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  • ASTROPHYSICS  (12)
  • Astrophysics
  • 1980-1984  (12)
Collection
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Year
  • 1
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    Grain destruction in interstellar shocks (1984)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: The destruction of interstellar grains by nonthermal sputtering, thermal sputtering, and grain-grain collisions is discussed. It is concluded that large grains are easily destroyed by shocks in the interstellar medium, but that small grains are much more persistent. Since the MRN model has many more small grains than large ones, this means that the total number of grains is not significantly reduced even when half or more of the grain material is returned to the gas phase. Thus, the small grains are always available as condensation cores for mantle formation or redepletion of refractory grain materials.
    Keywords: ASTROPHYSICS
    Type: Royal Observatory Lab. and Observational Infrared Spectra of Interstellar Dust; p 26-32
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  • 2
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    Radio and UV Observations of High Latitude HI (1984)
    In:  CASI
    Details
    Publication Date: 2014-09-27
    Description: Interstellar Lyman alpha observations toward high latitude stars, which give the column density of H I between Earth and the star, and 21 cm radio observations, which give the total H I column density to and beyond the star, can be combined to map out the amount of neutral gas as a function of distance from the Sun. Observations of the 21 cm and Lyman alpha lines made toward approximately 50 high latitude OB stars are presented. These data are used to discuss the distribution of H I in the solar neighborhood.
    Keywords: ASTROPHYSICS
    Type: NASA. Goddard Space Flight Center Local Interstellar Medium, No. 81; p 290
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  • 3
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    Shock processing of interstellar grains (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Theoretical and observational evidence is presented that shock processing of interstellar dust grains by supernova blast waves affects both heavy element depletions and ultraviolet extinction curves. By coupling a realistic model of grain sizes and populations with a radiative shock code, significant grain destruction at velocities as low as 40 km/s is demonstrated. Nonthermal sputtering and grain-grain collisions destroy relatively more large grains than small, and more silicates than graphite. Consequently, both the 2175 A extinction 'bump' and the far-ultraviolet normalized extinction are increased in strength. Ultraviolet extinction studies with the International Ultraviolet Explorer of nine stars near three supernova remnants (the Monoceros Loop, Shajn 147, and Vela) exhibit strong 2175 A bumps and normal or high far-ultraviolet extinction. Diffuse bands, if they are created by small grains, should show little correlation with such activity.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 275; 652-660
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  • 4
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    The X-ray spectrum of a hot interstellar plasma (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The soft X-ray spectrum of a hot, low density, optically thin plasma in collisional ionization equilibrium is calculated using current atomic data. Many Fe collision strengths differ from previous calculations. The spectra from individual elements are presented for use in abundance determinations from X-ray observations. Collision strengths and line energies, sorted both according to energy and according to ion, are tabulated.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal Supplement Series; 46; May 1981
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  • 5
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    Intergalactic shells at large redshift (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The intergalactic shells produced by galactic explosions at large redshift, whose interiors cool by inverse Compton scattering off the cosmic background radiation, have a characteristic angular size of about 1 arcmin at peak brightness. At z values lower than 2, the shells typically have a radius of 0.5 Mpc, a velocity of about 50 km/sec, a metal abundance of about 0.0001 of cosmic values, and strong radiation in H I(Lyman-alpha), He II 304 A, and the IR fine-structure lines of C II and Si II. The predicted extragalactic background emission from many shells, strongly peaked toward the UV, sets an upper limit to the number of exploding sources at z values of about 10. Shell absorption lines of H I, C II, Si II, and Fe II, which may be seen at more recent epochs in quasar spectra, may probe otherwise invisible explosions in the early universe.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 249
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  • 6
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    The ultraviolet spectrum of Herbig-Haro object 2H (1982)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: IUE spectra of Herbig-Haro object 2H are presented. The spectra show a strong 'excess' UV continuum and prominent emission lines of C, N, O, Si, Mg, and possibly Al. The continuum, F(lambda), exhibits a turnover shortward of about 1450 A, confirming for the first time the H0 two-photon nature of the emission source. A possible absorption feature near 1680 A, which could result from a new grain or molecular constituent in these protostellar objects is also noted. Recently computed models of steady shocks into partially ionized gas reproduce the two-photon spectral shape, but its observed intensity relative to H-beta and the Balmer continuum is anomalously high. It is suggested that a range of shock velocities, 70-100 km/s, or nonsteady, 'truncated' shocks may be responsible. Future high-sensitivity UV observations of HH objects may be used to probe grain extinction curves in star-forming regions.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 262
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  • 7
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    Observationally determined Fe II oscillator strengths (1983)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Absorption oscillator strengths for 21 Fe II resonance lines, have been determined using a curve-of-growth analysis of interstellar data from the Copernicus and International Ultraviolet Explorer (IUE) satellites. In addition to slight changes in strengths of the far-UV lines, new f-values are reported for wavelength 1608.45, a prominent line in interstellar and quasar absorption spectra, and for wavelength 2260.08, a weak, newly identified linen in IUE interstellar spectra. An upper limit on the strength of the undetected line at 2366.867 A (UV multiplet 2) is set. Using revised oscillator strengths, Fe II column densities toward 13 OB stars are derived. The interstellar depletions, (Fe/H), relative to solar values range between factors of 10 and 120.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 271; Aug. 1
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  • 8
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    Observationally determined silicon II oscillator strengths (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Si II oscillator strengths for eight resonance lines from the ground state have been determined using a curve-of-growth analysis of interstellar data from the Copernicus and International Ultraviolet Explorer satellites. The oscillator strengths for 1526 A and 1808 A increase by factors of 3.0 and 1.5, respectively, from previously tabulated values; those for 989 A and 1020 A decrease by factors of 0.55 and 0.60, respectively. These new values result in better silicon abundances for interstellar clouds and resolve a long-standing puzzle in the interpretation of quasar absorption line data.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 246
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  • 9
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    The ionization equilibrium of astrophysically abundant elements (1982)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The paper presents new calculations of ionization equilibrium fractions of 11 abundant elements (C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, Ni) as functions of temperature. Convenient coefficients for fitting the rates of collisional ionization, radiative recombination, and dielectronic recombination are also tabulated. Many of the ionization rates are based on recent experimental measurements of cross sections for collisional ionization and autoionization following inner-shell excitation. These rates are used elsewhere in computations of nonequilibrium ionization, radiative cooling, radiative shock models, and plasma emission diagnostics.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal Supplement Series; 48; Jan. 198
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  • 10
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    Interstellar molecular hydrogen (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Recent progress in the detection of H2 in the interstellar medium is reviewed. The hydrogen molecule is discussed in terms of properties, and collisional excitation and dissociation rates. Molecular hydrogen formation and cooling and heating by H2 are also discussed. Ultraviolet studies provided information for the study of the formation-destruction equilibrium of H2, the kinetic temperature of the cloud, the hydrogen particle density, and the ultraviolet radiation field. Infrared observations in Orion and the planetary nebula NGC 7027 reveal two detections of vibrationally excited H2. In addition, thermal and nonthermal excitation mechanisms are discussed. Finally, it is noted that increased ultraviolet sensitivity is necessary for thorough H2 studies.
    Keywords: ASTROPHYSICS
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