Analysis of Two Carbon Stars in the Carina Dwarf Spheroidal Galaxy

摘要

We present a chemical analysis of two metal-poor carbon stars belonging to the Carina Dwarf Spheroidal galaxy using high resolution optical spectra obtained with the VLT/UVES instrument. Their derived properties and chemical composition are compatible with these stars being in the TP-AGB phase undergoing third dredge-up episodes, although their extrinsic nature cannot be definitively discarded. The analysis indicates that these stars have large s-element enhancements, which are compatible with theoretical s-process nucleosynthesis predictions in low-mass AGB stars (~1.5M), and also large carbon enhancements (C/O5), much larger than the values typically found in galactic AGB carbon stars (C/O~1–2). This is also in agreement with the theoretical prediction that carbon stars are formed more easily through third dredge-up episodes as the initial stellar metallicity drops. We speculate on the possibility that the abundance pattern found in the low-mass s-elements (Y, Zr) may be revealing the existence of some chemical depletion into dust grains.