Deuterium enrichment in interstellar HCN and HNC

Abstract

THE cosmic abundance of deuterium, expressed as the ratio D/H, is thought to be approximately 2 × 10⁻⁵ (ref. 1). Observations on deuterium-containing molecules, such as DCN (refs 2,3) and DNC (ref. 4), however, indicate much higher values for the molecular ratios DCN/HCN and DNC/HNC. In the case of DCN this was attributed to chemical enrichment involving equilibria such as⁵ or to other mechanisms⁶. But it is difficult to reconcile the observed enrichment of DNC with any of these suggestions. Here we explore the alternatives and correct previous suggestions about the mechanism of formation of interstellar HCN. We consider conditions appropriate to dense molecular clouds such as that in Ori A or NGC2264 where the visual/ultraviolet optical depth is very high and ionisation is predominantly by cosmic rays.