Deuterium in interstellar molecules

Isotope shifts of the Lyman bands of the HD molecule relative to those of H2 are sufficient that the ultraviolet radiation which photodissociates HD in interstellar clouds is not appredably attenuated by absorption due to H2. In dense clouds this causes a larger fraction of hydrogen to exist as H2 than deuterium to exist as HD if the rates of formation are equal. At the low temperatures of gas and grains in interstellar clouds, H2 and HD are probably chemically unreactive. Then since H2 and HD form first and lock up these atoms, the abundances of H and D in other molecules reflect their abundances in atomic form. We show that the ratio (atomic D/atomic H) / (tota D/ total H) approaches Y[ (total H/total D)] in the more dense regions (visual optical depths Tvi8> 2 typically) of clouds, and thus predict a similar enhancement for deuterium in molecules other than HD. For the usual abundance of deuterium (deuterium/hydrogen lo- ) the predicted enhancement is 102, in excellent agreement with the observed (DCN/HCN) ratio. Subject headings:abundances - molecules, interstellar