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Letter to the Editor
Detection of doubly deuterated formaldehyde towards the low-luminosity protostar IRAS 16293-2422
C. Ceccarelli 1,
A. Castets 1,
L. Loinard 1, 2,
E. Caux 3 and
A.G.G.M. Tielens 4
Received 30 June 1998 / Accepted 12 August 1998
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 1990). The line searched was that corresponding to the transition at = 236.102 GHz, 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 342.522 GHz 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" ( cm) from the central source.
We used multi-transition H2CO observations of van Dishoeck et al. (1995) to estimate the density and temperature (T K) 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) 1014 cm-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 IRAS16293-2422 is comparable to that observed towards the Hot Cores in Orion ( 10-8; Blake et al. 1987) and if the D2CO 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.
Key words: ISM: abundances ISM: individual: IRAS 16293-2422 ISM: molecules stars: formation
Send offprint requests to: Cecilia.Ceccarelli@obs.ujf-grenoble.fr
Online publication: September 14, 1998