Deuterium Fractionation in Protoplanetary Disks

Abstract

We investigate the chemistry of deuterium-bearing molecules in outer regions of protoplanetary disks, where comets may form. We find that molecules formed in the disk have higher D/H ratios (by which we mean abundance ratios between the singly deuterated species and the normal species) than the elemental D/H ratio in protosolar abundance, which is consistent with observations of recent comets. Despite the higher densities in protoplanetary disks, deuterium fractionation occurs in a similar way as in molecular clouds; because of the differences in zero-point energies and the existence of rapid ion-molecule isotopic exchange reactions, species such as H have high D/H ratios, which propagate to other molecules via gas-phase chemical reactions. Our results depend on a variety of factors such as the ionization rate and the temperature of the disk, which is somewhat uncertain because of possible variations in stellar luminosity and the existence of accretion shocks. In order to reproduce the observed cometary values of DCN/HCN and HDO/H₂O, a lower ionization rate (~10^-18 s^-1) than is "standard" in molecular clouds is more favorable. The calculated DCN/HCN ratio also depends on whether the products assumed for the grain surface recombination reaction between HCO⁺ and electrons are dissociative or not.