s‐Process Nucleosynthesis in Carbon Stars

We present the first detailed and homogeneous analysis of the s-elementcontent in Galactic carbon stars of N-type. Abundances of Sr,Y, Zr (low-masss-elements, or ls) and of Ba, La, Nd, Sm and Ce (high-mass s-elements, hs) arederived using the spectral synthesis technique from high-resolution spectra.The N-stars analyzed are of nearly solar metallicity and show moderates-element enhancements, similar to those found in S stars, but smaller thanthose found in the only previous similar study (Utsumi 1985), and also smallerthan those found in supergiant post-AGB stars. This is in agreement with thepresent understanding of the envelope s-element enrichment in giant stars,which is increasing along the spectral sequence M-->MS-->S-->SC-->C during theAGB phase. We compare the observational data with recent $s$-processnucleosynthesis models for different metallicities and stellar masses. Goodagreement is obtained between low mass AGB star models (M < 3 M_o) ands-elements observations. In low mass AGB stars, the 13C(alpha, n)16O reactionis the main source of neutrons for the s-process; a moderate spread, however,must exist in the abundance of 13C that is burnt in different stars. Bycombining information deriving from the detection of Tc, the infrared coloursand the theoretical relations between stellar mass, metallicity and the finalC/O ratio, we conclude that most (or maybe all) of the N-stars studied in thiswork are intrinsic, thermally-pulsing AGB stars; their abundances are theconsequence of the operation of third dredge-up and are not to be ascribed tomass transfer in binary systems.Comment: 31 pages, 10 figures, 6 tables. Accepted in Ap