Discovery of solid HDO in grain mantles

The observation of deuterium fractionation in the molecular gas in hot core s is currently interpreted as evaporation of deuterium-rich molecules from the icy grain mantles where they accumulated during the cold, dense cloud c ore stage. The predicted enrichment in deuterium of the ices has never been observed. This work provides the first detections of solid HDO in grain ma ntles. We find column densities in the solid state of N-s(HDO) = (3.23 +/- 0.03) x 10(16) cm(-2) for W33A, and N-s(HDO) = (1.78 +/- 0.03) x 10(16) cm( -2) for NGC7538 IRS9. The estimated [HDO]/[H2O] ratios range from 8 x 10(-4 ) in W33A to 10(-2) in NGC7538 IRS9. These values are comparable to the pre dictions by chemical models, and provide support for the assumption that th e origin of the deuterium enhancement in "hot cores" is evaporation from gr ain mantles.