The Chemistry of Chlorine-bearing Species in the Diffuse Interstellar Medium, and New SOFIA/GREAT Observations of HCl+

We have revisited the chemistry of chlorine-bearing species in the diffuse interstellar medium with new observations of the HCl⁺ molecular ion and new astrochemical models. Using the GREAT instrument on board SOFIA, we observed the ²Π_3/2 J = 5/2 - 3/2 transition of HCl⁺ near 1444 GHz toward the bright THz continuum source W49N. We detected absorption by diffuse foreground gas unassociated with the background source, and were able to thereby measure the distribution of HCl⁺ along the sight line. We interpreted the observational data using an updated version of an astrochemical model used previously in a theoretical study of Cl-bearing interstellar molecules. The abundance of HCl⁺ was found to be almost constant relative to the related H₂Cl⁺ ion, but the observed n(H₂Cl⁺)/n(HCl⁺) abundance ratio exceeds the predictions of our astrochemical model by an order of magnitude. This discrepancy suggests that the rate of the primary destruction process for H₂Cl⁺, dissociative recombination, has been significantly overestimated. For HCl⁺, the model predictions can provide a satisfactory fit to the observed column densities along the W49N sight line while simultaneously accounting for the OH⁺ and H₂O⁺ column densities. * GREAT is a development by the MPI für Radioastronomie and the KOSMA/Universität zu Köln, in cooperation with the MPI für Sonnensystemforschung and the DLR Institut für Planetenforschung.