s-process nucleosynthesis in carbon stars

We present the first detailed and homogeneous analysis of the s-element content in Galactic carbon stars of N type. Abundances of Sr, Y, Zr (low-mass s-elements, or ls), Ba, La, Nd, Sm, and Ce (high-mass s-elements, or hs) are derived using the spectral synthesis technique from high-resolution spectra. The N stars analyzed are of nearly solar metallicity and show moderate s-element enhancements, similar to those found in S stars, but smaller than those found in the only previous similar study (Utsumi 1985), and also smaller than those found in supergiant post asymptotic giant branch (post-AGB) stars. This is in agreement with the present understanding of the envelope s-element enrichment in giant stars, which is increasing along the spectral sequence M --> MS --> S --> SC --> C during the AGB phase. We compare the observational data with recent s-process nucleosynthesis models for different metallicities and stellar masses. Good agreement is obtained between low-mass AGB star models (M less than or similar to 3 M.) and s-element observations. In low-mass AGB stars, the C-13(alpha, n)O-16 reaction is the main source of neutrons for the s-process; a moderate spread, however, must exist in the abundance of C-13 that is burnt in different stars. By combining information deriving from the detection of Tc, the infrared colors, and the theoretical relations between stellar mass, metallicity, and the final C/O ratio, we conclude that most (or maybe all) of the N stars studied in this work are intrinsic, thermally pulsing AGB stars; their abundances are the consequence of the operation of third dredge-up and are not to be ascribed to mass transfer in binary systems.