An analysis of IRAS low resolution spectra of S stars

Abstract

The Infrared Astronomical Satellite (IRAS) carried a low resolution spectrometer (LRS) with a wavelength response covering the 8 to 22 micrometer region. About 5000 LRS spectra exist of point sources brighter than 10 Jy at 12 and 25 micrometers. Spectra of 67 known S stars were identified by the IRAS catalogue search. We have analyzed the 32 highest quality S star spectra and find that 19 show measurable emission features in the 9 to 13 micrometer region. Most of the emission features were relatively weak, the strongest being about 30% of the strength of the underlying continuum and the weakest about 10%. Four stars, Y Lyn, RS Cnc, W And, and RAFGL-2425, show emission features resembling the silicate features of M stars. One star, UY Cen, shows a nearly pure SiC feature. Most (14) S Star emission features were centered near 10.5 micrometers, intermediate between the silicate emission features of M stars and the silicon carbide (SIC) features of C stars (see the paper by Little-~renin and Wilton at this conference). The identification of the intermediate wavelength feature is at present unknown, but two possible interpretations may be suggested: i) A previously unknown molecular species may occur in circumstellar shells only when stars are progressing from M to S to C by stellar evolution on the asymptotic giant branch, and while the photospheric ratio of C/0 changes respectively from i. We believe that the M to S to C change occurs on a time sca~e of greater than thousands of years, and that the circumstellar shell, with a lifetime of 10 to 100 years, should represent the chemical abundances in the photosphere. It has been suggested at this conference (see following remarks) that the unknown molecular species may be a form of polycyclic hydrocarbon. Our intermediate wavelength feature is similar to that observed by Gehrz, et al. (Ap. J., 281, 303, 1984) in Nova Aql 1982 as it evolved during its dust shell development phase. 2) The intermediate wavelength feature shown by our 14 S stars may be a blend of SiC and silicate emission. We are able to model the intermediate wavelength features of our 14 S stars with mixtures ranging