2004A&A...422.1045D

We present a spectroscopic study of D461, a giant star belonging to Draco dwarf spheroidal galaxy. From spectral synthesis in LTE we derive a lithium abundance of log ε(Li)=3.5±0.4 and a C/O ratio between 3 and 5. This is the first detection of a lithium rich C-star in a dwarf spheroidal galaxy. Basing on stellar models of appropriate chemical composition, we show that a similar C enrichment is compatible with that expected for a low mass low metallicity thermally pulsing AGB star, undergoing few third dredge up episodes. The position in the logg-logT_eff diagram of D461 is also compatible with this theoretical scenario. In particular, the low effective temperature, lower than that expected for a low metallicity giant star, is a consequence of the huge increase of the envelope opacity occurring after the carbon dredge up. The Li enrichment may be explained if a deep circulation would take place during the interpulse period, the so called cool bottom process. In spite of the low resolution of our spectra, we derive a lower limit for the carbon isotopic ratio, namely ¹²C/¹³C>40, and a constraint for the Ba abundance, namely 0.5<[Ba/Fe]< 2. The proposed scenario also fits these further constraints. Then, we estimate that the mass of D461 ranges between 1.2 and 2M_☉, which corresponds to an age ranging between 1 and 3Gyr. We conclude that this star is more massive and younger than the typical stellar population of Draco.