<i>Herschel</i>/HIFI discovery of interstellar chloronium (H₂Cl⁺)

Abstract

We report the first detection of chloronium, H<sub>2<sub/>Cl<sup>+<sup/>, in the interstellar medium, using the HIFI instrument aboard the <i>Herschel<i/> Space Observatory. The 2<sub>12<sub/>–1<sub>01<sub/> lines of ortho-HCl<sup>+<sup/> and ortho-HCl<sup>+<sup/> are detected in absorption towards NGC 6334I, and the 1<sub>11<sub/>–0<sub>00<sub/> transition of para-HCl<sup>+<sup/> is detected in absorption towards NGC 6334I and Sgr B2(S). The H<sub>2<sub/>Cl<sup>+<sup/> column densities are compared to those of the chemically-related species HCl. The derived HCl/H<sub>2<sub/>Cl<sup>+<sup/> column density ratios, ~1–10, are within the range predicted by models of diffuse and dense photon dominated regions (PDRs). However, the observed H<sub>2<sub/>Cl<sup>+<sup/> column densities, in excess of 10<sup>13<sup/> cm<sup>-2<sup/>, are significantly higher than the model predictions. Our observations demonstrate the outstanding spectroscopic capabilities of HIFI for detecting new interstellar molecules and providing key constraints for astrochemical models.