Partial mixing and the formation of 13C pockets in AGB stars: effects on the s-process elements

Abstract

The production of the elements heavier than iron via slow neutron captures (the s process) is a main feature of the contribution of asymptotic giant branch (AGB) stars of low mass (< 5 Msun) to the chemistry of the cosmos. However, our understanding of the main neutron source, the 13C(alpha,n)16O reaction, is still incomplete. It is commonly assumed that in AGB stars mixing beyond convective borders drives the formation of 13C pockets. However, there is no agreement on the nature of such mixing and free parameters are present. By means of a parametric model we investigate the impact of different mixing functions on the final s-process abundances in low-mass AGB models. Typically, changing the shape of the mixing function or the mass extent of the region affected by the mixing produce the same results. Variations in the relative abundance distribution of the three s-process peaks (Sr, Ba, and Pb) are generally within +/-0.2 dex, similar to the observational error bars. We conclude that other stellar uncertainties - the effect of rotation and of overshoot into the C-O core - play a more important role than the details of the mixing function. The exception is at low metallicity, where the Pb abundance is significantly affected. In relation to the composition observed in stardust SiC grains from AGB stars, the models are relatively close to the data only when assuming the most extreme variation in the mixing profile.

17 pages, 8 figures, 6 tables, accepted for publications on Monthly Notices of the Royal Astronomical Society

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Cited by in corpus (11)

• The s Process and Beyond
• s-Processing in AGB Stars Revisited. III. Neutron captures from MHD mixing at different metallicities and observational constraints
• Barium stars as tracers of s-process nucleosynthesis in AGB stars I. 28 stars with independently derived AGB mass
• Origin of large meteoritic SiC stardust grains in metal-rich AGB stars
• Unusual neutron-capture nucleosynthesis in a carbon-rich Galactic bulge star
• The most metal-rich stars in the universe: chemical contributions of low and intermediate mass asymptotic giant branch stars with metallicities between $0.04\le Z\le \mathrm{0.10.}$
• The formation of barium giants via mass accretion in binary systems
• Galactic Chemical Evolution of Radioactive Isotopes with an s-process Contribution
• First models of s process in AGB stars of solar metallicity for the stellar evolutionary code ATON with a novel stable explicit numerical solver
• s-process Enriched Post-AGB Star J003643.94-723722.1 in the SMC with an Extreme C/O Ratio and the First Precise Detection of Lead
• Impact of $\text{T}$- and $\rho$-dependent decay rates and new (n,$\gamma$) cross sections on the $\text{s}$ process in low-mass AGB stars