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 × 10-8. This corresponds to a formaldehyde abundance with respect to water vapor of x(H2CO)/x(H2O) = (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 CH3OH (methanol) around IRC +10216, and our upper limit of x(CH3OH)/x(H2O) < 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 Al37Cl 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 H2; 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.