Circumstellar Properties of S Stars. I. Dust Features

The 1435 stars currently classified as of S spectral type have been searched for association with the IRAS Point Source Catalog. Among the 835 S stars with IRAS counterparts, 149 are found to have usable IRAS low-resolution spectra. The circumstellar properties of these S stars are classified using the LRS. We find that the circumstellar properties of S stars are very similar to those of M stars. For the S stars that have LRS, silicate emission objects form the largest group. Only four objects of the whole sample show the SiC emission feature commonly found in carbon stars. Some of the S stars that show a photospheric continuum with no infrared excess may not be thermal pulsing AGB stars but are mass transfer binaries. We propose a new classification scheme for AGB stars, taking into account both the photospheric and circumstellar spectral properties. This scheme recognizes the importance of mass loss on the evolution of AGB stars. The evolutionary status of S stars is discussed in terms of their circumstellar properties, and an evolutionary scenario is proposed to reconcile the visible photospheric properties and the circumstellar properties of S stars.