Implications of a new triple-α nuclear reaction rate. Consequences for Cepheids
Résumé
Context. Recently the triple-α reaction rate has been re-evaluated. In the temperature range, 107-108 K, with respect to the NACRE rate, the new rate is enhanced by up to 20 orders-magnitude. Aims: To validate this new rate, we investigate its consequences for the evolution of Cepheid models. Methods: The stellar evolutionary tracks are calculated with the CESAM code and displayed in the domain 4-10 M_ȯ. Results: With the new rate, the first dredge-up does not occur. For masses larger than gse 4.5 M_ȯ each evolutionary track crosses the instability strip only once. The luminosities are higher than with the previous rate, then leading to smaller theoretical masses that better agree with the pulsational mass. Conversely, and inconsistently with one century of observations of more than two hundred Cepheids, the temporal derivative of the period keeps a positive sign. Moreover the observed depletions of atmospheric lithium and C/N ratio do not occur. A slight modification of only a few percents of the new nuclear rate allows us however to restore the loops inside the instability strip and the changes of sign of the temporal derivative of periods. Conclusions: This preliminary work indicates that the new rate may solve some of the long-lasting unsolved theoretical problems of Cepheids. Yet indisputable observations argue against its pertinence. Nonetheless, with regard to its theoretical importance, the triple-α new reaction rate still needs to be confirmed or revisited.
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