The Composition of the Diffuse Interstellar Medium

Open Access
Publisher Website
Google Scholar
Abstract
Recent Goddard High Resolution Spectrograph measurements of Si, S, Cr, Mn, Fe, and Zn in interstellar clouds along lines of sight in the Galactic disk and into the lower halo are discussed. The gas-phase abundance of S relative to H in the interstellar clouds appears to be indistinguishable from the solar value. For the other elements, we find well-defined upper limits in the gas-phase abundances at significantly subsolar values. For Fe, Mn, and Cr (and probably Ti), there are no convincing cases in which the relative gas-phase abundances exceed roughly -0.5 dex, i.e., these elements are not seen in interstellar gas with an abundance greater than about one-third solar. For Si, the limit is roughly -0.15 dex, and for Zn a constant abundance of -0.13 dex is found from seven clouds along one halo sight line. These subsolar maximum abundances have two possible interpretations: (1) they indicate the presence of an essentially indestructible component of interstellar dust, which contains about two-thirds of the Ti, Mn, Cr, and Fe and about one-third of the Si (based on solar composition), or (2) they indicate that the true total abundances of these elements are substantially less than in the Sun.