Chemical analysis of carbon stars in the Local Group - II. The Carina dwarf spheroidal galaxy
Résumé
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. 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. 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]$\sim -1.8$ 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]$=+1.6$, $+1.5$, respectively. These enhancements and the derived $s$-process indexes [ls/Fe], [hs/Fe] and [hs/ls] are compatible with theoretical $s$-process nucleosynthesis predictions in low mass AGB stars ($\sim 1.5$ \Mo) on the basis that the $^{13}$C($\alpha$,n)$^{16}$O is the main source of neutrons. Furthermore, the analysis of C$_2$ and CN bands reveals a large carbon enhancement (C/O $\sim 7$ and 5, respectively), much larger than the values typically found in galactic AGB carbon stars (C/O $\sim 1-2$). 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.
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