Detection of Formaldehyde toward the Extreme Carbon Star IRC +10216

We report the detection of H 2 CO (formaldehyde) around the carbon-rich asymptotic giant branch star IRC +10216. We find a fractional abundance with respect to molecular hydrogen of x (H 2 CO) = 1.3 × 10 -8 . This corresponds to a formaldehyde abundance with respect to water vapor of x (H 2 CO)/ x (H 2 O) = (1.1 ± 0.2) × 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 CH 3 OH (methanol) around IRC +10216, and our upper limit of x (CH 3 OH)/ x (H 2 O) < 7.7 × 10 -4 (3 σ) 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 Al 37 Cl 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 (2-8) × 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 ~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.