Discovery of Deuterated Water in a Young Protoplanetary Disk

Abstract

We report the first detection of the ground transition of deuteratedwater at 464 GHz in the young protoplanetary disk surrounding thesolar-type protostar DM Tau. The line is observed in absorption againstthe continuum from the cold dust in the disk midplane, with a line-to-continuumratio close to unity. The observation implies that deuterated gaseouswater is present, with a relatively large abundance (~3 × 10-9),in the outer disk above the midplane, where the density is ~106 cm-3,within a factor of 10, and the temperature is lower than about 25K. In these conditions, the H2O condensation timescale is much smallerthan the DM Tau disk age, and, therefore, water should be fully frozenonto the grain mantles. We suggest that UV photons and/or X-rayssublimate part of the mantles, reinjecting the ices into the gasphase. Even though there is currently no measurement of H2O, we providearguments that the HDO/H2O ratio should be about 0.01 or larger,which would be hundreds of times larger than the values measuredin solar system objects. This suggests the need for strong cautionin comparing and linking the HDO/H2O in solar system and star-formingenvironments.