A Closer Look at Some Gas-phase Depletions in the ISM: Trends for O, Ge, and Kr versus , f(H₂), 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 H 2 f (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 (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 α-process ones.

Large Metallicity Variations in the Galactic Interstellar Medium

“…In fact, we observe sometimes large discrepancies between the observed abundances of the highly volatile elements (e.g. O, Kr, C, N) taken from the literature ( [8], [48], see Extended Data Table 6) on one hand and on the other hand the observed abundances of the mildly volatile elements (e.g. Zn) and the refractory elements, which is hard to interpret with the classic knowledge of dust depletion.…”

“…Extended Data Table 6: Column densities of the volatile elements. These values are taken from [8] and [48].…”

The Origin of Galactic Cosmic Rays as Revealed by their Composition

“…Germanium. The gas-phase depletion of Ge was recently measured by both Ritchey et al (2018) and Jenkins (2019), and we obtained from these measurements 𝑓 𝑑 = (67 ± 21)% and (65 ± 20)%, respectively. We take the weighted mean value of these results in our calculations: 𝑓 𝑑 = (66 ± 14)%.…”

“…Argon and Krypton. The abundances of Kr in the cool gas-phase reported in Ritchey et al (2018) and Jenkins (2019) exhibit some depletions with respect to solar values, although the origin of these subsolar abundances remains unclear. Depletions of Ar were also reported, but Sofia & Jenkins (1998) showed that the measurements of the Ar abundance from its neutral form are not always reliable.…”

“…As these elements are expected to be in the same proportions in all phases of the ISM, we estimated the mean fraction of each element contained in ISM dust considering the same elemental composition for dust in the various phases of the ISM. The mean elemental fractions in ISM dust are obtained from the large data set of gas-phase abundances reported in Jenkins (2009Jenkins ( , 2019 and Ritchey et al (2018), with the help of the interstellar dust modelling framework THEMIS (Jones et al 2017) and some general properties of primitive interplanetary dust. We now discuss our results for each element.…”