The Homogeneity of Interstellar Krypton in the Galactic Disk

Abstract

We present an analysis of high resolution HST Space Telescope Imaging Spectrograph (STIS) observations of Kr I λ1236 absorption in seven sight lines that probe a variety of interstellar environments. In combination with krypton and hydrogen column densities derived from current and archival STIS and Far-Ultraviolet Spectroscopic Explorer data, the number of sight lines with reliable Kr/H ISM abundance ratios has been increased by 50 % to 26—including paths that sample a range of nearly 5 orders of magnitude in f(H2), over 2 orders of magnitude in 〈nH〉, and extending up to 4.8 kpc in length. For sight lines contained entirely within the local spiral arm (the Orion Spur), the spread of Kr/H ratios about the mean of log10[N(Kr)/N(H)]ISM = −9.02 ± 0.02 is remarkably tight (0.06 dex), less than the typical datapoint uncertainty. Intriguingly, the only two sight lines that extend through neighboring structures, in particular gas associated with the Carina/Sagittarius Arm, exhibit relatively large, near-solar krypton abundances (log10[N(Kr)/N(H)]combined = −8.75 +0.09 −0.11). Although these deviations are only measured at the 2σ level, they suggest the possibility that krypton abundances beyond the Orion Spur may differ from the local value.