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

Abstract

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.

23 pages, 19 figures

References in corpus (9)

• Gaia Data Release 2. Summary of the contents and survey properties
• Gaia Data Release 1. Summary of the astrometric, photometric, and survey properties
• Estimating distances from parallaxes IV: Distances to 1.33 billion stars in Gaia Data Release 2
• Stellar yields from metal-rich asymptotic giant branch models
• A critical test of empirical mass loss formulae applied to individual giants and supergiants
• Hertzsprung-Russell diagram and mass distribution of barium stars
• When binaries keep track of recent nucleosynthesis: The Zr - Nb pair in extrinsic stars as an s-process diagnostic
• A mid-IR interferometric survey with MIDI/VLTI: resolving the second-generation protoplanetary disks around post-AGB binaries
• 3D Interstellar Extinction Map within the Nearest Kiloparsec

Cited by in corpus (20)

• Barium & related stars and their white-dwarf companions I. Giant stars
• Low-mass low-metallicity AGB stars as an efficient i-process site explaining CEMP-rs stars
• s-Processing in AGB Stars Revisited. III. Neutron captures from MHD mixing at different metallicities and observational constraints
• Characterisation of Galactic carbon stars and related stars from Gaia-EDR3
• The Gaia-ESO Survey: detection and characterization of single line spectroscopic binaries
• Barium and related stars, and their white-dwarf companions. III. The masses of the white dwarfs
• Binary evolution along the Red Giant Branch with BINSTAR: The barium star perspective
• Observational evidence of third dredge-up occurrence in S-type stars with initial masses around 1 Msun
• Discovery of technetium- and niobium-rich S stars: the case for bitrinsic stars
• The carbon star mystery: forty years later
• S stars and s-process in the Gaia era II. Constraining the luminosity of the third dredge-up with Tc-rich S stars
• Carbon and Oxygen Isotopic Ratios. II. Semiregular Variable M Giants
• S-type stars discovered in Medium-Resolution Spectra of LAMOST DR9
• S-type stars from LAMOST DR10: classification of intrinsic and extrinsic stars
• Weighing stars from birth to death: mass determination methods across the HRD
• S-Process Nucleosynthesis in Chemically Peculiar Binaries
• A comprehensive Gaia view of ellipsoidal and rotational red giant binaries
• Discovery of an optical cocoon tail behind the runaway HD 185806
• Technetium-rich M Stars: Prime diagnostics of recent third dredge-up events on the Asymptotic Giant Branch
• Full abundance study of two newly discovered barium giants