S stars and s-process in the Gaia era II. Constraining the luminosity of the third dredge-up with Tc-rich S stars

Abstract

S stars are late-type giants that are transition objects between M-type stars and carbon stars on the asymptotic giant branch (AGB). They are classified into two types: intrinsic or extrinsic, based on the presence or absence of technetium (Tc). The Tc-rich or intrinsic S stars are thermally-pulsing (TP-)AGB stars internally producing s-process elements (including Tc) which are brought to their surface via the third dredge-up (TDU). Tc-poor or extrinsic S stars gained their s-process overabundances via accretion of s-process-rich material from an AGB companion which has since turned into a dim white dwarf. Our goal is to investigate the evolutionary status of Tc-rich S stars by locating them in a Hertzsprung-Russell (HR) diagram using the results of Gaia early Data Release 3 (EDR3). We combine the current sample of 13 Tc-rich stars with our previous studies of 10 Tc-rich stars to determine the observational onset of the TDU in the metallicity range [-0.7; 0]. We also compare our abundance determinations with dedicated AGB nucleosynthesis predictions. The stellar parameters are derived using an iterative tool which combines HERMES high-resolution spectra, accurate Gaia EDR3 parallaxes, stellar evolution models and tailored MARCS model atmospheres for S-type stars. Using these stellar parameters we determine the heavy-element abundances by line synthesis. In the HR diagram, the intrinsic S stars are located at higher luminosities than the predicted onset of the TDU. These findings are consistent with Tc-rich S stars being genuinely TP-AGB stars. The comparison of the derived s-process abundance profiles of our intrinsic S stars with the nucleosynthesis predictions provide an overall good agreement. Stars with highest [s/Fe] tend to have the highest C/O ratios.

22 pages, 10 figures, accepted for publication in Section 7. Stellar structure and evolution of Astronomy and Astrophysics (A&A)

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Cited by in corpus (12)

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• Barium and related stars, and their white-dwarf companions. III. The masses of the white dwarfs
• A Fresh Look at AGB stars in Galactic Open Clusters with Gaia: Impact on Stellar Models and the Initial-Final Mass Relation
• WIYN Open Cluster Study. LXXXVII. HST Ultraviolet Detection of Hot White Dwarf Companions to Blue Lurkers in M67
• The carbon star mystery: forty years later
• S-type stars from LAMOST DR10: classification of intrinsic and extrinsic stars
• The impact of third dredge-up on the mass loss of Mira variables
• S-Process Nucleosynthesis in Chemically Peculiar Binaries
• The evolutionary state of the red giant star L${}_2$ Puppis
• CGCS 6306, another X-ray-emitting asymptotic giant branch star confirmed to be a symbiotic binary
• Technetium-rich M Stars: Prime diagnostics of recent third dredge-up events on the Asymptotic Giant Branch
• Abundance of radioactive technetium in Przybylski's star revisited