Herschel/HIFI line surveys: Discovery of interstellar chloronium (H2Cl+)

Abstract

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-H235Cl+ and ortho-H237Cl+ were detected in absorption toward NGC 6334I, and the 111-000 transition of para-H235Cl+ 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 1013 cm−2, 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.