Observations of Absorption Lines from Highly Ionized Atoms

Abstract

Well before space-borne instruments were launched to observe ultraviolet absorption lines from the interstellar medium, it was suggested that features from highly ionized atoms might provide a valuable insight on the magnitude and character of ionizing and heating processes which were responsible for the observed physical states of gases in space. Early proposals centered on the notion that primarily neutral gas (HI regions) occupied most of the space, and that low energy cosmic ray particles or X-rays were responsible for the observed temperature and small traces of ionization (Pikel’ner, 1967; Spitzer, 1968; Spitzer and Tomasko, 1968; Field et al., 1969; Spitzer and Scott, 1969, Goldsmith et al., 1969; Silk and Werner, 1969; Dalgarno and McCray, 1972). Since low energy cosmic rays are shielded from us by interplanetary magnetic fields and low energy X-rays were difficult to measure with existing experiments, we could not measure the strength of these proposed sources of heating and partial ionization. Instead, it was suggested that the fluxes could be ascertained indirectly by observing the interstellar abundances of some highly ionized forms of particular heavy elements, such as C III, C IV, N III, N V, Si III, Si IV, S III, and S IV, all which had strong resonance lines in the ultraviolet (Silk, 1970; Silk and Brown, 1971; Weisheit, 1973).

This article is an updated version of one which appeared in Local Interstellar Medium (IAU Colloquium 81), Y. Kondo, F. C. Bruhweiler, and B. D. Savage (eds.), NASA CP-2345, p. 155 (1984). Much new material has been added in the text which follows Section 3.