S stars and s-process in the Gaia era

Abstract

Context. 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) that are brought to their surface via the third dredge-up (TDU). The Tc-poor or extrinsic S stars gained their s-process overabundances via the accretion of s-process-rich material from an AGB companion that has since turned into a dim white dwarf. Aims. 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. Methods. We derived the stellar parameters using an iterative tool that 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 determined the heavy-element abundances by line synthesis. Results. 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 genuine 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.