The abundance of 36 S in IRC + 10216 and its production in the Galaxy

The J = 2 − 1 and 3 − 2 rotational lines of the rare isotopomer CS and the J = 5 − 4 and 6 − 5 transitions of SiS were detected in the carbon star IRC+10216 (CWLeo). These are the first detections of S bearing molecules in a star and the first spectroscopic detection of SiS. From a comparison of S and S bearing isotopomers, the S/S isotopic ratio is 107(±15). This value is comparable to values in the interstellar medium of the inner Galactic disk (115) but is smaller than the solar value of 288 (Ding et al. 2001). The increase of the S abundance relative to S only qualitatively follows model predictions of a low mass AGB star. Quantitative agreement of the observed S/S ratio with the stellar models can be reached if the age of IRC+10216 and Galactic chemical evolution are taken into account. Other less likely possibilities are the presence of considerable inhomogeneities in the interstellar medium and either IRC+10216 or the Sun started with a peculiar S abundance. Other production mechanisms potentially capable of enhancing the Galactic interstellar medium are discussed. From the observed line density toward IRC+10216 and toward Galactic star forming regions, we estimate the confusion limit toward those sources.