Interstellar Atomic Abundances

Abstract

A broad array of interstellar absorption features that appear in the ultraviolet spectra of bright sources allows us to measure the abundances and ionization states of many important heavy elements that exist as free atoms in the interstellar medium. By comparing these abundances with reference values in the Sun, we find that some elements have abundances relative to hydrogen that are approximately consistent with their respective solar values, while others are depleted by factors that range from a few up to around 1000. These depletions are caused by the atoms condensing into solid form onto dust grains. Their strengths are governed by the volatility of compounds that are produced, together with the densities and velocities of the gas clouds. We may characterize the depletion trends in terms of a limited set of parameters; ones derived here are based on measurements of 15 elements toward 144 stars with known values of N(H i) and N(H₂). In turn, these parameters may be applied to studies of the production, destruction, and composition of the dust grains. The interpretations must be done with care, however, since in some cases deviations from the classical assumptions about missing atoms in unseen ionization stages can create significant errors. Our experience with the disk of our Galaxy offers important lessons for properly unravelling results for more distant systems, such as high- and intermediate-velocity clouds in the Galactic halo, material in the Magellanic Stream and Magellanic Clouds, and otherwise invisible gas systems at large redshifts (detected by absorption features in quasar spectra). Inferences about the total (gas plus dust) abundances of such systems offer meaningful information on their origins and/or chemical evolution.