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

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 ϵ(Li) =  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 – 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/, and a constraint for the Ba abundance, namely . The proposed scenario also fits these further constraints. Then, we estimate that the mass of D461 ranges between 1.2 and 2 , 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.