IAU Symposium Abstract S186-015P

Are the Bulge carbon stars related to the Sagittarius dwarf galaxy?

Y.K. Ng
Padova Astronomical Observatory, Italy (yuen@astrpd.pd.astro.it)

If the Sagittarius dwarf galaxy (SDG) is crossing the Galactic disc once per Gyr (Ibata et al., 1997, AJ 113, 635) a star formation burst could be the signature of each passage. Carbon stars are due to their brightness ideal tracers for such a burst. Ng & Schultheis (1997, A&AS in press) suggested that the bulge carbon stars from Azzopardi et al. (1991, A&AS 88, 265) might be located in the SDG. This possibility is analyzed together with the (candidate) carbon stars found in the SDG (Whitelock et al., 1996, New Astronomy 1, 57). It might shed some light on the star formation history of the SDG in the last 4 Gyr.

The results indicate that the carbon stars are not metal-rich as previously thought. They have a metallicity comparable to the LMC with an age between 0.1-1 Gyr. A significant fraction of the carbon stars have a luminosity fainter than the lower LMC and SMC limit of respectively Mbol ~ -3.5 and Mbol ~ -3.0. At present, the TP-AGB models cannot explain this, even if carbon stars form immediately after they enter the TP-AGB phase. Mass transfer through binary evolution is suggested as a possible scenario to explain the origin of these low luminosity carbon stars.