Astrophys. J., 614, 990-1006 (2004/October-3)
Detection of formaldehyde toward the extreme carbon star IRC +10216.
FORD K.E.S., NEUFELD D.A., SCHILKE P. and MELNICK G.J.
Abstract (from CDS):
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.
Comets: General - Kuiper Belt - Stars: Planetary Systems - Radio Lines: Stars - Stars: AGB and Post-AGB - Stars: Individual: Alphanumeric: IRC +10216
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