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

Abstract

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, 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 ${\simeq} 4\times 10^{-3}$, 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 ($6.05 \,{\rm km\,s}^{-1}$), makes it clear that the absorption features attributed to HDO and C$_6$H 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 ($\tau >10^4$). This ratio is typically 0.1–0.3 in proto-planetary disks and is ${\simeq} 0.15$ in DM Tau, much smaller than that for the alleged absorption features. We also show that the detection of H$_2$D$^+$ 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.