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 ( ) .
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 { 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 .