Discovery of Deuterated Water in a Young Protoplanetary Disk

Abstract

We report the first detection of the ground transition of deuterated water at 464 GHz in the young protoplanetary disk surrounding the solar-type protostar DM Tau. The line is observed in absorption against the continuum from the cold dust in the disk midplane, with a line-to-continuum ratio close to unity. The observation implies that deuterated gaseous water 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 25 K. In these conditions, the H2O condensation timescale is much smaller than the DM Tau disk age, and, therefore, water should be fully frozen onto the grain mantles. We suggest that UV photons and/or X-rays sublimate part of the mantles, reinjecting the ices into the gas phase. Even though there is currently no measurement of H2O, we provide arguments that the HDO/H2O ratio should be about 0.01 or larger, which would be hundreds of times larger than the values measured in solar system objects. This suggests the need for strong caution in comparing and linking the HDO/H2O in solar system and star-forming environments.