Deuterium chemistry in the Orion Bar PDR - "warm" chemistry starring
  CH2D+
  High levels of deuterium fractionation in gas-phase molecules are usually
associated with cold regions, such as prestellar cores. Significant
fractionation ratios are also observed in hot environments such as hot cores or
hot corinos, where they are believed to be produced by the evaporation of the
icy mantles surrounding dust grains, and thus are remnants of a previous cold
(either gas-phase or grain surface) chemistry. The recent detection of DCN
towards the Orion Bar, in a clump at a characteristic temperature of 70K, has
shown that high deuterium fractionation can also be detected in PDRs. The Orion
Bar clumps thus appear as a good environment for the observational study of
deuterium fractionation in luke-warm gas, allowing to validate chemistry models
in a different temperature range, where dominating fractionation processes are
predicted to be different than in cold gas (< 20K). We aimed at studying
observationally in detail the chemistry at work in the Orion Bar PDR, to
understand if DCN is produced by ice mantle evaporation, or is the result of
warm gas-phase chemistry, involving the CH2D+ precursor ion (which survives
higher temperatures than the usual H2D+ precursor). Using the APEX and the IRAM
30m telescopes, we targetted selected deuterated species towards two clumps in
the Orion Bar. We confirmed the detection of DCN and detected two new
deuterated molecules (DCO+ and HDCO) towards one clump in the Orion Bar PDR.
Significant deuterium fractionations are found for HCN and H2CO, but a low
fractionation in HCO+. We also give upper limits for other molecules relevant
for the deuterium chemistry. (...)
  We show evidence that warm deuterium chemistry driven by CH2D+ is at work in
the clumps.
