The D/H Ratio of Water Ice at Low Temperatures
  We present the modeling results of deuterium fractionation of water ice, H2,
and the primary deuterium isotopologues of H3+ adopting physical conditions
associated with the star and planet formation process. We calculated the
deuterium chemistry for a range of gas temperatures (T_gas ~ 10 - 30 K),
molecular hydrogen density (n(H2)~ 10^4 - 10^7), and ortho/para ratio (opr) of
H2 based on state-to-state reaction rates and explore the resulting
fractionation including the formation of a water ice mantle coating grain
surfaces. We find that the deuterium fractionation exhibits the expected
temperature dependence of large enrichments at low gas temperature. More
significantly the inclusion of water ice formation leads to large D/H ratios in
water ice (>= 10^-2 at 10 K) but also alters the overall deuterium chemistry.
For T < 20 K the implantation of deuterium into ices lowers the overall
abundance of HD which reduces the efficiency of deuterium fractionation at high
density. In agreement with an earlier study, under these conditions HD may not
be the primary deuterium reservoir in the cold dense interstellar medium and
H3+ will be the main charge carrier in the dense centers of pre-stellar cores
and the protoplanetary disk midplane.
