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

TL;DR

This paper reports the first detection of interstellar chloronium (H₂Cl⁺) using Herschel/HIFI, revealing higher-than-expected abundances and providing new insights into interstellar chemistry and the capabilities of space-based spectroscopy.

Contribution

First detection of interstellar chloronium (H₂Cl⁺) with Herschel/HIFI, offering new data to refine astrochemical models of interstellar molecules.

Findings

H₂Cl⁺ detected in absorption towards NGC 6334I and Sgr B2(S)

H₂Cl⁺ column densities exceed model predictions

HCl/H₂Cl⁺ ratios are consistent with PDR models

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.