Hydrogen chloride in diffuse interstellar clouds along the line of sight to W31C (G10.6-0.4)

TL;DR

This paper reports the detection of hydrogen chloride in diffuse interstellar clouds along the line of sight to W31C, revealing higher-than-expected chlorine abundance and providing new insights into chlorine chemistry in diffuse gas.

Contribution

First detection of HCl in diffuse clouds along this line of sight, challenging existing chemical models with higher observed chlorine abundance.

Findings

HCl accounts for ~0.6% of gas-phase chlorine

HCl detection exceeds model predictions by a factor of ~6

Provides constraints on chlorine chemistry in diffuse interstellar gas

Abstract

We report the detection of hydrogen chloride, HCl, in diffuse molecular clouds on the line of sight towards the star-forming region W31C (G10.6-0.4). The J = 1-0 lines of the two stable HCl isotopologues, H35Cl and H37Cl, are observed using the 1b receiver of the Heterodyne Instrument for the Far-Infrared (HIFI) aboard the Herschel Space Observatory. The HCl line is detected in absorption, over a wide range of velocities associated with diffuse clouds along the line of sight to W31C. The analysis of the absorption strength yields a total HCl column density of few 10^13 cm^-2, implying that HCl accounts for ~0.6 % of the total gasphase chlorine, which exceeds by a factor of ~6 the theoretical model predictions. This result is comparable to those obtained from the chemically-related species H2Cl+ and HCl+, for which large column densities have also been reported on the same line of sight. The source of discrepancy between models and observations is still unknown; however, the detection of these Cl-bearing molecules, provides key constraints for the chlorine chemistry in the diffuse gas.