On the nature of the intercloud medium

Details On the nature of the intercloud medium On the nature of the intercloud medium

Jun 1975

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975apj...198..299h&link_type=abstract

Astrophysical Journal, vol. 198, June 1, 1975, pt. 1, p. 299-306. Research supported by the Alfred P. Sloan Foundation;

Astronomy and Astrophysics

Astrophysics

22

Astronomical Models, Interstellar Extinction, Nebulae, Stellar Spectra, Abundance, Gas Ionization, Hydrogen Clouds, Line Spectra, Steady State, X Rays

Scientific paper

Previous analyses of Copernicus observations of interstellar absorption in the spectra of unreddened stars are reviewed in the framework of steady-state ionization models for the intercloud (low-density) component of the interstellar medium. A simple homogeneous model is described which accounts for the H, C, N, S, and Si interstellar lines in the spectrum of lambda Sco in terms of an extensive low-density medium photoionized by Lyman continuum radiation from the II Sco OB association. Quantitative constraints are placed on the maximum permissible flux of diffuse soft X-rays, and the model is generalized to interpret the observed ion, electron, and hydrogen column densities in the low-density medium. Severe constraints are found to be imposed on ionization models by the ratios Si III/Si, Si IV/Si, and S IV/S. It is concluded that the observations may be explained if 10 to 30% of a representative line of sight passes through a photoionized medium of density 0.1 to 0.2/cu cm, if H I constitutes the bulk of the gas along the line of site, and if the gas is at a similar density or is in the form of shielded clumps or clouds.

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