Barium stars as tracers of s-process nucleosynthesis in AGB stars

I. 28 stars with independently derived AGB mass

¹ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly Thege M. út, 15–17, 1121 Budapest, Hungary
e-mail: cseh.borbala@csfk.org
² ELTE Eötvös Loránd University, Institute of Physics, Pázmány Péter sétány 1/A, Budapest 1117, Hungary
³ Observatório Nacional/MCTI, Rua General José Cristino, 77 Sao Cristovao, Rio de Janeiro, Brazil
⁴ INAF Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁵ School of Physics and Astronomy, Monash University, VIC 3800, Australia
⁶ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
⁷ Laboratory of Observational Astrophysics, Saint Petersburg State University, Universitetski Pr. 28, 198504 Saint Petersburg, Russia
⁸ Laboratório Nacional de Astrofísica/MCTI, Rua dos Estados Unidos 154, Bairro das Nações, 37504-364 Itajubá, Brazil

Received: 17 October 2021
Accepted: 25 January 2022

Abstract

Context. Barium (Ba) stars are polluted by material enriched in the slow neutron capture (s-process) elements synthesised in the interior of their former asymptotic giant branch (AGB) companion star, which is now a white dwarf.

Aims. We aim to compare individual Ba star abundance patterns to AGB nucleosynthesis model predictions to verify if the AGB model mass is compatible with independently derived AGB mass, which was previously estimated using binary parameters and Gaia parallax data.

Methods. We selected a sample of 28 Ba stars for which both self-consistent spectroscopic observation and analysis were performed and, additionally, stellar mass determinations, via positioning the star on the Hertzsprung-Russell (HR) diagram and comparing with evolutionary tracks are available. For this sample of stars, we considered both previously (Y, Zr, Ce, and Nd) and recently derived (Rb, Sr, Nb, Mo, Ru, La, Sm, and Eu) elemental abundances. Then, we performed a detailed comparison of these s-process elemental abundances to different AGB nucleosynthesis models from the Monash and the FRUITY theoretical data sets. We simplified the binary mass transfer by calculating dilution factors to match the [Ce/Fe] value of each star when using different AGB nucleosynthesis models, and we then compared the diluted model abundances to the complete Ba-star abundance pattern.

Results. Our comparison confirms that low-mass (with initial masses roughly in the range 2−3 M_⊙), non-rotating AGB stellar models with ¹³C as the main neutron source are the polluters of the vast majority of the considered Ba stars. Out of the 28 stars, in 21 cases the models are in good agreement with both the determined abundances and the independently derived AGB mass, although in 16 cases higher observed abundances of Nb, Ru, Mo, and/or Nd, Sm than predicted were present. For three stars, we obtain a match to the abundances only by considering models with masses lower than those independently determined. Finally, four stars show much higher first s-process peak abundance values than the model predictions, which may represent the signature of a physical (e.g. mixing) and/or nucleosynthetic process that is not represented in the set of models considered here.