Herschel/HIFI discovery of interstellar chloronium (H${}_2$Cl${}^{+}$)

Abstract

We report the first detection of chloronium, H${}_2$Cl${}^{+}$, in the interstellar medium, using the HIFI instrument aboard the \emph{Herschel} Space Observatory. The $2_{12}-1_{01}$ lines of ortho-H${}_2^{35}$Cl${}^{+}$ and ortho-H${}_2^{37}$Cl${}^{+}$ are detected in absorption towards NGC~6334I, and the $1_{11}-0_{00}$ transition of para-H${}_2^{35}$Cl${}^{+}$ is detected in absorption towards NGC~6334I and Sgr~B2(S). The H${}_2$Cl${}^{+}$ column densities are compared to those of the chemically-related species HCl. The derived HCl/H${}_2$Cl${}^{+}$ column density ratios, $\sim$1--10, are within the range predicted by models of diffuse and dense Photon Dominated Regions (PDRs). However, the observed H${}_2$Cl${}^{+}$ column densities, in excess of $10^{13}$~cm${}^{-2}$, 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.

Accepted for publication in A&A, HIFI Special Issue; 4 pages, 2 figures, 1 table