Detection of formaldehyde toward the extreme carbon star IRC+10216

We report the detection of H2CO (formaldehyde) around the carbon-rich asymptotic giant branch star IRC + 10216. We find a fractional abundance with respect to molecular hydrogen of x(H2CO) = 1.3(-0.8)(+1.5). This corresponds to a formaldehyde abundance with respect to water vapor of x(H2CO)/x(H2O) = (1.1 +/- 0.2) x 10(-2), in line with the formaldehyde abundances found in solar system comets, and indicates that the putative extrasolar cometary system around IRC + 10216 may have a similar chemical composition to solar system comets. However, we also failed to detect CH3OH (methanol) around IRC + 10216, and our upper limit of x(CH3OH)/x(H2O) < 7.7 x 10(-4) (3 sigma) indicates that methanol is substantially underabundant in IRC + 10216 compared to solar system comets. We also conclude, on the basis of offset observations, that formaldehyde has an extended source in the envelope of IRC + 10216 and may be produced by the photodissociation of a parent molecule, similar to the production mechanism for formaldehyde in solar system comet comae. Preliminary mapping observations also indicate a possible asymmetry in the spatial distribution of formaldehyde around IRC + 10216, but higher signal-to-noise ratio observations are required to confirm this finding. By serendipity, our observations have led to the detection of the J = 17-16 transition of (AlCl)-Cl-37 at 241.855 GHz. Our analysis of the measured line flux, along with those of previously observed lower frequency transitions, yields a total AlCl (aluminum monochloride) abundance in the range (2x8) x 10(-8) relative to H-2; this range, which is a factor of 10 smaller than an abundance estimate that has appeared previously in the literature, amounts to similar to 4%-16% of the solar elemental abundance of chlorine, a fraction that is in accord with the predictions of thermochemical equilibrium models for cool stellar photospheres.