Deuterated molecules in DM Tau: DCO, but no HDO

We report the detection of the J=2-1 line of DCO in the proto-planetary disk of DM Tau and re-analyze the spectrum covering the 465 GHz transition of HDO in this source, recently published by Ceccarelli et al. (2005). A modelling of the DCO line profile with the source parameters derived from high resolution HCO observations yields a DCO/HCO abundance ratio of ≃ 4×10−3, an order of magnitude smaller than that derived in the low mass cores. The re-analysis of the 465 GHz spectrum, using the proper continuum flux (0.5 Jy) and source systemic velocity (6.05 km s), makes it clear that the absorption features attributed to HDO and C6H are almost certainly unrelated to these species. We show that the line-to-continuum ratio of an absorption line in front of a Keplerian disk can hardly exceed the ratio of the turbulent velocity to the projected rotation velocity at the disk edge, unless the line is optically very thick (τ > 104). This ratio is typically 0.1-0.3 in proto-planetary disks and is ≃ 0.15 in DM Tau, much smaller than that for the alleged absorption features. We also show that the detection of H2D in DM Tau, previously reported by these authors, is only a 2-sigma detection when the proper velocity is adopted. So far, DCO is thus the only deuterated molecule clearly detected in proto-planetary disks.