A Closer Look at Some Gas-phase Depletions in the ISM: Trends for O, Ge, and Kr versus {F}_{* }, f(H 2 ), and Starlight Intensity

Abstract

In a survey of archived ultraviolet spectra of 100 stars recorded by the echelle spectrograph of the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST), we measure the strengths of the weak absorption features of O I, Ge II and Kr I in the interstellar medium. Our objective is to undertake an investigation that goes beyond earlier abundance studies to see how these elements are influenced independently by three different environmental properties: (1) values of a generalized atomic depletion factor $F_*$ due to condensations onto dust grains (revealed here by the abundances of Mg and Mn relative to H), (2) the fraction of H atoms in the form of molecular hydrogen, $f({\\rm H}_2)$, and (3) the ambient intensity $I$ of ultraviolet starlight relative to an average value in our part of the Galaxy $I_0$. As expected, the gas-phase abundances of all three elements exhibit negative partial correlations with $F_*$. The abundances of free O atoms show significant positive partial correlations with log $f({\\rm H}_2)$ and $\\log (I/I_0)$, while Ge and Kr exhibit negative partial correlations with $\\log (I/I_0)$ at marginal levels of significance. After correcting for these trends, the abundances of O relative to H show no significant variations with location, except for the already-known radial gradient of light-element abundances in the Milky Way. A comparison of Ge and O abundances revealed no significant regional enhancements or deficiencies of neutron-capture elements relative to alpha-process ones.