CARBON-BEARING MOLECULES IN OXYGEN-RICH CIRCUMSTELLAR ENVELOPES

We are currently conducting a search of oxygen-rich circumstellar enve lopes for carbon-bearing molecules with the ISO SWS instrument. CH4 is the molecule in which we are most interested, because it has been sho wn to be capable of providing the reactive carbon which is needed prod uce the carbon-bearing molecules in these envelopes (Willacy & Millar 1997). However, CO2 and C2H2 are also possible candidates. The spectra l ranges for our observations have been chosen specifically to include ro-vibrational bands in these molecules, but many other molecules, su ch as HCN, NH3, OH and HO2 have bands in these regions and hopefully t here will be a few interesting surprises. To date we have received the observations of three of our objects, R Gas, IRC+10420 and the S-type star W Aql, which we are observing to study the effect of the C/O rat io on chemical composition. Preliminary results for W Aql show little evidence of CH4, a tentative detection of CO2, and clear evidence of O H and HCl.