Composición química y estado evolutivo de las estrellas de carbono de tipo espectral r

Résumé

In this work we have performed a detailed study of 23 galactic early and late-type R carbon stars, The sample was selected from the $Hipparcos$ catalogue of cool stars with reprocessed parallaxes according to Knapp et al. (2001). We have analysed their kinematics, photometric and chemical properties, this later based on high-resolution (R $\sim$ 20,000--40,000), high signal-to-noise ratio spectra. For the chemical analysis we use the spectral synthesis technique in the one-dimensional LTE approximation and the state of the art of carbon-rich spherical model atmospheres. Their location in the Galaxy and kinematics properties show that the late-type R stars belong to the galactic thin disk, while the early-type to the thick disk. This implies that the two types of R stars constitute different stellar populations, the late-type being typically more massive and younger than the early-type ones. In the chemical analysis, we derive the C/O and $^{12}$C/$^{13}$C ratios, average metallicity, lithium and $s-$element abundances (including technetium) and, in some stars, the absolute carbon, nitrogen and oxygen abundances, independently. From the abundance patterns obtained, kinematics and photometric characteristics we conclude that i) the late-type R stars are identical to the normal (N-type) AGB carbon stars, ii) a significant number of the early-type R stars are misclassified K giants or carbon stars of CH-type and, iii) for the remaining {\it true} early-type R stars, our chemical analysis confirms the previous by Dominy (1984), i.e.: they have near solar metallicity, N enhanced, C/O ratios slightly larger than one, low $^{12}$C/$^{13}$C ratios and no $s-$element enhancements. We suggest, despite that our stellar sample is small, that the fraction of the real R-type stars among all giant carbon stars types seems to be significantly lower than previously thought and, thus, that they do not constitute a frequent stage during the evolution of low mass stars. The observed characteristics of the early-type R are discussed in the framework of the proposed scenarios for their formation: pollution of primordial origin and non-standard carbon mixing triggered by an anomalous He-\it{flash}, whether as single stars or as a consequence of the coalescence of two degenerate He cores in a binary system. While the observational evidence clearly discard the former scenario, our preliminary smooth particle hydrodynamic simulations of the merging and one dimension hydrostatic calculations, do not result in such carbon mixing. This still keeps the origin of the early-type R stars both a mystery and a challenge for modern stellar evolution. Complementary, we study the chemical composition of three galactic carbon stars of SC-type. We find a good agreement with previous chemical analysis by Abia & Wallerstein (1998) except in the average metallicity, which is $\sim$ 0.4 dex lower on average in the present analysis. For the stars in the sample with some $s-$element enhancements (late-R and the reclassified stars as CH-type), the abundance patterns nicely agree with the theoretical $s-$process nucleosynthesis predictions in low-mass AGB stars where the $^{13}$C$(\alpha,n)^{16}$O reaction is the main source of neutrons. Nevertheless, the $s-$process abundance pattern derived in the SC stars put some doubts on their location in the spectral sequence M$\rightarrow$MS$\rightarrow$S$\rightarrow$SC$\rightarrow$C(N) along the AGB phase. We suggest that SC stars are intermediate-mass stars (M $>3$ M$_\odot$) in the later stages of the AGB phase that become carbon rich for a short period of time. Detailed nucleosynthesis calculations in intermediate-mass stars are needed to confirm such hypothesis.