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  • ASTROPHYSICS  (13)
  • 1975-1979  (13)
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  • ASTROPHYSICS  (13)
Years
Year
  • 1
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    Orion's Cloak - A rapidly expanding shell of gas centered on the Orion OB1 association (1979)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The structure of the interstellar gas surrounding the Orion OB1 association and the neighboring lambda Orionis association is detailed. UV absorption lime spectra of various ionization stages of C, N, Si and S in the directions of 12 stars were obtained by means of the spectrometer on board the Copernicus satellite. The presence of a shell of material surrounding the two associations and expanding at 100 to 120 km/sec, designated Orion's Cloak, was revealed, together with sporadically occurring higher column density matter at lower velocities. Results are interpreted to indicate the presence of a rapidly moving radiative shock outside the H II region of the association stars and inside this feature, a lower velocity, higher column density cloud which appears to be directly ionized by association stars. It is suggested that the gas features are caused by the effects of a recent supernova and of multiple supernovae, stellar winds and rocket-accelerated clouds in addition to stellar ionization.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 230
    Format: text
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  • 2
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    Radiative regulation of gas flow within clusters of galaxies - A model for cluster X-ray sources (1977)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 215
    Format: text
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  • 3
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    The velocity distribution of interstellar gas observed in strong UV absorption lines (1978)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Observations of three strong interstellar UV absorption lines of N I (1199 A), N II (1083 A), and Si III (1206 A) in 47 stars of widely varying distance and a variety of spectral types are analyzed to obtain a velocity distribution function for the interstellar gas. A technique based on the maximum and minimum velocities observed along a line of sight is adopted because of heavy line blending, and results are discussed for both power-law and exponential distribution functions. The expected distribution of radiative-phase supernova remnants (SNRs) in the interstellar medium is calculated as a function of SNR birthrate and of the interstellar density in which they evolve. The results are combined with observed distance estimates, and it is shown that an interstellar density in excess of 0.1 per cu cm would be required to keep the SNRs sufficiently confined so that their cross sections are consistent with the observed number of components. The alternative possibility is considered that SNRs do not enter the radiative phase before escaping from the Galaxy or colliding with neighboring remnants.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 223
    Format: text
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  • 4
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    Intermediate-velocity gas in the local interstellar medium (1978)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Scans of interstellar ultraviolet absorption lines of N I, N II, and Si III for 17 stars are combined with previously published data for 30 stars. The extremal velocities at which detectable absorption occurs are tabulated, and it is shown that these are correlated for the three species. The data suggest that intermediate-velocity gas (20 to 60 km/s), best known from Na I and Ca II absorption, contains both neutral and ionized hydrogen. Features characteristic of intermediate-velocity isothermal shocks (greater than 60 km/s) are conspicuously rare. The intermediate-velocity gas may be in the form of clouds containing both H I and H II regions or of radiative shocks propagating in the interstellar medium; in the latter case the gas should be detectable in H-alpha emission.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 220
    Format: text
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  • 5
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    The early evolution of supernova remnants in a homogeneous medium - The effects of electron thermal conduction (1977)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 215
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  • 6
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    The evaporation of spherical clouds in a hot gas. II - Effects of radiation (1977)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The effects of radiation on the evaporation of spherical clouds in a hot medium are considered. The critical cloud radius at which radiative losses balance conductive heating is determined as a function of the external temperature and density. Smaller clouds evaporate, and larger clouds condense. The conditions under which the surfaces of the clouds may be detected are discussed. Net radiative losses for evaporating clouds are calculated, and an effective cooling function for a cloudy medium is obtained. The results may be applied to clouds in supernova remnants, in the interstellar medium, and in clusters of galaxies.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 215
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  • 7
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    An investigation of the stability of the Bondi-Hoyle model of accretion flow (1977)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The Bondi-Hoyle-Lyttleton (1944) accretion model is considered which involves accretion onto a massive body moving at a high Mach number with respect to the ambient medium and the production of a high-density accretion column along the axis where particle orbits intersect. The stability of steady-state solutions with respect to short-wavelength perturbations is analyzed using the WKB approximation, and the accretion column is shown to be unstable toward such perturbations. It is noted that this instability is not affected by the position of the stagnation point in the steady-state solution.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society; vol. 180
    Format: text
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  • 8
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    O VI absorption in interstellar cloud surfaces (1979)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The velocity profiles of O VI absorption lines of 24 stars, observed in early Copernicus surveys, have been compared with the line profiles of Si III (1206.51 A) and N II (1083.99 A). The velocity structures of the O VI lines appear to be correlated with those of the material in the lower ionization stages. It is argued that the O VI absorption arises in the coronal gas of the conductive interface between hot gas, responsible for extended, soft X-ray emission, and cooler interstellar clouds. The velocity broadening of both sets of lines is attributed to motions of the cloud surfaces induced by pressure fluctuations in the interstellar medium.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 232
    Format: text
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  • 9
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    Refractory grain destruction in low-velocity shocks (1978)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Recent abundance determinations in intermediate-velocity clouds near the Orion association have shown that silicon abundances may be nearly cosmic even in clouds with a velocity as low as 40 to 50 km/sec. It is argued that grain destruction in radiative shocks traveling perpendicular to the magnetic field may be much more effective than was previously considered. The destruction mechanism consists of acceleration of the gyromotion of the grains about their drift center owing to magnetic field compression, followed by sputtering of the high-velocity grains by He and (CNO) atoms. It is found that MgSiO3 grains may be almost fully destroyed in shocks of velocity greater than 80 km/sec and graphite grains substantially destroyed at shock velocities of 100 km/sec.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 225
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  • 10
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    Time-dependent spherically symmetric accretion onto compact X-ray sources (1978)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Analytical arguments and a numerical hydrodynamic code are used to investigate spherically symmetric accretion onto a compact object, in an attempt to provide some insight into gas flows heated by an outgoing X-ray flux. It is shown that preheating of spherically symmetric accretion flows by energetic radiation from an X-ray source results in time-dependent behavior for a much wider range of source parameters than was determined previously and that there are two distinct types of instability. The results are compared with observations of X-ray bursters and transients as well as with theories on quasars and active galactic nuclei that involve quasi-spherically symmetric accretion onto massive black holes. Models based on spherically symmetric accretion are found to be inconsistent with observations of bursters and transients.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 226
    Format: text
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