HYDROGEN-CHLORIDE DETECTION IN ORION-A AND MONOCEROS-R2 AND DERIVATION OF THE (HCL)-CL-35 (HCL)-CL-37 ISOTOPIC RATIO/

We have detected the J = 1-0 transition of (HCl)-Cl-35 in the Orion A and Monoceros R2 giant molecular clouds, using the Caltech Submillimet er Observatory and a new heterodyne receiver built at the Jet Propulsi on Laboratory. We find a HCl fractional abundance of a few times 10(-1 0) and a depletion of gasphase chlorine by a factor of 550-640 for Ori on and 65-125 for Mon R2. For both sources, we derived the line optica l depth and excitation temperature using the relative intensities of t he hyperfine components and determined the abundance and excitation co nditions with a large velocity gradient (LVG) model. The LVG model ind icates that radiative pumping of the molecules by moderately warm dust is required to explain the observed line intensities and optical dept hs. In addition, we report the first detection of the isotope (HCl)-Cl -37 from its J = 1-0 emission in Orion. Because the chemical reactions involving hydrogen chloride in molecular clouds are relatively simple , [(HCl)-Cl-35]/[(HCl)-Cl-37] is believed to provide a good estimate o f [Cl-35]/[Cl-37]. We find an isotopic ratio about 1.3-2 times that of the solar neighborhood, suggesting that this ratio is not constant th roughout the galactic interstellar medium.