A comprehensive survey of hydrogen chloride in the galaxy

We report new observations of the fundamental J = 1–0 transition of HCl (at 625.918 GHz) toward a sample of 27 galactic star-forming regions, molecular clouds, and evolved stars, carried out using the Caltech Submillimeter Observatory. Fourteen sources in the sample are also observed in the corresponding H^(37)Cl J = 1–0 transition (at 624.978 GHz). We have obtained clear detections in all but four of the targets, often in emission. Absorptions against bright background continuum sources are also seen in nine cases, usually involving a delicate balance between emission and absorption features. From RADEX modeling, we derive gas densities and HCl column densities for sources with HCl emission. HCl is found in a wide range of environments, with gas densities ranging from 10^5 to 10^7 cm^(−3). The HCl abundance relative to H_2 is in the range of (3–30) × 10^(−10). Comparing with the chlorine abundance in the solar neighborhood, this corresponds to a chlorine depletion factor of up to ~400, assuming that HCl accounts for one-third of the total chlorine in the gas phase. The [^(35)Cl]/[^(37)Cl] isotopic ratio is rather varied, from unity to ~5, mostly lower than the terrestrial value of 3.1. Such variation is highly localized, and could be generated by the nucleosynthesis in supernovae, which predicts a ^(37)Cl deficiency in most models. The lower ratios seen in W3IRS4 and W3IRS5 likely confine the progenitors of the supernovae to stars with relatively large mass (≳25 M_⊙) and high metallicity (Z ~ 0.02)