Deuterium fractionation mechanisms in interstellar clouds

Abstract

In the theory of the fractionation of deuterated molecules, it is assumed that almost all of the deuterium in dense clouds exists in the form of HD molecules and none in atomic form. In the present investigation it is suggested that a significant fraction of the deuterium is atomic and that the fractionation is assisted by reactions such as D + H3(+) yields H2D(+) + H and D + HCO(+) yields H + DCO(+). The theory is extended on the basis of the considered suggestions. Attention is given to the derivation of the upper and lower bounds to the cosmic deuterium-hydrogen abundance ratio, the derivation of the upper limits for the fractional ionization, and the conditions for a number of warm clouds and cold clouds.