HST STIS Observations of Interstellar Chlorine

Among the dominant ions of abundant elements in the diffuse interstellar medium, only chlorine (Cl II) has a rapid exothermic reaction with molecular hydrogen (H₂) that should lead to the dominance of its atomic form (Cl I) in clouds where most of the hydrogen is in H_2. We present the results of an archival study of the interstellar Cl I λ1347.24 absorption observed at high spectral resolution toward 41 stars with the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST). Our key goals in this survey are to explore the relationship between interstellar N(Cl I) and N(H₂) with a larger sample and a larger N(H₂) range (16.44 < log N(H₂) < 20.87) than the Copernicus interstellar survey of Moomey et al. (2012). We additionally contrast it with the high-z QSO damped Lyman-alpha system (DLA) findings of Balashev et al. (2015). We find that for log N(H₂) > 19.0, the HST STIS sample is consistent with the Copernicus data and high-z DLA samples in indicating a linear trend of increasing N(Cl I) with increasing N(H₂). Furthermore, all of the interstellar sightlines with log f(H₂) > -0.5 have log N(Cl I) > 13.5, and those with log f(H₂) < -1.5 have log N(Cl I) < 13.5, where f(H₂)=2N(H₂)/[2N(H₂)+N(H I)] is the fractional amount of H₂ in H. Consequently, observations of interstellar Cl I can potentially trace the H₂ fraction of the “CO-dark” gas marking the transition between diffuse atomic and dense CO molecular clouds.