S stars and s-process in the Gaia era

S. Shetye , S. van Eck (1) , A. Jorissen (1) , H. van Winckel (2) , L. Siess (1) , S. Goriely (3) , A. Escorza , D. Karinkuzhi , Bertrand Plez (4)

1 Institut d'Astronomie et d'Astrophysique
2 Instituut voor Sterrenkunde [Leuven]
3 ULB - Université libre de Bruxelles
4 LUPM - Laboratoire Univers et Particules de Montpellier

S stars are transition objects between M-type giants and carbon stars on the asymptotic giant branch (AGB). They are characterized by overabundances of s-process elements. Roughly half of them are enhanced in technetium (Tc), an s-process element with no stable isotope, while the other half lack technetium. This dichotomy arises from the fact that Tc-rich S stars are intrinsically producing s-process elements and have undergone third dredge-up (TDU) events, while Tc-poor S stars owe their s-process overabundances to a past pollution by a former AGB companion. Our aim is to analyse the abundances of S stars and gain insights into their evolutionary status and on the nucleosynthesis of heavy s-process elements taking place in their interior. In particular, the location of extrinsic and intrinsic S stars in the HR diagram will be compared with the theoretical onset of the TDU on the thermally-pulsing AGB. A sample of 19 S-type stars was analysed by combining HERMES high-resolution spectra, accurate Gaia Data Release 2 (GDR2) parallaxes, stellar-evolution models, and newly-designed MARCS model atmospheres for S-type stars. Combining the derived parameters with GDR2 parallaxes allows a joint analysis of the location of the stars in the Hertzsprung-Russell diagram and of their surface abundances. For all 19 stars, Zr and Nb abundances are derived, complemented by abundances of other s-process elements for the three Tc-rich S stars. These abundances agree within the uncertainties with nucleosynthesis predictions for stars of corresponding mass, metallicity and evolutionary stage. Most extrinsic S stars lie close to the tip of the red giant branch (RGB), and a few are located along the early AGB. The location of intrinsic S stars in the HR diagram is compatible with them being thermally-pulsing AGB stars.

Domaines: Astrophysique stellaire et solaire [astro-ph.SR]

S. Shetye, S. van Eck, A. Jorissen, H. van Winckel, L. Siess, et al.. S stars and s-process in the Gaia era: I. Stellar parameters and chemical abundances in a sub-sample of S stars with new MARCS model atmospheres. Astronomy and Astrophysics - A&A, 2018, 620, pp.A148. ⟨10.1051/0004-6361/201833298⟩. ⟨hal-01971966⟩