2000A&A...359.1169L

Ceccarelli et al. (1998A&A...338L..43C) recently reported the detection of D₂CO in the low-luminosity protostar IRAS 16293-2422. Using the data available at the time, they found that the abundance of D₂CO might be as high as 1/10th that of its hydrogenated counterpart H₂CO. Here we describe and analyse new multi-transition observations of D₂CO, HDCO, H₂CO, and H₂¹³CO towards IRAS 16293-2422. Correcting for the opacity of the H₂CO lines, we find that the abundance of D₂CO is ∼5% that of H₂CO. In addition, we find a component in absorption - also associated to IRAS 16293-2422, but at larger radius - where the abundance of D₂CO compared to H₂CO could be even higher. Though slightly lower than initially claimed, the abundance of D₂CO in IRAS 16293-2422 is extremely high, more than one order of magnitude higher than in Orion KL, the only other source where D₂CO has ever been detected. Because the gas temperature (T=20-50K) is too high, deuteration in the gas-phase is very insufficient to explain such high abundances. We conclude that D₂CO is most likely not currently formed in the gas phase, but is evaporated from the dust grains, where it has been accumulating during the cold, dense pre-collapse period.