RADIO OBSERVATIONS OF D-I AND FRACTIONATION

We report observations to detect the 327 MHz line of atomic deuterium in the primarily molecular clouds towards Cas A and Sgr A, making appr opriate choices of spectral and spatial resolution. Our best results f or Cas A, based on 1253 hr of observing with the 85 foot (26 m) Hat Cr eek telescope and 45 hr with the VLA, yield an upper limit for the fra ction of deuterium in atomic form, D I, of 0.14. We present an approxi mate analytical analysis of deuterium fractionation in translucent clo uds, which shows that gas-phase ion-molecule reactions, assisted by du st and HD line self-shielding, are efficient in converting deuterium t o HD. We conclude that little atomic deuterium is present in the molec ular clumps in the Cas A clouds and that much higher sensitivity obser vations would be required to detect the 327 MHz line in molecular clou ds. We have also attempted to detect the 72 GHz line of DCO+ toward Ca s A. The observed upper limit to the DCO+/HCO+ ratio of approximately 0.03 is not much larger than our theoretical estimate, and the DCO+ li ne should be detectable with existing instrumentation, unless the temp erature of these clouds is much larger than usually assumed.