Astronomy and Astrophysics, volume 422, 1045-1052 (2004/8-2)
First detection of a lithium rich carbon star in the Draco dwarf galaxy: Evidence for a young stellar population.
DOMINGUEZ I., ABIA C., STRANIERO O., CRISTALLO S. and PAVLENKO Y.V.
Abstract (from CDS):
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-logTeff 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.
stars: carbon - stars: abundances - stars: evolution - stars: AGB and post-AGB - galaxies: local group
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