Unveiling the nature of barium stars: I. Asteroseismic masses and the evolutionary link between Ba dwarfs and giants

Abstract

Context. Barium star systems are excellent sites for studying asymptotic giant branch (AGB) nucleosynthesis, binary evolution, and mass transfer processes. However, an accurate estimation of their fundamental stellar parameters is still lacking. Aims. We measure accurate and precise masses of Ba stars using asteroseismology. This enables us to constrain the nature, origin, and evolution of these binary systems. Methods. Using data from the Transiting Exoplanet Survey Satellite, we made the first extensive asteroseismic mass measurements of Ba stars. Our sample comprises 31 Ba giants and 13 Ba dwarfs. For some, we were able to measure ∆P, thereby ascertaining their evolutionary phase. With reliable asteroseismic masses, we then constructed a grid of stellar models across the relevant mass range, where we accreted AGB material using composition from existing yields. Results. We found that the average masses of the Ba dwarfs and Ba giants are significantly different (1.29 ± 0.09 M⊙ versus 1.96 ± 0.16 M⊙, respectively; with typical individual mass uncertainties of ~10%). However, their mass distributions peak at about the same mass (~1.3 M⊙). While our sample of Ba giants spans the low- and intermediate-mass regime, we found no intermediate-mass Ba dwarfs. The abundance trends of s-process elements ([s/Fe], [hs/Fe], and [ls/Fe]) show an overall anti-correlation with stellar mass, particularly in the low-mass regime (<2 M⊙), for giants and dwarfs. The stellar models adopting Monash AGB yields can satisfactorily reproduce the observed light elements, s, and heavy-s abundance trends simultaneously, with an accreted mass of 0.1--0.5 M⊙ for the majority of the Ba stars. However, the models fail to explain the light-s abundances and, consequently, the [hs/ls] ratio. We found that most Ba stars had AGB companions in the mass range 1--4 M⊙. Conclusions. Our results support an evolutionary scenario in which Ba giants evolve from Ba dwarfs, with mass accretion occurring while the progenitor Ba star is still on the main sequence. In this scenario, a substantial number of intermediate-mass Ba dwarfs are expected. We argue that they remain undetected due to observational bias. We found that post-accretion additional mixing in our models is critical to explain the observed s-process abundances in Ba dwarfs and the low C isotopic ratio (<30) in Ba giants. The mismatch between the model and the observed [hs/ls] ratio suggests that the chemical enrichment of Ba stars cannot be explained by standard single-star AGB yields alone. This may be due to (i) their binary nature altering AGB evolution, (ii) missing or modified nucleosynthesis processes in AGB models, or (iii) additional sources of pollution.