Chemical analysis of carbon stars in the Local Group^*

II. The Carina dwarf spheroidal galaxy

¹ Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18 071 Granada, Spain e-mail: cabia@ugr.es
² Observatoire de la Côte d'Azur, Dpt. Cassiopée UMR6 202, 06 304 Nice Cedex 4, France
³ Department of Astronomy and Space Physics, Box 515, 75120 Uppsala, Sweden

Received: 21 November 2007
Accepted: 8 January 2008

Abstract

Aims.We present new results of our ongoing chemical study of carbon stars in Local Group galaxies to test the critical dependence of s-process nucleosynthesis on the stellar metallicity.

Methods.We collected optical spectra with the VLT/UVES instrument of two carbon stars found in the Carina Dwarf Spheroidal (dSph) galaxy, namely ALW-C6 and ALW-C7. We performed a full chemical analysis using the new generation of hydrostatic, spherically symmetric carbon-rich model atmospheres and the spectral synthesis method in LTE.

Results.The luminosities, atmosphere parameters and chemical composition of ALW-C6 and ALW-C7 are compatible with these stars being in the TP-AGB phase undergoing third dredge-up episodes, although their extrinsic nature (external pollution in a binary stellar system) cannot be definitively excluded. Our chemical analysis shows that the metallicity of both stars agree with the average metallicity ([Fe/H] ~ -1.8 dex) previously derived for this satellite galaxy from the analysis of both low resolution spectra of RGB stars and the observed colour magnitude diagrams. ALW-C6 and ALW-C7 present strong s-element enhancements, [ s/Fe] , +1.5, respectively. These enhancements and the derived s-process indexes [ ls/Fe] , [ hs/Fe] and [ ] are compatible with theoretical s-process nucleosynthesis predictions in low mass AGB stars (~) on the basis that the ¹³C()¹⁶O is the main source of neutrons. Furthermore, the analysis of C₂ and CN bands reveals a large carbon enhancement (C/O ~ 7 and 5, respectively), much larger than the values typically found in galactic AGB carbon stars (C/O ~ ). This is also in agreement with the theoretical prediction that AGB carbon stars are formed more easily through third dredge-up episodes as the initial stellar metallicity drops. However, theoretical low-mass AGB models apparently fail to simultaneously fit the observed s-element and carbon enhancements. On the other hand, Zr is found to be less enhanced in ALW-C7 compared to the other elements belonging to the same s-peak. Although the abundance errors are large, the fact that in this star the abundance of Ti (which has a similar condensation temperature to Zr) seems also to be lower than those of others metals, may indicate the existence of some depletion into dust-grains in its photosphere.