Letter to the Editor

Deuterated molecules in DM Tauri: DCO⁺, but no HDO

¹ L3AB, Observatoire de Bordeaux, 2 rue de l'Observatoire, BP 89, 33270 Floirac, France e-mail: guilloteau@obs.u-bordeaux1.fr
² IRAM, 300 rue de la piscine, 38406 Saint Martin d'Hères Cedex, France

Received: 3 November 2005
Accepted: 19 January 2006

Abstract

We report the detection of the 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, ApJ, 631, L81). A modelling of the DCO⁺ line profile with the source parameters derived from high resolution HCO⁺ observations yields a DCO⁺/HCO⁺ abundance ratio of , 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 (), 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 (). This ratio is typically 0.1–0.3 in proto-planetary disks and is 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.