Oxygen-rich AGB IRAM spectra

We surveyed a sample of 30 oxygen-rich AGB stars in the {lambda} 2mm band using the IRAM 30m telescope. We performed excitation and radiative transfer calculations based on the large velocity gradient (LVG) method to model the observed lines of the molecules and to derive their fractional abundances in the observed envelopes.

We detected SiO in all 30 targeted envelopes, as well as CS, SiS, SO, and SO 2 in 18, 13, 26, and 19 sources, respectively. Remarkably, SiS is not detected in any envelope with a mass loss rate below 10^-6^M_{sun}_/yr, whereas it is detected in all envelopes with mass loss rates above that threshold. From a comparison with a previous, similar study on C-rich sources, it becomes evident that the fractional abundances of CS and SiS show a marked differentiation between C-rich and O-rich sources, being two orders of magnitude and one order of magnitude more abundant in C-rich sources, respectively, while the fractional abundance of SiO turns out to be insensitive to the C/O ratio.