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  • ASTROPHYSICS  (20)
  • Astrophysics
  • 1985-1989  (8)
  • 1980-1984  (12)
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  • 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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    IUE absorption studies of broad- and narrow-line gas in Seyfert galaxies (1986)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: The interstellar medium of a galaxy containing an active nucleus may be profoundly affected by the high energy (X-ray, EUV) continuum flux emanating from the central source. The energetic source may photoionize the interstellar medium out to several kiloparsecs, thereby creating a global H II region. The International Ultraviolet Explorer (IUE) satellite has attempted to observe in several Seyfert galaxies (NGC 3516, NGC 4151, NGC 1068, 3C 120) the narrow absorption lines expected from such global H II regions. Instead, in two of the galaxies (NGC 3516, NGC 4151) broad, variable absorption lines at C IV lambda 1550, N V lambda 1240, and Si IV lambda 1400 were found, as well as weaker absorption features at O I lambda 1302 and C II lambda 1335. These features swamp any possible global H II region absorption. Such broad absorption features have previously been observed in IUE data, but their origin is still not well understood.
    Keywords: ASTROPHYSICS
    Type: NASA. Ames Research Center Summer School on Interstellar Processes: Abstracts of Contributed Papers; p 77-78
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  • 3
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    Shock processing of interstellar grains (1986)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: Shock processing plays an important role in the life of a typical interstellar grain. Shocks of 100 km/s-l or greater can destroy about 50% of the grain material under appropriate preshock conditions of density and magnetic field. The destruction occurs by grain-grain collisions and nonthermal sputtering for steady state radiative shocks and by thermal sputtering for fast adiabatic shocks. The evaluation of the lifetime of grains against shock destruction depends on models of the interstellar medium (ISM) structure and on supernova remnants (SNR) evolution. Results from various authors give lifetimes between 10 to the 8th and 10 to the 9th power years, compared to typical injection times for new grains of a few times 10 to the 9th power years. These numbers require that a major portion of the interstellar silicon bearing grain material must be formed by grain growth in the ISM. At the same time, the presence of isotopic anomalies in some meteorites implies that at least some grains must survive from their formation in SNRs or red giant winds through incorporation into the solar system.
    Keywords: ASTROPHYSICS
    Type: NASA, Washington. Interrelationships among Circumstellar, Interstellar and Interplanetary Dust; p 37-54
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  • 4
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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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  • 5
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    IUE/IRAS studies of metal abundances and infrared cirrus (1986)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: A survey is reported of interstellar densities, abundances, and cloud structure in the Galaxy, using the IUE and IRAS satellites. Heavy element depletions are discussed along with their correlations with mean density, reddening, and galactic location. Interesting correlations between the Fe/Si abundance ratio and the infrared diffuse cirrus is also reported, which may provide information on the history and formation of grains in the galactic halo.
    Keywords: ASTROPHYSICS
    Type: NASA. Ames Research Center Summer School on Interstellar Processes: Abstracts of Contributed Papers; p 163-164
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  • 6
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    Galactic interstellar abundance surveys with IUE. I - Neutral hydrogen (1985)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: A survey is initiated of interstellar densities, abundances, and cloud structure in the Galaxy, using the International Ultraviolet Explorer. From high-resolution (0.1-A) spectra, interstellar column densities are derived toward 244 early-type stars by fitting the damping wings of the Ly-alpha absorption line. Published B-V photometry is used to derive E(B-V) color excesses, stellar distances r, and the mean hydrogen densities, n-bar = N(H I)/r. The data set includes stars out to r = 8.5 kpc and E(B-V) = 0.86, with 68 stars in the galactic halo (absolute b greater than 20 deg). The statistical sample consists of 205 stars of type O3-B2.5 within 5 kpc (53 stars in the halo). The column densities, log N(H I), range from less than 19.44 per square centimeter for alpha Vir to 21.81 for HD 37061. A statistical analysis of the E(B-V) distribution suggests variations in the line-of-sight cloud density and the mean reddening per cloud.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 294; 599-614
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  • 7
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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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  • 8
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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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  • 9
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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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  • 10
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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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