The homogeneity of interstellar krypton in the Galactic disk

We present an analysis of high-resolution Hubble Space Telescope 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) and over 2 orders of magnitude in 〈n _H〉 and extend 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 data-point 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_-0.11 ^+0.09). Although these deviations are only measured at the 2 σ level, they suggest the possibility that krypton abundances beyond the Orion spur may differ from the local value.