First Detection of Hydrogen Chloride Toward Protostellar Shocks

Abstract

We present the first detection of hydrogen chloride in a protostellar shock by observing the fundamental transition at 626 GHz with the HIFI spectrometer. We detected two of the three hyperfine lines from which we derived a line opacity <=1. Using a non-local thermodynamic equilibrium large velocity gradient 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 because it would imply a very dense gas. Conversely, assuming that the emission originates in the 10''-15'' size shocked gas previously observed at the IRAM Plateau de Bure Interferometer, we obtain N(HCl) = 0.7-2 × 10¹³ cm^-2, temperature >15 K, and density >3 × 10⁵ cm^-3. Combining these with the Herschel HIFI CO(5-4) observations allows us to further constrain the gas density and temperature, 10⁵-10⁶ cm^-3 and 120-250 K, respectively, as well as the HCl column density, 2 × 10¹³ cm^-2, and, finally, the abundance ~3-6 × 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 grain components are enhanced, suggests either that HCl is not the main reservoir of chlorine in the gas phase, which goes against previous chemical model predictions, or that the elemental chlorine abundance is low in L1157-B1. Astrochemical modeling suggests that HCl is in fact formed in the gas phase at low temperatures prior to the occurrence of the shock; the latter does not enhance its abundance.