Dissociative recombination of H2D+ and the cosmic abundance of deuterium

Deuterated molecules in interstellar clouds constitute a unique source of information on three interstellar parameters: the deuterium/hydrogen abundance ratio [D]/[H], the fractional ionization in dense clouds, and the galactic ionizing flux (Dalgarno & Lepp 1984, Millar 1990). The molecular ion H2D+ plays a key role in the formation of deuterated molecules. Dissociative recombination with electrons is a major loss channel and a potential source of deuterium atoms. An ion storage ring has been used to obtain the total recombination rate coefficient, and the probabilities for decay into H+H+D, HD+H and H-2+D. Using them in a model of the cloud L1529 we derive from observational data on HCO+ and DCO+ an upper limit to the fractional ionization of 2 x 10(-6), a [D]/[H] ratio of 1.7 x 10(-5) and an atomic D/H ratio of 2.7 x 10(-3).