THE DISCRIMINATION BETWEEN O-RICH AND C-RICH CIRCUMSTELLAR ENVELOPES FROM MOLECULAR OBSERVATIONS

We have observed a wide sample of circumstellar envelopes around evolved stars in 10 radio lines of the molecules HCN, HNC, SiS, HC3N, CS, SiO, SO, and (CO)-C-13. Th, species form two groups: O-like molecules (SiO and SO, which are relatively stronger in O-rich stars) and C-like molecules (HCN, HNC, SiS, HC3N, and CS, which are stronger in C-rich sources). We find statistically significant criteria that, from the line intensity ratios of O-/C-like molecules, allow us to classify evolved stars as O- or C-rich. Such criteria are also useful to establish a meaningful comparison between peculiar chemistry objects and the main chemical classes. For line pairs including (CO)-C-12 or (CO)-C-13, criteria can also be settled, but the discrimination is less significant. Some observations of S-type stars show that these objects present line intensity properties that are almost exactly equidistant between those of C- and O-rich sources.