We report the detection of H _ { 2 } CO ( formaldehyde ) around the carbon-rich AGB star , IRC+10216 .
We find a fractional abundance with respect to molecular hydrogen of x ( { H _ { 2 } CO } ) = 1.3 ^ { +1.5 } _ { -0.8 } \times 10 ^ { -8 } .
This corresponds to a formaldehyde abundance with respect to water vapor of x ( { H _ { 2 } CO } ) / x ( { H _ { 2 } O } ) = ( 1.1 \pm 0.2 ) \times 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 \times 10 ^ { -4 } ( 3 \sigma ) indicates that methanol is substantially underabundant in IRC+10216 , compared to Solar System comets .

We also conclude , based on 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 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 \times 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 \sim 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 .

This study is based on observations carried out with the IRAM 30m telescope .
IRAM is supported by INSU/CNRS ( France ) , MPG ( Germany ) and IGN ( Spain ) .