Detection of doubly deuterated formaldehyde towards the low-luminosity protostar IRAS 16293-2422

Details Detection of doubly deuterated formaldehyde towards the low-luminosity protostar IRAS 16293-2422 Detection of doubly deuterated formaldehyde towards the low-luminosity protostar IRAS 16293-2422

Oct 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998a%26a...338l..43c&link_type=abstract

Astronomy and Astrophysics, v.338, p.L43-L46 (1998)

Astronomy and Astrophysics

Astrophysics

77

Ism: Abundances, Ism: Individual: Iras 16293-2422, Ism: Molecules, Stars: Formation

Scientific paper

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 D_2CO in any astronomical source other than Orion (Turner 1990). The line searched was that corresponding to the 4_{2,2}-3_{2,1} transition at nu_ {0} = 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 D_2CO 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 D_2CO emission is located at less than about 10'' ( ~ 2 * 10(16) cm) from the central source. We used multi-transition H_2CO observations of van Dishoeck et al. (1995) to estimate the density (nH_2 ~ 10(7) cm(-3) ) and temperature (T ~ 100 K) of the emitting gas. From those values, we can derive a lower limit to the column density required for the D_2CO line we observed to be self-absorbed: N(D_2CO) ~ 10(14) cm(-2) . Combining the previous and present observations of H_2CO, HDCO and D_2CO we obtain the following upper limits to the abundance ratios of the three species: [HDCO] / [H_2CO] lkern -4.1ptlower2pt char19 kern -0.8pteq 0.25 and [D_2CO]/[HDCO] lkern -4.1ptlower2pt char19 kern -0.8pteq 0.5. If the abundance of H_2CO in IRAS16293-2422 is comparable to that observed towards the Hot Cores in Orion ( ~ 10(-8) ; Blake et al. 1987) and if the D_2CO 4_{2,2}-3_{2,1} line is only moderately thick, D_2CO is only 1/10 less abundant than H_2CO. We finally speculate that such a large amount of warm D_2CO 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.

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