A Comprehensive Survey of Hydrogen Chloride in the Galaxy
  We report new observations of the fundamental $J=1-0$ transition of HCl (at
625.918GHz) toward a sample of 25 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\tscl\ $J=1-0$ transition (at 624.978GHz). 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 $(3-30)\times10^{-10}$. Comparing with the
chlorine abundance in the solar neighborhood, this corresponds to a chlorine
depletion factor of up to $\sim$400, assuming that HCl accounts for one third
of the total chlorine in the gas phase. The [\tfcl]/[\tscl] isotopic ratio is
rather varied, from unity to $\sim$5, mostly lower than the terrestrial value
of 3.1. Such variation is highly localized, and could be generated by the
nucleosynthesis in supernovae, which predicts a \tscl\ deficiency in most
models. The lower ratios seen in W3IRS4 and W3IRS5 likely confine the
progenitors of the supernovae to stars with relatively large mass
($\ga$25M$_\sun$) and high metallicity (Z$\sim$0.02).
