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.