Barium stars as tracers of s-process nucleosynthesis in AGB stars I. 28 stars with independently derived AGB mass

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 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 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 are available and stellar mass determinations, via positioning the star on the HR diagram and comparing with evolutionary tracks. For this sample 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 13 C 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. For 3 stars we obtain a match to the abundances only by considering models with masses lower than those independently determined. Finally, 4 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.

Origin of the elements

“…More details can be found, e.g. in Karakas and Lugaro (2016); Kobayashi et al (2020); Cseh et al (2022). While we discuss the contribution of planetary nebulae in an integrated way to the inventory of the overall solar system abundances and the abundance evolution of galaxies in later sections, direct proofs for their ejecta composition can be found in the isotopic composition of meteorites, containing inclusions of dust condensates from their winds (Zinner and 2014), copyright by the authors Amari 1999; Lugaro and Gallino 2006;Lugaro and Chieffi 2011;Kobayashi et al 2020;Busso et al 2021), for a more general treatment of meteoritic dust inclusions, containing also the imprint from other sources see Nittler et al (1996); Zinner (1998Zinner ( , 2008bZinner ( , 2008a; Nittler and Ciesla (2016).…”

Learning about AGB stars by studying the stars polluted by their outflows

“…2018) are responsible for the pollution of Ba stars. One could now use this information together with the abundance pattern on HD 76225 and put constraints on mass transfer and dilution mechanisms (as done for example by Stancliffe 2021 andCseh et al 2022). Finally, when the masses of the full sample are available, we will be able to look for correlations between the orbital parameters of Ba star systems and the WD masses and hopefully put constraints on mass-transfer mechanisms too.…”

“…Charbonnel et al 2007;Stancliffe et al 2007;Aoki et al 2008) in mind, one can combine the properties of the two stellar components and the Ba star abundances to learn about nucleosynthesis models as well (e.g. Cseh et al 2022). When one tries to use Ba star observations to constrain models, the largest observational uncertainty comes from the WD companions since they are cool, dim, and directly undetectable in most systems.…”