HD depletion in starless cores
  Aims: We aim to investigate the abundances of light deuterium-bearing species
such as HD, H2D+ and D2H+ in a gas-grain chemical model including an extensive
description of deuterium and spin state chemistry, in physical conditions
appropriate to the very centers of starless cores. Methods: We combine a
gas-grain chemical model with radiative transfer calculations to simulate
density and temperature structure in starless cores. The chemical model
includes deuterated forms of species with up to 4 atoms and the spin states of
the light species H2, H2+ and H3+ and their deuterated forms. Results: We find
that HD eventually depletes from the gas phase because deuterium is efficiently
incorporated to grain-surface HDO, resulting in inefficient HD production on
grains. HD depletion has consequences not only on the abundances of e.g. H2D+
and D2H+, whose production depends on the abundance of HD, but also on the spin
state abundance ratios of the various light species, when compared with the
complete depletion model where heavy elements do not influence the chemistry.
Conclusions: While the eventual HD depletion leads to the disappearance of
light deuterium-bearing species from the gas phase in a relatively short
timescale at high density, we find that at late stages of core evolution the
abundances of H2D+ and D2H+ increase toward the core edge and the disributions
become extended. The HD depletion timescale increases if less oxygen is
initially present in the gas phase, owing to chemical interaction between the
gas and the dust predecing the starless core phase. Our results are greatly
affected if H2 is allowed to tunnel on grain surfaces, and therefore more
experimental data not only on tunneling but also on the O + H2 surface reaction
in particular is needed.
