Deuterated Methanol in the Orion Compact Ridge

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Abstract
We have investigated the chemistry of hot cores containing methanol, formaldehyde, and their deuterated forms. We have computed the D fractionation ratios attainable by the addition of H and D atoms to CO on cold grain surfaces. We can easily reproduce the observed HDCO/H2CO and D2CO/H2CO ratios but find that, as there is an additional path to CH2DOH, the CH2DOH/CH3OD ratio is typically 3. This result is in conflict with the conclusion of Jacq et al. that the observed gas-phase CH2DOH/CH3OD ratio of ≈ 1.1-1.5 in the Orion Compact Ridge source is consistent with their production in grain mantles. We show that, when these mantles are evaporated, the CH2DOH/CH3OD ratio can be altered by gas-phase reactions that preferentially form CH3OD via reaction of H2DO+ with CH3OH. We find that an unrealistically high HDO/H2O ratio of ≈ 0.1 is necessary to drive the CH2DOH/CH3OD ratio to within the observed range. Before concluding that the observed gas-phase CH2DOH/CH3OD ratio is inconsistent with these grain-surface reactions, we recommend that more accurate determinations of this ratio be made in the Compact Ridge and in other hot cores.