The s-process in stellar population synthesis: a new approach to understanding AGB stars

Abstract

Thermally pulsating asymptotic giant branch (AGB) stars are the main producers of slow neutron capture (s-) process elements, but there are still large uncertainties associated with the formation of the main neutron source, 13C, and with the physics of these stars in general. Observations of s-process element enhancements in stars can be used as constraints on theoretical models. For the first time we apply stellar population synthesis to the problem of s-process nucleosynthesis in AGB stars, in order to derive constraints on free parameters describing the physics behind the third dredge-up and the properties of the neutron source. We utilize a rapid evolution and nucleosynthesis code to synthesize different populations of s-enhanced stars, and compare them to their observational counterparts to find out for which values of the free parameters in the code the synthetic populations fit best to the observed populations. These free parameters are the amount of third dredge-up, the minimum core mass for third dredge-up, the effectiveness of 13C as a source of neutrons and the size in mass of the 13C pocket. We find that galactic disk objects are reproduced by a spread of a factor of two in the effectiveness of the 13C neutron source. Lower metallicity objects can be reproduced only by lowering by at least a factor of 3 the average value of the effectiveness of the 13C neutron source needed for the galactic disk objects. Using observations of s-process elements in post-AGB stars as constraints we find that dredge-up has to start at a lower core mass than predicted by current theoretical models, that it has to be substantial ($\lambda$ >~ 0.2) in stars with mass M <~ 1.5 M_sun and that the mass of the 13C pocket must be about 1/40 that of the intershell region.

16 pages, 15 figures, accepted for publication in Astronomy & Astrophysics

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Cited by in corpus (10)

• The s process in asymptotic giant branch stars of low metallicity and the composition of carbon-enhanced metal-poor stars
• The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models
• Wind Roche-lobe overflow: Application to carbon-enhanced metal-poor stars
• Population Synthesis of Binary Carbon-enhanced Metal-poor Stars
• s-Process in Low Metallicity Stars. III. Individual analysis of CEMP-s and CEMP-s/r with AGB models
• Orbital eccentricities of binary systems with a former AGB star
• Origin of the early-type R stars: a binary-merger solution to a century-old problem?
• Infrared Photometry and Evolution of Mass-Losing AGB Stars. II. Luminosity and Colors of MS and S Stars
• SALT reveals the barium central star of the planetary nebula Hen 2-39
• Nucleosynthesis in helium-enriched asymptotic giant branch models: Implications for Heavy Element Enrichment in Omega Centauri