The Homogeneity of Interstellar Krypton in the Galactic Disk

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 FarUltraviolet 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. Subject headings: ISM: abundances — ultraviolet: ISM Based on observations with the NASA/ESA Hubble Space Telescope (HST) and the NASA-CNES-CSA