A New Infrared Photometric Study of Intrinsic and Extrinsic S-type Stars

We collect all known intrinsic and extrinsic S-type stars to discuss their infrared properties and find their difference in the infrared using photometric data from the Two Micron All Sky Survey, Wide Field Infrared Survey Explorer, and Infrared Astronomical Satellite missions. Then we look for the diagnosis to extract intrinsic S-type stars from the large unclassified sample. We found that, statistically, intrinsic S-type stars have larger infrared excesses than extrinsic S-type stars in the wavelength region of 1-60 mu m due to thicker dusty circumstellar envelopes. We also found that only intrinsic S-type stars occupy the reddest color areas in all of the two-color diagrams we presented. Finally, 172 new intrinsic S-type stars are presented in this paper. This makes the number of known intrinsic S-type stars almost double. In addition, some intrinsic and extrinsic S-type stars have power-law distributions in some two-color diagrams with the wavelength longer than 5 mu m. The possible reason for this is also discussed.