1998A&A...338L..43C

We report the detection of the doubly deuterated form of formaldehyde towards IRAS 16293-2422, an extremely young protobinary system. This is the only detection of D₂CO in any astronomical source other than Orion (Turner, 1990ApJ...362L..29T). The line searched was that corresponding to the 4_2,2-3_2,1 transition at ν₀=236.102GHz, and was detected at three positions, one centered on the source itself, and the other two 10" West and 10" East respectively. A survey of the literature revealed that two additional lines of D₂CO at 245.532 and 342.522GHz were detected towards IRAS 16293-2422, but classified as ``unidentified''.We failed to detect any signal towards the outflow (at offsets (-20";0") and (-30";+10")) where low-J CO emission lines are quite strong. The line profiles at the three positions show a dip at the zero rest velocity. We rule out the possibility that this dip may be due to a pure kinematic effect and show that it is actually due to self absorption. The gas responsible for the D₂CO emission is located at less than about 10" (∼2x10¹⁶cm) from the central source. We used multi-transition H₂CO observations of van Dishoeck et al. (1995ApJ...447..760V) to estimate the density (n_H2∼10⁷cm^–3) and temperature (T∼100K) of the emitting gas. From those values, we can derive a lower limit to the column density required for the D₂CO line we observed to be self-absorbed: N(D₂CO)∼10¹⁴cm^–2. Combining the previous and present observations of H₂CO, HDCO and D₂CO we obtain the following upper limits to the abundance ratios of the three species: [HDCO]/[H₂CO]≤0.25 and [D₂CO]/[HDCO]≤0.5. If the abundance of H₂CO in IRAS 16293-2422 is comparable to that observed towards the Hot Cores in Orion (∼10^–8; Blake et al., 1987ApJ...315..621B) and if the D₂CO 4_2,2-3_2,1 line is only moderately thick, D₂CO is only 1/10 less abundant than H₂CO. We finally speculate that such a large amount of warm D₂CO can only be produced by evaporation of this molecule from the grain mantles, where it was presumably copiously formed during the long pre-collapse period.