Chlorine Chemistry in the Interstellar Medium

Prior to the launch of Herschel, HCl was the only chlorine-containing molecule detected in the interstellar medium (ISM). However, chemical models have identified chloronium, H2Cl, as a relatively abundant species that is potentially detectable. H2Cl was predicted to be most abundant in the environments where the ultraviolet radiation is strong: in diffuse clouds, or near the surfaces of dense clouds illuminated by nearby O and B stars. The HIFI instrument on Herschel provided the first detection of H2Cl in the ISM. The 212−101 lines of ortho-H2Cl and ortho-H2Cl were detected in absorption toward NGC 6334I, and the 111−000 transition of para-H2Cl was detected in absorption toward NGC 6334I and Sgr B2(S). The derived HCl/H2Cl column density ratios, ∼1–10, are within the range predicted by models of diffuse and dense Photon Dominated Regions (PDRs). However, the observed H2Cl column densities, in excess of 10 cm, are significantly higher than the model predictions. These observations demonstrate the outstanding spectroscopic capabilities of HIFI for detecting new interstellar molecules and providing key constraints for astrochemical models.