Nature of the intercloud medium

Earlier interpretation of the Copernicus observations of interstellar absorption lines in the spectra of un reddened stars are reviewed in the context of steady-state ionization models of the low-density (intercloud) component of the interstellar medium. Except for the highest ionization states (which may be produced in the immediative vicinity of the central star), a simple homogeneous model is described that accounts satisfactorily for the interstellar lines of H, C, N, S, and Si observed in the spectrum of lambda Sco in terms of an extensive low-density medium that is photoionized by Lyman continum radiation from the II Sco OB association. Quantitative constraints are given on the maximum permissible flux of diffuse soft X-radiation. This model is generalized to provide an interpretation of the observed ion, electron, and hydrogen column densities in the low-density interstellar medium. It is that severe constraints on ionization models are imposed by the ratios Si(III)/Si, Si(IV)/Si, and S(IV)/S. We conclude that the observations can be adequately explained if between 10 and 30 percent of a representative line of sight passes through a photoionized medium of density 0.1--0.2 cm^-3; the bulk of the gas along the line of sight is H(I), and can either be at a similar density, in which case the mean primary soft x-ray ionization rate cannot exceed 1 x 10^-18 s^-1 (H atom)^-1, or else it must be in shielded clumps or clouds.