2004ApJ...614..990F

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^+1.5_–0.8x10^–8. This corresponds to a formaldehyde abundance with respect to water vapor of x(H₂CO)/x(H₂O)=(1.1±0.2)x10^–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)<7.7x10^–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)x10^–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.