Astron. Astrophys. 338, L43-L46 (1998)

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 <sup>4

1</sup> Laboratoire d'Astrophysique, Observatoire de Grenoble - B.P. 53, F-38041 Grenoble cedex 09, France
² Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, F-38406 St.-Martin-d'Hères, France
³ CESR CNRS-UPS, B.P. 4346, F-31028 Toulouse cedex 04, France
⁴ SRON, P.O. Box 800, 9700 AV Groningen, The Netherlands

Received 30 June 1998 / Accepted 12 August 1998

Abstract

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

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Contents

• 1. Introduction
• 2. Observations and results
• 3. Discussion
  • 3.1. Origin of the 4₂₂ - 3₂₁ line emission
  • 3.2. D₂CO column density
  • 3.3. Relative abundances of D₂CO, HDCO and H₂CO
  • 3.4. Implications
• 4. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998
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