Hydrogen Chloride Detection in Orion A and Monoceros R2 and Derivation of the H35Cl/H37Cl Isotopic Ratio

We have detected the J = 1-0 transition of H³⁵Cl in the Orion A and Monoceros R2 giant molecular clouds, using the Caltech Submillimeter Observatory and a new heterodyne receiver built at the Jet Propulsion Laboratory. We find a llCl fractional abundance of a few times 10^-10 and a depletion of gas- phase chlorine by a factor of 550-640 for Orion and 65-125 for Mon R2. For both sources, we derived the line optical depth and excitation temperature using the relative intensities of the hyperfine components and determined the abundance and excitation conditions with a large velocity gradient (LVG) model. The LVG model indicates that radiative pumping of the molecules by moderately warm dust is required to explain the observed line intensities and optical depths. In addition, we report the first detection of the isotope H³⁷Cl from its J = 1-0 emission in Orion. Because the chemical reactions involving hydrogen chloride in molecular clouds are relatively simple, [H³⁵Cl]/[H³⁷Cl] is believed to provide a good estimate of [³⁵Cl]/[³⁷Cl]. We find an isotopic ratio about 1.3-2 times that of the solar neighborhood, suggesting that this ratio is not constant throughout the galactic interstellar medium.