s‐Process Nucleosynthesis in Carbon Stars

Abstract

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 selements, or ls) and of Ba, La, Nd, Sm and Ce (high-mass s-elements, 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-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 3M ⊙ ) and s-elements observations. In low mass AGB stars, the 13 C(α, n) 16 O reaction is the main source of neutrons for the s-process; a moderate spread, however, must exist in the abundance of 13 C that is burnt in different stars. By combining information deriving from the detection of Tc, the infrared colours 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.

Effects of nuclear cross sections on19F nucleosynthesis at low metallicities

“…3), owing to the higher temperatures attained by these stars. Moreover, the expected surface s-process distributions of intermediate AGB stars are characterized by low or even negative [hs/ls] indexes and high [Rb/Sr] ratios, in striking contrast to the observed spectra (Abia et al 2002).…”

“…This model is representative of low-mass AGB stars, i.e. the most promising candidates to reproduce the majority of the observed s-process distributions in AGB stars (see, e.g., Abia et al 2002). Very similar results can be obtained for different metallicities or slightly different masses (in the range 1.5−2.5 M ).…”

Short-lived nuclei in the early Solar System: Possible AGB sources

“…This choice corresponds to 4 × 10 −6 M ⊙ of 13 C in LMS and to 4 × 10 −7 M ⊙ of 13 C in IMS (see Section 3). We note that the existence of a wide intrinsic spread of 13 C concentrations, from a few 10 −6 M ⊙ down to the complete absence of the 13 C pocket in the intershell zone, is demonstrated by observations of a wide range of s-process enhancements at a given metallicity in AGB stars [205] [206] [207]. This is also shown by the isotopic patterns in presolar grains (see e.g.…”

Post-AGB stars in the SMC as tracers of stellar evolution: the extreme s-process enrichment of the 21 μm star J004441.04-732136.4

“…This is mainly 12 C as the primary product of the triple alpha reaction, but also the products of neutron-capture synthesis. There are two main neutron sources in AGB stars: 1) the 22 Ne(α, n) 25 Mg reaction which is activated at temperatures of T 300 × 10 6 K, and 2) the 13 C(α, n) 16 O reaction, which is activated at much lower temperatures of T 90 × 10 6 K. Observational and theoretical evidence has shown that the 13 C(α, n) 16 O reaction is the main neutron source in low-mass AGB stars of ≈1−3 M (Straniero et al 1995;Gallino et al 1998;Abia et al 2002).…”