Herschel/HIFI Line Surveys: Discovery of Interstellar Chloronium (H_2Cl~+)

摘要

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, H_2Cl~+, as a relatively abundant species that is potentially detectable. H_2Cl~+ 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 H_2Cl~+ in the ISM. The 2_(12)?1_(01) lines of ortho-H_2~(35)Cl~+ and ortho-H_2~(37)Cl~+ were detected in absorption toward NGC 6334I, and the 1_(11)?0_(00) transition of para-H_2~(35)Cl~+ was detected in absorption toward NGC 6334I and Sgr B2(S). The derived HCl/H_2Cl~+ column density ratios, ～1–10, are within the range predicted by models of diffuse and dense Photon Dominated Regions (PDRs). However, the observed H_2Cl~+ column densities, in excess of 10~(13) 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.