First detection of Hydrogen Chloride towards protostellar shocks

TL;DR

This study reports the first detection of hydrogen chloride in a protostellar shock, providing insights into its abundance, formation, and the shock's physical conditions, challenging previous chemical models of chlorine reservoirs.

Contribution

It presents the first detection and analysis of HCl in a protostellar shock, constraining its abundance and formation mechanisms using Herschel observations and astrochemical modeling.

Findings

HCl detected with a column density of 0.7-2 x 10^13 cm^-2.

Gas temperature > 15 K and density > 3 x 10^5 cm^-3 in the shock region.

HCl abundance is consistent with previous protostellar observations, but challenges existing chemical models.

Abstract

We present the first detection of hydrogen chlorine in a protostellar shock, by observing the fundamental transition at 626 GHz with the Herschel HIFI spectrometer. We detected two of the three hyperfine lines, from which we derived a line opacity < 1. Using a non-LTE LVG code, we constrained the HCl column density, temperature and density of the emitting gas. The hypothesis that the emission originates in the molecular cloud is ruled out, as it would imply a too dense gas. Conversely, assuming that the emission originates in the 10"-15" size shocked gas previously observed at the IRAM PdB interferometer, we obtain: N(HCl)=0.7-2 x 10(13) cm-2, temperature > 15 K and density > 3 x 10(5) cm-3}. Combining with the Herschel HIFI CO(5-4) observations allows to further constrain the gas density and temperature, 10(5)-10(6) cm-3 and 120-250 K, as well as the HCl column density, 2 x 10(13) cm-2, and, finally, abundance: 3-6 x 10(-9). The estimated HCl abundance is consistent with that previously observed in low- and high- mass protostars. This puzzling result in the L1157-B1 shock, where species from volatile and refractory grains components are enhanced, suggests either that HCl is not the main reservoir of chlorine in the gas phase, against previous chemical models predictions, or that the elemental chlorine abundance is low in L1157-B1. Astrochemical modelling suggests that HCl is in fact formed in the gas phase, at low temperatures, prior to the occurance of the shock, and that the latter does not enhance its abundance.