Other
Scientific paper
May 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...427..274d&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 427, no. 1, p. 274-287
Other
146
Atmospheric Density, B Stars, Data Processing, Interplanetary Dust, Interstellar Gas, Interstellar Matter, Lyman Alpha Radiation, Ultraviolet Spectra, Absorption Spectra, Astronomical Observatories, Emission Spectra, Iue, Line Of Sight, Oao 3, Spectral Resolution
Scientific paper
We present an analysis of interstellar neutral hydrogen column densities obtained from archival Ly alpha absorption line data toward the complete sample of B2 and hotter stars observed at high spectral resolution with the IUE satellite. The full sample includes 554 stars and more than doubles the number of lines of sight previously observed. Many of the B2 and B1.5 stars exhibit contamination from stellar Ly alpha absorption, and were excluded from the subsequent analysis. The final working sample includes 393 stars. We present statistical averages for a number of quantities derived for the sample. The stars range in distance from 0.12 to 11 kpc with an average distance of 2.1 kpc. Values for average sight-line density range from 0.017 to 8.62 atoms cm-3, with an overall average of 0.23 atoms cm-3. Neutral hydrogen and dust are well correlated, with N(H I)/E(B - V) = 4.93 x 1021 cm-2 mag-1, and N(H I)/E(Bump) = 2.17 x 1021 cm-2 mag-1. There is an increase in the H I to dust ratios for the densest sight lines. Evidently, the H I to dust ratios in dense clouds are higher than in the lower density medium between the clouds and in interarm directions. The effect is probably an indication of dust modification in the dense clouds. The H I to dust ratios are approx. 17% smaller for the sight lines to O stars compared to the ratios found toward B stars. We have identified sight lines with anomalously large and small values of N(H I)/E(B - V) and H(H I)/E(Bump). Some of these sight lines are well-known examples of peculiar extinction (i.e., HD 147933 and HD 37061 (NU Ori)). Others are less well known and may provide important new examples of regions with highly modified dust. The number of stars with anomalously low H I to dust ratios is disproportionately larger than the number of stars with anomalously high H I to dust ratios. The distribution of the gas away form the Galactic plane for the sample of objects is complex. For the subset of 375 stars whose lines of sight have a statistically low probablility of intersecting a large cloud, the distribution may be roughly approximated by an exponential density distribution, with a midplane density of 0.366 atoms cm-3, a scale height of 195 pc, and a random logarithmic scattering parameter sigma p of 0.159 dex.
Diplas Athanassios
Savage Blair. D.
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