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  • Other Sources  (21)
  • 2005-2009
  • 1985-1989  (9)
  • 1975-1979  (12)
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  • 1
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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
    Format: application/pdf
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  • 2
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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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  • 3
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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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  • 4
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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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  • 5
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    Model emission spectra from primeval galaxies (1979)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The paper computes models of the UV luminosity produced by a distribution of supernova remnants resulting from a burst of star formation in primeval galaxies at large redshift. The evolution of low metal abundance remnants is described analytically, and their UV emission is found from a velocity grid of radiative shock models. The implications of these results for the detection of line emission from young galaxies and for the diffuse background radiation are discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 234
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  • 6
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    Copernicus observations of interstellar matter toward the Orion OB1 association. I - Epsilon and Pi-5 Orionis (1979)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Copernicus UV data on interstellar lines toward Epsilon Ori and Pi-5 Ori are analyzed to study abundances and physical conditions in both low- and intermediate-velocity components. Clouds at -8 and +5 km/s (LSR) toward Epsilon Ori show typical depletions of Fe, Ti, Mg, and Si in dense (H number density about 100 per cu cm) gas. Low-column-density intermediate-velocity clouds toward both stars, with low densities (hydrogen number density less than 1 per cu cm) and near-cosmic Si abundances, are consistent with a widespread pattern of high-velocity gas over a 15-deg area surrounding the Orion region. Such activity may be attributed to the repeated action of supernovae in a patchy low-density region of interstellar gas.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 233
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  • 7
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    The unexpected ultraviolet variability of Herbig-Haro object 1 (1985)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The line fluxes of the C IV 1550 A and semiforbidden C III 1909 A emission lines in HH 1 are noted to have monotonically decreased by factors of 4-6 between 1979 and 1983, despite a lack of indications of optical range changes. The thickness of the shocked layers and preshock density can be estimated on the basis of these rapid changes, with results suggesting a clumpy medium that leads to truncated shock waves with column densities insufficient for the development of complete recombination zones.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X); 292; L75-L78
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  • 8
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    The extent of the local hi halo (1985)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Forty-five high-latitude, OB stars have been observed in the Ly alpha and 21 cm lines of HI in an effort to map out the vertical distribution and extent of the local HI halo. The 25 stars for which a reliable HI colum density can be obtained from Ly alpha lie between 60 and 3100 pc from the plane. The principal result is that the total column density of HI at z 1 kpc is, on the average, 5 + or - 3 x 10 the 19th power/sq cm, or 15% of the total sub HI. At relatively low z the data toward some stars suggest a low effective scale height and fairly high average foreground density, while toward others the effective scale height is large and the average density is low. This can be understood as the result of irregularities in the interstellar medium. A model with half of the HI mass in clouds having radii of a few pc and a Gaussian vertical distribution with sigma sub 2 = 135 pc, and half of the mass in an exponential component with a scale height of 500 pc, gives a satisfactory fit to the data. The technique of comparing Ly alpha and 21 cm column densities is also used to discuss the problem of estimating the distance to several possibly subluminous stars.
    Keywords: ASTRONOMY
    Type: NASA-CR-176115 , NAS 1.26:176115
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  • 9
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    Thermal phases of interstellar and quasar gas (1985)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Interstellar gas may be in a variety of thermal phases, depending on how it is heated and ionized; here a unified picture of the equation of state of interstellar and quasar gas is presented for a variety of such mechanisms over a broad range of temperatures, densities, and column densities of absorbing matter. It is found that for select ranges of gas pressure, photoionizing flux, and heating, three thermally stable phases are allowed: coronal gas (T above 100,000 K); warm gas (T about 10,000 K); and cold gas (T less than 100 K). With attenuation of ultraviolet and X-ray radiation, the cold phase may undergo a transition to molecules. In quasar broad-line clouds, this transition occurs at column density N(H) = about 10 to the 23rd/sq cm and could result in warm molecular cores and observable emission from H2 and OH. The underlying atomic physics behind each of these phase transitions and their relevance to interstellar matter and quasars are discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 288; 58-64
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  • 10
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    Vibrational excitation of H2 in intense ultraviolet fluxes (1978)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The vibrational excitation of H2 in intense ultraviolet radiation fields is studied. The radiative cascade through bound levels of the ground electronic state, following absorption of Lyman- and Werner-band photons, is modified to include effects of 'multiple pumping' and direct photodissociation from the high vibrational levels. Although the column densities in the excited vibrational levels produced by UV pumping appear too low to explain the recently observed Orion quadrupole emission lines, it is suggested that the far-red lines in the (3-0) and (4-0) bands, produced by UV pumping from O stars near dense molecular clouds, may be detectable.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 219
    Format: text
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