2002A&A...381L..17C

We present new observations of the D₂CO emission towards the small cloud L1689N in the ρ Ophiuchus complex. We surveyed five positions, three being a cut across a shock site and two probing the quiescent gas of the molecular cloud. We detected D₂CO emission in the first three positions. The measured [D₂CO]/[H₂CO] is about 3%, whereas it is ≤2% in the quiescent gas. We discuss the implications of these new observations, which suggest that the bulk of the D₂CO molecules is stored in grain mantles, and removed from the cold storage by the shock at the interface between the outflowing and quiescent gas. We review the predictions of the published models proposed to explain the observed high deuteration of formaldehyde. They fall in two basic schemes: gas phase and grain surface chemistry. None of the reviewed models is able to account for the observed [D₂CO]/[H₂CO] abundance ratio. A common characteristics shared by the models is apparently that all underestimate the atomic [D]/[H] ratio in the accreting gas.