Technetium Abundances in S-type AGB Stars to Constrain Mixing and Nucleosynthesis Processes

摘要

Intrinsic S stars are cool giants of similar temperature as M stars but their atmospheres are enriched in carbon and s-process elements due to ongoing nucleosynthesis (e.g. the s-process) and mixing (e.g. partial mixing, third dredge-up) processes during their evolution on the asymptotic giant branch (AGB). However, these processes are subjected to large uncertainties in stellar evolution AGB models. In order to set stronger constraints, derived abundances of Zr and the unstable isotope ~(99)Tc (t_(1/2) = 2.1 × 10~5 yr) are compared with predictions of post-processing AGB models. Tc/Zr and [Zr/Fe] abundances are derived for a sample of 28 intrinsic S stars using dedicated MARCS model atmospheres for S stars. It was found that (1) the derived Zr abundances from lines with laboratory oscillator strengths yielded lower abundances (0.3 dex on average) than those with computed gf-values, and (2) after correcting for this, the derived abundances fit well with low-mass (≤3 M_⊙) model predictions.