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  • ASTROPHYSICS  (1,236)
  • General Chemistry  (1,091)
  • Inorganic Chemistry  (773)
  • 2015-2019
  • 1990-1994  (3,100)
  • 1991  (3,100)
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  • 2015-2019
  • 1990-1994  (3,100)
Year
  • 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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    Cosmic evolution of Quasar radio structure (1991)
    In:  Other Sources
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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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  • 4
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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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  • 5
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    Early phases of LMC star clusters? (1991)
    In:  Other Sources
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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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  • 6
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    Why did Halley hiccup? (1991)
    In:  Other Sources
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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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  • 7
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    Horizontal-branch evolution with time-dependent convective overshooting (1991)
    In:  Other Sources
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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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  • 8
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    Molecular hydrogen emission in L1448 associated with a highly collimated molecular outflow (1991)
    In:  Other Sources
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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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  • 9
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    Ultraviolet flares from RS CVn-type stars and related systems (1991)
    In:  Other Sources
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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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  • 10
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    The cosmic neutrino background (1991)
    In:  Other Sources
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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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  • 11
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    The X-ray flux dipole of AGNs as an indicator of the local gravitational field (1991)
    In:  Other Sources
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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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  • 12
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    The missing mass in clusters of galaxies and elliptical galaxies (1991)
    In:  Other Sources
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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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  • 13
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    Evidence for evolution of the luminosity function of clusters of galaxies (1991)
    In:  Other Sources
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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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  • 14
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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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  • 15
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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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  • 16
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    Hydrodynamic study of Supernova 1987A: The phase of a wave of cooling and recombination (1991)
    In:  CASI
    Details
    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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  • 17
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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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  • 18
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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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  • 19
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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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  • 20
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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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  • 21
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    Extragalactic x ray source counts (1991)
    In:  CASI
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    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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  • 22
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    Ultraluminous infrared galaxies (1991)
    In:  CASI
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    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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  • 23
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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
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    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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  • 24
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    On the observational appearances of a freely precessing neutron star in Hercules X-1 (1991)
    In:  CASI
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    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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  • 25
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    The large-scale surface brightness distribution of the x ray background (1991)
    In:  CASI
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    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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  • 26
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    The evolution of the gravitational radiation from stellar components of galaxies (1991)
    In:  CASI
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    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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  • 27
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    Quasi-periodic oscillations in low-mass x ray binaries (1991)
    In:  CASI
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    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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  • 28
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    Gravitational lenses: The current sample, recent results, and continuing searches (1991)
    In:  CASI
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    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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  • 29
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    On two-dimensional relativistic stellar winds (1991)
    In:  CASI
    Details
    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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    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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    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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    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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    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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    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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  • 35
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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
    Details
    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
    Details
    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 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
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    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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    The accumulation and structure of comets (1991)
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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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    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)
    In:  Other Sources
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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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  • 62
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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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    Dynamical capture of field stars by interstellar clouds (1991)
    In:  Other Sources
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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)
    In:  Other Sources
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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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    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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    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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    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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    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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    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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    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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    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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  • 72
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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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    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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    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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    Molecular clouds in starburst galaxy nuclei (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: New CO observations of a complete sample of starburst galaxies are presented in order to determine the range of molecular gas masses and star-formation efficiencies that characterize these systems. The properties of the galaxies in the sample are given, and the H-alpha and R-band CCD images for six of the galaxies are shown. The novel feature of this study is that the galaxies in this study are infrared selected as opposed to optically selected. They exhibit levels of nuclear star formation activity which are comparable to the starburst galaxies M82 and NGC 253. A subset of galaxies with unusually narrow CO line widths, high star-formation efficiencies, and compact 10-micron distributions is revealed. It is proposed that, if gas distributions in these galaxies are also compact, as the narrow linewidths suggest, the high yields of massive stars per unit mass of molecular gas may be attributed to a star-formation mechanism which becomes more efficient as the gas density increases.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 373; 414-422
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    Origin of the burst of TeV gamma-rays from SN 1987 A (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: An electromagnetic origin of the TeV gamma-ray burst from SN 1987 A is proposed. The TeV gamma-rays result from inverse Compton scattering in the extreme Klein-Nishina limit of nuclear Co-56 gamma-ray lines and soft X-rays by monoenergetic TeV electrons. The model accounts for the observed time delay between the TeV burst and the peak X-ray intensity of about 3 days, and the different burst duration at TeV and X-ray energies.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics (ISSN 0004-6361); 243; 1, Ma; L1-L4
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    Spectral transformation of the unusual variable star MWC560 to resemble a nova (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A dramatic change has occurred in the ultraviolet spectrum of the emission-line star MWC560, so that it now closely resembles the spectrum of a nova shortly after outburst. This event may signal a major mass-ejection episode such as presumably occurred in past centuries in the symbiotic star R Aquarii to produce the well-known bipolar nebula, and it may herald the emergence of a standard symbiotic-star emission-line spectrum in MWC560, corresponding to a change in evolutionary state.
    Keywords: ASTROPHYSICS
    Type: Nature (ISSN 0028-0836); 350; 404-406
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    Comet Halley remote plasma tail observations and in situ solar wind properties - Vega-1/2 IMF/plasma observations and ground-based optical observations from 1 December 1985 to 1 May 1986 (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: General features of the solar wind plasma and the interplanetary magnetic field (IMF) are studied for the time period of December 1, 1985 - May 1, 1986. Characteristics studied include corotating events, shocks, and sector boundaries in the IMF. For the same period, Comet Halley was near its perihelion and many ground-based observations of the plasma tail were made. They show interesting events such as tail substorms and disconnection events where the whole distant plasma tail seems to be disrupted from the part that is still attached to the cometary head. Several mechanisms have been produced to explain the cause of these events. The correlation between the IMF sector boundaries measured by Vega and the observed disconnection events are studied, noting that such a correlation would support the dayside reconnection explanation. It is found that only in 50 percent of the considered events does a correlation between these two phenomena exist. For the other cases, a sector boundary of the IMF sweeping over the comet cannot explain the occurrence of disruptions of the main plasma tail.
    Keywords: ASTROPHYSICS
    Type: Planetary and Space Science (ISSN 0032-0633); 39; 697-708
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    Organic chemical analysis on a microscopic scale using two-step laser desorption/laser ionization mass spectrometry (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The distribution of PAHs in the Allende meteorite has been measured using two-step laser desorption and laser multiphoton-ionization mass spectrometry. This method enables in situ analysis (with a spatial resolution of 1 mm or better) of selected organic molecules. Results show that PAH concentrations are locally high compared to the average concentration found by analysis of pulverized samples, and are found primarily in the fine-grained matrix; no PAHs were detected in the interiors of individual chondrules at the detection limit (about 0.05 ppm).
    Keywords: ASTROPHYSICS
    Type: Space Science Reviews (ISSN 0038-6308); 56; 191-195
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    A high-resolution radio image of a young supernova (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A VLBI radio images of the bright supernova 1986J, which occurred in the galaxy NGC891 at a distance of about 12 Mpc, is presented. No detailed image of any supernova or remnant has been obtained before so soon after the explosion. The image shows a shell of emission with jetlike protrusions. Analysis of the images should advance understanding of the dynamics of the expanding debris, the dissipation of energy into the surrounding circumstellar medium, and the evolution of the supernova into the remnant.
    Keywords: ASTROPHYSICS
    Type: Nature (ISSN 0028-0836); 350; 212-214
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    IRAS 15099 - 5856 and the composite supernova remnant G320.4 - 1.2 (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The supernova remnant (SNR) G320.4 - 1.2 is a composite SNR, containing both a shock-dominated shell, and a pulsar-dominated plerion. G320.4 - 1.2 is one of the few SNRs in which a pulsar has been detected. This paper reports on IRAS observations of this SNR. A pointlike source (IRAS 15099 - 5856) near the position of the pulsar appears quite prominently at 25 microns. The region appears more complex at 60 and 100 microns. Three models that may account for the IR emission of IRAS 15099 - 5856 are examined; none of these is entirely satisfactory. Some IR emission is detected from the brightest portion of the shell of this SNR, but this emission is nearly hidden by other confusing sources.
    Keywords: ASTROPHYSICS
    Type: Astronomical Journal (ISSN 0004-6256); 101; 2160-216
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  • 82
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    Ultraviolet high-excitation Fe II fluorescence lines excited by O VI, C IV, and H I resonance emission as seen in IUE spectra (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Archival high-dispersion spectra from the IUE are used in a search for Bowen emission lines of Fe II excited by the stronger transition of the O VI resonance doublet. The possibility of using these Fe II emission lines as a diagnostic of the strength of the far-ultraviolet emission of O VI at 1032 A is explored. It is found that the Fe II emission lines are quite common and strong in symbiotic stars, particularly those of the type known as 'symbiotic novae', as well as in normal novae. The lines are observed in central stars of some planetary nebulae of the O VI sequence besides a few central stars of type WR. High density, high excitation, and high temperature are suggested to be requirements for the excitation of the Fe II fluorescence lines. It is pointed out that while these lines were observed in PG 1159-035 and K1-16, they were not observed in AGNs.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 373; 649-656
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    Far-infrared properties of NGC 6822 (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: IRAS FIR maps of NGC 6822, one of the few noninteracting Magellanic irregular galaxies resolved in the FIR by IRAS, are examined and compared to optical and H I data. The diffuse FIR emission at large radii from the center of the galaxy is found to be due to Galactic cirrus. There is a general correspondence between H-alpha and FIR emission in that FIR peaks are associated with major H II complexes, but the ratio of the FIR to the H-alpha emission varies between these complexes by about a factor of five. H I mass-to-FIR luminosity ratios cover a similar range for the three major H I cloud complexes in NGC 6822. More than about half of the FIR flux is produced in regions near H II region complexes. The remaining flux comes from diffuse FIR emission that extends over the entire optically visible galaxy. A model for the galaxy is discussed in which the galaxy is fairly transparent and the FIR is strongest where there is a strong UV source and higher optical depth due to dust in gas clouds.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 371; 142-147
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    C IV line profile correlations in NGC 4151 (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Novel correlations are presented between the peak emission of the two components of the strong UV resonance line C IV in the Seyfert 1 galaxy NGC 4151 and the total luminosity of the line. These correlations are distinctly different for the red and the blue peaks of the double-peaked C IV profile, implying that the physical parameters involved in their formation are quite different. The behavior of the red component appears more in accordance with the behavior of lines observed in other objects, while that of the blue peak appears more directly and strongly related to C IV luminosity and shows greater sensitivity to C IV flux changes. These findings imply the existence of specific, hitherto unknown, underlying physical processes.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 371; 136-141
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    Anisotropic inverse Compton emission (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The angular distribution of the power and spectrum of Compton-scattered radiation in the case of an anisotropic distribution of soft photons is considered. Results obtained show that both the intensity and spectrum of the scattered radiation are a function of the Lorentz factor of the electrons, the geometry of the soft X-ray source and the viewing angle. The intensity of the hard X-rays scattered back into the bowl can be significantly greater (by a factor of up to 5.5) than that scattered toward the observer for an isotropic distribution of relativistic electrons at the center of a hemispherical bowl of soft X-ray emission. This factor is increased as the solid angle of the soft X-ray source subtended at the location of the electrons is reduced. The results obtained here have inmplications for models invoking a further 'reflection' of the resultant hard X-rays from the surface of a disk.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 248; 14-19
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  • 86
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    The formation of molecules in protostellar winds (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The production and destruction processes for molecules in very fast protostellar winds are analyzed and modeled with a one-dimensional chemical kinetics code. Radial density and temperature distributions suggested by protostellar theory are explored as are a range of mass-loss rates. The efficiency of in situ formation of heavy molecules is found to be high if the wind temperature falls sufficiently rapidly, as indicated by theory. The degree of molecular conversion is a strong function of the mass-loss rate and of density gradients associated with the acceleration and collimation of the wind. Even in cases where essentially all of the heavy atoms are processed into molecules, a significant fraction of atomic hydrogen remains so that hghly molecular, protostellar winds are able to emit the 21-cm line. Although CO has a substantial abundance in most models relevant to very young protostars, high abundances of other molecules such as SiO and H2O signify more complete association characteristic of winds containing regions of very high density. Although the models apply only to regions close to the protostar, they are in qualitative accord with recent observations at much larger distances of both atomic and molecular emission from extremely high-velocity flow.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 373; 254-265
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  • 87
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    Two-temperature accretion disks with winds in a fluid approximation (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The polytropic relation between pressure and density of wind gases is used to obtain solutions for optically thin hot accretion disks with winds. It is assumed that the cooling mechanisms in the disks are bremsstrahlung and Compton scattering. This extends the previous work of Takahara et al. (1989) which did not analyze winds within a self-consistent disk model. It is shown that the disks have winds for accretion rates higher than about 10 percent of the Eddington rate, depending on the viscosity parameter of the disks and the polytropic exponent of wind gases. The poloidal velocity of winds at the foot-point is shown to be as high as 10 percent of light speed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 370; 505-510
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  • 88
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    Absolute magnitude calibration using trigonometric parallax - Incomplete, spectroscopic samples (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A new numerical algorithm is used to calibrate the absolute magnitude of spectroscopically selected stars from their observed trigonometric parallax. This procedure, based on maximum-likelihood estimation, can retrieve unbiased estimates of the intrinsic absolute magnitude and its dispersion even from incomplete samples suffering from selection biases in apparent magnitude and color. It can also make full use of low accuracy and negative parallaxes and incorporate censorship on reported parallax values. Accurate error estimates are derived for each of the fitted parameters. The algorithm allows an a posteriori check of whether the fitted model gives a good representation of the observations. The procedure is described in general and applied to both real and simulated data.
    Keywords: ASTROPHYSICS
    Type: Astronomical Journal (ISSN 0004-6256); 101; 1075-108
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  • 89
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    What COBE might see - The far-infrared cosmological background (1991)
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    Publication Date: 2011-08-19
    Description: The redshifted emission from infrared luminous galaxies should produce a diffuse background around 300 microns detectable by the Cosmic Background Explorer (COBE). The detection of such emission would provide evidence for evolution in the density or luminosity of galaxies in the infrared.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 370; L1-L4
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  • 90
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    The number counts and infrared backgrounds from infrared-bright galaxies (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Extragalactic number counts and diffuse backgrounds at 25, 60, and 100 microns are predicted using new luminosity functions and improved spectral-energy distribution density functions derived from IRAS observations of nearby galaxies. Galaxies at redshifts z less than 3 that are like those in the local universe should produce a minimum diffuse background of 0.0085, 0.038, and 0.13 MJy/sr at 25, 60, and 100 microns, respectively. Models with significant luminosity evolution predict backgrounds about a factor of 4 greater than this minimum.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 367; L49-L53
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  • 91
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    Photoionization modeling of Magellanic Cloud planetary nebulae. I (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The results of self-consistent photoionization modeling of 38 Magellanic Cloud PNe are presented and used to construct an H-R diagram for the central stars and to obtain both the nebular chemical abundances and the physical parameters of the nebulae. T(eff)s derived from nebular excitation analysis are in agreement with temperatures derived by the classical Zanstra method. There is a linear correlation between log T(eff) and the excitation class. The majority of the central stars in the sample with optically thick nebulae have masses between 0.55 and 0.7 solar mass and are observed during their hydrogen-burning excursion toward high temperatures. Optically thin objects are found scattered throughout the H-R diagram, but tend to have a somewhat smaller mean mass. Type I PN are found to have high core masses and to lie on the descending branch of the evolutionary tracks. The nebular mass of the optically thick objects is closely related to the nebular radius, and PN with nebular masses over one solar are observed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 367; 115-125
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  • 92
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    Excitation of millimeter and submillimeter water masers (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The excitation of maser emission in millimeter and submillimeter transitions of interstellar and circumstellar water is considered. An escape probability method is used to determine the equilibrium populations in 349 rotational states of both ortho- and para-water under varying conditions of gas temperature, density, water abundance, and radiation field. It is shown that, under those conditions believed to prevail around late-type stars and within star-forming regions, strong millimeter and submillimeter water maser emission can be generated by collisional excitations by H2. Several maser transitions can have strengths close to that of the 22 GHz line. The water maser line which can be observed from mountaintop facilities and those which will require air- or space-borne platforms are indicated. The exact portion of parameter space in which each maser transition exhibits peak emission is shown.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 368; 215-230
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  • 93
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    Negative ions in the coma of Comet Halley (1991)
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    Publication Date: 2011-08-19
    Description: The detection of negatively charged cometary ions in the inner coma of Comet Halley is reported. These ions are observed in three broad mass peaks at 7-19, 22-65, and 85-110 AMU, with densities reaching greater than about 1/cu cm, about 0.05/cu cm, and about 0.04/cu cm, respectively, at a distance of about 2300 km from the nucleus. The ion species thought to be present include O(-), OH(-), C(-), CH(-), CN(-) and heavier complex CHO molecular ions. As negative ions are easily destroyed by solar radiation at about 1 AU, an efficient production mechanism, so far unidentified, is required to account for the observed densities. The detection of negative ions in the coma near 1 AU implies that negative ions should also be present in similar neutral gas and dust environment farther away from the sun. If the negative-ion densities are large enough, they could play an important part in physical processes such as radiative transfer or charge exchange.
    Keywords: ASTROPHYSICS
    Type: Nature (ISSN 0028-0836); 349; 393-396
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  • 94
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    Using SN 1987A light echoes to determine mass loss from the progenitor (1991)
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    Publication Date: 2011-08-19
    Description: The hypothesis that the blue progenitor of SN 1987A passed through a blue supergiant phase ending with the expulsion of the outer envelope is tested. The many light echoes seen near SN 1987A were used to search for a mass flow from the progenitor and for abrupt density changes at the limits of this smooth mass flow. The progenitor needed roughly a million yr to create these structures, assuming a constant mass loss at 15 km/s. The dust in the region is small-grained and isotropically scattering. Interaction between the progenitor blue supergiant and red supergiant winds is probably contained within a roughly spherical structure 1.5 pc in diameter.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 366; L73-L77
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  • 95
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    A non-local thermodynamic equilibrium, line-blanketed synthetic spectrum of Iota Herculis - C, Al, and Si lines (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A non-LTE line-blanketed model stellar atmosphere is used to compute a model of I Herculis (B3 IV) with a Teff of 17,500 K and a log g of 3.75, following the conclusions of Peters and Polidan (1985). Detailed profiles of a number of lines of C, Al, and Si in the 1200-2000-A region are computed, including the resonance lines of C II, Al II, and Al III. These profiles are compared to observations obtained from the coaddition of eight IUE SWP images, using a technique developed by Leckrone and Adelman (1989). Comparison of carbon lines with a model that is underabundant in carbon by a factor of 2 relative to the sun indicates that the C abundance of Iota Her is at most one-half solar. Non-LTE effects are examined by comparing an LTE model possessing identical atmospheric parameters with the non-LTE model. Substantial differences in the populations of the model atomic states are found, but differences in the temperature structure of the two models often mask the non-LTE effects in the synthetic spectra.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 380; 606-616
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  • 96
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    Polarimetric imaging of the Galactic center at 12.4 microns - The detailed magnetic field structure in the northern arm and the east-west bar (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Recent closely sampled imaging polarimetry of the central 0.5 pc of the Galaxy at 12.4 microns with a 58 x 62 array camera system is presented. The results clearly show that the magnetic field is a property of the diffuse material of the large-scale structures in Sgr A (the northern arm and east-west bar) rather than the embedded sources. The magnetic field is independent of the changes of density and temperature in the compact source environments. The magnetic field direction in much of the east-west bar is complex, and its polarization is probably the result of the superposition of differently oriented filaments. The abrupt decrease of polarization southwest of IRS 1 observed previously is confirmed and shown to reverse sign. This is due either to a rapid change of magnetic field direction close to the dynamical center, or to the superposition of nearly orthogonal fields. The field direction changes smoothly across IRS 1, and no discontinuity of motion here is found.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 380; 419-428
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  • 97
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    Comparison of magnetosonic wave and water group ion energy densities at Comet Giacobini-Zinner (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Measurements of the Comet Giacobini-Zinner (GZ) are presented to determine to what extent wave-particle scattering redistributed the initial pick-up energy of the ion population. Also examined is the difference between the ion thermal energy and the energy in the magnetic fields of the waves. In spite of uncertainty of about a factor of 2 noted in the pick-up and mass-loaded regions, it is shown that less than approximately 50 percent of the pick-up energy is converted into wave magnetic energy in the inbound pick-up region.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 11; 9, 19; 83-86
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  • 98
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    Gamma-ray lines from SN1987A (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Results of gamma-ray observations of SN1987A conducted over a period of 800 days using balloon-borne high-resolution germanium detectors are discussed. Most of the measurements show red-shifted line centroids (about 500 km/s) and broadened profiles, contradicting the standard homogeneous and spherically symmetric models, which predict blue-shifted lines and narrower line widths. To explain this apparent contradiction, it may be necessary to invoke fragmentation and nonspherical geometries.
    Keywords: ASTROPHYSICS
    Type: Advances in Space Research (ISSN 0273-1177); 11; 8, 19
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  • 99
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    A tentative identification of methanol as the progenitor of the 3.52-micron emission feature in several comets (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Continuum emission has been detected during 3.44-3.64 micron spectral region observations of Comets P/Brorsen-Metcalf, Okazaki-Levy-Rudenko, and Levy. The spatial brightness profile of the 3.52-micron emission feature was measured and found to be consistent with uniform outflow from a central, unresolved source; the molecule generating the emission is either directly sublimated from the cometary surface or is a short-lived parent specie's dissociation product. The modeled IR spectrum of CH3OH implies the consistency of the observed spectra with a methanol origin for the 3.52 emission.
    Keywords: ASTROPHYSICS
    Type: Icarus (ISSN 0019-1035); 93; 122-134
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  • 100
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    A 9.1-hour candidate orbital period for X1556-605 (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: V-band photometry of the low-mass X-ray binary X1556-605 obtained in May 1988 is presented in an attempt to determine the orbital period of the system. The source is seen to be variable by up to 0.6 magnitude on a time scale of hours. Combining the data with those obtained one month later by Schmidtke (1990), Fourier techniques and a recently improved version of the standard period-folding analysis are used to find a probable period of 0.3807 + or - 0.0003 day, with a semiamplitude of about 0.1 magnitude. Both methods indicate that the formal significance of this period detection is greater than 99.9 percent. While independent confirmation is advised before accepting this to be the definite orbital period of the system, a period of this length would not be inconsistent with the X-ray properties of X1556-605 and would, in addition, suggest that the mass-donating companion may be beginning to evolve away from the main sequence.
    Keywords: ASTROPHYSICS
    Type: Astronomical Society of the Pacific, Publications (ISSN 0004-6280); 103; 636-641
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