First detection of a lithium rich carbon star in the Draco dwarf galaxy: Evidence for a young stellar population

I. Domínguez; C. Abia; O. Straniero; S. Cristallo; Ya. V. Pavlenko

doi:10.1051/0004-6361:20040289

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Volume 422 / No 3 (August II 2004)

A&A, 422 3 (2004) 1045-1052

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Issue		A&A Volume 422, Number 3, August II 2004


Page(s)		1045 - 1052
Section		Stellar structure and evolution
DOI		https://doi.org/10.1051/0004-6361:20040289
Published online		16 July 2004

A&A 422, 1045-1052 2004

First detection of a lithium rich carbon star in the Draco dwarf galaxy: Evidence for a young stellar population

I. Domínguez¹, C. Abia¹, O. Straniero², S. Cristallo² and Ya. V. Pavlenko³

¹ Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain e-mail: inma@ugr.es
² INAF-Osservatorio Astronomico di Collurania, 64100 Teramo, Italy
³ Main Astronomical Observatory, National Academy of Sciences, Zabolotnoho 27, Kiev-127 03680, Ukranie

Received: 17 February 2004
Accepted: 29 April 2004

Abstract

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 $\rm log$ ϵ(Li) = $3.5\pm0.4$ 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 $\log~g$ – $\log~T_{\rm{eff}}$ 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/ $^{13}\rm C>40$ , and a constraint for the Ba abundance, namely $\rm 0.5<[Ba/Fe]< 2$ . The proposed scenario also fits these further constraints. Then, we estimate that the mass of D461 ranges between 1.2 and 2 $M_\odot$ , which corresponds to an age ranging between 1 and 3 Gyr. We conclude that this star is more massive and younger than the typical stellar population of Draco.

Key words: stars: carbon / stars: abundances / stars: evolution / stars: AGB and post-AGB / galaxies: local group

© ESO, 2004

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