 371; 696-707
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  • 90
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    The carbon-rich proto-planetary nebula IRAS 22272 + 5435 (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Ground-based photometry and spectroscopy is presented for IRAS 22272 + 5435, a 9th mag star with a large infrared excess. The flux distribution is 'double-peaked', with a visible and near-infrared component due to the reddened photosphere and a far-infrared component presumably due to a detached dust shell. About equal amounts of energy are detected from each. The spectrum is that of a peculiar G supergiant, Gp Ia. In addition, strong molecular bands of C3 and C2 are observed. The flux distribution is modeled and, together with published molecular-line radio observations, allows the determination of basic parameters of the central star and the dust shell. The source has the characteristics one would expect of a proto-planetary nebula, an object in transition from the asymptotic giant branch to the planetary nebula phase.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 371; 631-635
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  • 91
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    A picosecond accuracy relativistic VLBI model via Fermi normal coordinates (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Fermi normal coordinates are used to construct transformations relating solar system barycentric coordinates to local inertial geocentric coordinates. Relativistic corrections to terrestrial VLBI measurements are calculated, and this formalism is developed to include corrections needed for picosecond accuracy. A calculation of photon time delay which includes effects arising from the motion of gravitational sources is given.
    Keywords: ASTROPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 18; 1139-114
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  • 92
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    The interaction of a very large interplanetary magnetic cloud with the magnetosphere and with cosmic rays (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The observation of one of the largest magnetic clouds ever observed at a distance of 1 AU, with a diameter of greater than about 0.4 AU, is reported. The cloud is shown to be almost unchanged structurally by interaction with the earth bow shock. The first observations are reported of an auroral activity response to the passage of a magnetic cloud, with a nearly immediate increase in auroral activity when the IMF theta(B) angle reversed polarity to negative near the cloud center. The results provide strong evidence that turbulent magnetic fields behind interplanetary shocks are a possible cause of Forbush decreases, but contest the idea that relatively smooth, strong fields in clouds are a cause of such decreases. The cloud field modeling supports the existence of magnetic force-free fields in describing cloud structure.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 9425-943
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  • 93
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    Formation of cosmic structure by Doppler instability (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A new mechanism is described which can create an instability in homogeneous gaseous matter at very low density. When an isotropic background radiation field has, near an electronic resonance, a spectral feature for which photon occupation number increases with frequency, moving atoms increase their speed by taking energy from the photon distribution. In a cosmological setting, a sufficiently intense spectral feature can interact with neutral atomic gas, after recombination, to generate protogalactic perturbations of the scale and magnitude needed to explain large-scale cosmic structure.
    Keywords: ASTROPHYSICS
    Type: Nature (ISSN 0028-0836); 350; 469-473
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  • 94
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    Angular diameter measurements of stars (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Angular diameters determined with the Mark III Optical Interferometer are presented for 12 stars at wavelengths of 450 and 800 nm. The uniform disk diameters resulting from fits to the visibility observations have rms residuals of order 1 percent for the 800 nm measurements and less than 3 percent for the 450 nm measurements. The improvement over previous observations with this instrument is due to improved data analysis and the use of a wider range of baseline lengths. An analysis of the calibration systematics for the Mark III Optical Interferometer is included. There is good agreement between these measurements and previously published data. The changes in uniform disk diameter between wavelengths of 450 and 800 nm agree with models of stellar atmospheres.
    Keywords: ASTRONOMY
    Type: Astronomical Journal (ISSN 0004-6256); 101; 2207-221
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  • 95
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    Blue loops during core helium burning as the consequence of moderate convective envelope overshooting in stars of intermediate to high mass (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: New calculations of stellar evolutionary sequences without semiconvection in any phase have unexpectedly turned up, for stars in the mass range 3-30 solar masses, an irregular pattern of blue loops on the H-R diagram during the core helium-burning phase. Blue loops occur for 3 and 10 solar masses, but not for 5, 7, 15, and 30 solar masses. It is found, however, that the models near the bottom of the red supergiant branch are only marginallly stable against small inward displacements of the former base of the outer convection zone when it was deepest, for the stellar mass range 5-15 solar masses. As a consequence, convective envelope overshooting need only penetrate a distance of about 0.3 of a local pressure scale height to promote a delayed blue loop in these particular sequences. In conformity with previous work, the triggering mechanism for the blue loop appears to be proximity of the hydrogen-burning shell to the hydrogen discontinuity at the former base of the outer convection zone.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 374; 288-290
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  • 96
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    Speckle interferometry of Hipparcos link stars. III (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A third list of stars is presented which have been tested by speckle interferometry for use in the Hubble Space Telescope link between the Hipparcos astrometric reference frame and the extragalactic VLBI reference frame. Structural information on angular scales of 0.15-1.2 arcsec for 34 Southern Hemisphere stars is reported from observations made with the Imperial College Speckle Interferometer mounted on the Mount Stromlo 1.9-m telescope. Twenty-four percent of the stars (8 out of the 34) show evidence of multiplicity, in agreement with previous observations in this program.
    Keywords: ASTRONOMY
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 248; 411-413
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  • 97
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    Spectral types of stars in the SMC associations NGC456, NGC460a, b and NGC465 (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Spectral types of stars for an extended area (1200 sq arcmin) of the SMC, containing the associations NGC456, NGC460 a, b and NGC465 have been derived here. High-quality UJ and YJ film copies of objective prism plates taken with the 1.2-m UK Schmidt telescope were used. The medium dispersion spectral (830 A/mm) were digitized with a PDS microdensitometer and spectra types of stars were obtained down to about 16.5 mag. A computer interactive method of reduction has been developed, and the spectral types obtained are accurate to 0.3 spectral subtypes. Spectral classification for the low (2440 A/mm) dispersion spectra down to 18.5 mag was carried out visually using a binocular microscope. The frequency distributions of stars in the associations region, after subtracting the field stars, have revealed that only blue main sequence stars are their members.
    Keywords: ASTRONOMY
    Type: Astronomy and Astrophysics Supplement Series (ISSN 0365-0138); 87; 1, Ja; 97-108
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  • 98
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    Star formation and the nature of bipolar outflows (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: This paper presents a simple physical model for the bipolar molecular outflows that frequently accompany star formation. The model forges an intrinsic link between the bipolar flow phenomenon and the process of star formation, and it helps to explain many of the systematics known for existing sources.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 370; L31-L34
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  • 99
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    Two micron morphology of candidate protostars (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Results of a deep near-infrared imaging survey of low-luminosity cold IRAS sources in the Taurus dark cloud are discussed. The images involved identify the compact sources energizing the IRAS sources, identify infrared nebulosity around numbers of the invisible sources, and reveal the large-scale (about 1000 to 10,000 AU) morphology of this nebulosity. Some of the invisible sources show a clear bipolar or monopolar morphology suggesting a close relation of the nebulosity with a bipolar mass outflow. It is concluded that the nebulosity is likely due to scattering of radiation from the central source by the dust associated with the mass outflow extending to the poles of a circumstellar dust disk.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 374; L25-L28
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  • 100
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    Voyager 2 test of the radar time-delay effect (1991)
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    Publication Date: 2011-08-19
    Description: By analyzing radio range measurements generated during the solar conjunction of the Voyager 2 spacecraft in 1985, the relativistic time-delay effect has been tested accurately with a spacecraft at a distance of 3 x 10 to the 9th km from the earth. Because the range measurements were generated simultaneously at two frequencies on the downlink, it was possible to derive calibrations for the delay resulting from intervening solar plasma. In addition, a record of attitude thruster firings was used to model small accelerations of the spacecraft that could have produced a range error. It was thereby possible to determine the parameterized post-Newtonian parameter gamma to an accuracy of 3 percent with only a sparse amount of range data.
    Keywords: ASTRONOMY
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 373; 665-670
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