Doubly deuterated formaldehyde in star-forming regions: an observational approach

We present the results of a search for the doubly deuterated form of formaldehyde (D2CO) in a large sample of objects associated to star-formation. D2CO was detected in nearly 20 low-mass protostars, a success rate of 100%, with [D2CO]/[H2CO] abundance ratios of 2-40%. On the other hand, no detection was obtained towards more massive protostars (where [D2CO]/[H2CO] < 0.5%) nor towards the shocks along the molecular outflows powered by protostars (where [D2CO]/[H2CO] < 0.5-1%). The results reported here multiply the number of D2CO detections by a factor of 6. Since low temperatures and high densities are needed to obtain high levels of multiple deuteration, the D2CO detected towards low-mass protostars is most likely currently desorbed from dust grain mantles where it has accumulated and/or formed during the earlier and colder pre-stellar stage. The lack of detection towards molecular outflows is naturally explained in this scheme since the high-velocity gas there is mostly previously ambient material, that never went through the cold phase required to attain high deuteration levels. The lack of detection towards higher mass protostars indicates that the chemistry in low-mass protostars is significantly different from that in higher mass objects, and that much caution should be used when generalizing chemical results obtained in high-mass sources to low-mass objects. (C) 2002 Elsevier Science Ltd. All rights reserved.