Discovery of solid HDO in grain mantles

Abstract

The observation of deuterium fractionation in the molecular gas in hot cores is currently interpreted as evaporation of deuterium-rich molecules from the icy grain mantles where they accumulated during the cold, dense cloud core stage. The predicted enrichment in deuterium of the ices has never been observed. This work provides the first detections of solid HDO in grain mantles. 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) = (7.78 +/- 0.03) x 10(16) cm(-2) for NGC7538 IRS9. The estimated [HDO]/H_2 Oi] ratios range from 8x 10(-4) in W33A to 10(-2) in NGC7538 IRS9. These values are comparable to the predictions by chemical models, and provide support for the assumption that the origin of the deuterium enhancement in ``hot cores'' is evaporation from grain mantles. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.