Deuterium fractionation in warm dense interstellar clumps

Aims. In a recently published 1-millimeter line survey of the Orion Bar by Leurini et al., the confirmation of dense neutral clumps was reported based mainly on an analysis of methanol spectral lines. Within these clumps, a high abundance ratio was found between the deuterated isotopologue DCN and HCN. Methods. In this paper, we use steady-state chemical modelling to determine if the DCN/HCN abundance ratio can be understood in terms of gas-phase processes. Results. Our results show that, at temperatures (50-75 K) and densities (5-20 x 10(6) cm(-3)) in the vicinity of those measured, the calculated DCN/HCN abundance ratio can reach 0.01 or even higher, in good agreement with the measured range of 0.007-0.009. Moreover, the fractional abundance of HCN is also calculated to be in the vicinity of that reported. Other computed abundance ratios between deuterated and normal isotopologues are reported for the physical conditions of the clumps. The abundance ratios are analyzed as functions of temperature and density in terms of a simple formula. In general, calculated ratios can be somewhat dependent on the choice of two very different sets of elemental abundances, one of which is the standard low-metal set, while the other we term the warm- core set.