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

Abstract

Barium star systems are excellent sites for studying AGB nucleosynthesis, binary evolution, and mass transfer processes. However, an accurate estimation of their fundamental stellar parameters is still lacking. Using TESS data, we made the first extensive asteroseismic mass measurements of 31 Ba giants and 13 Ba dwarfs. For some, we were able to measure $ΔP$, ascertaining their evolutionary phase. We then constructed a grid of stellar models across the relevant mass range, where we accreted AGB material using composition from existing yields. We found that the average masses of the Ba dwarfs and Ba giants are significantly different ($1.29\pm0.09~\rm{M}_\odot$ versus $1.96\pm0.16~\rm{M}_\odot$, respectively). However, their mass distributions peak around $1.3~\rm{M}_\odot$. 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 show an overall anti-correlation with stellar mass, particularly in the low-mass regime. The stellar models adopting Monash AGB yields can satisfactorily reproduce the observed light elements, $s$, and heavy-$s$ abundance trends, with an accreted mass of $0.1-0.5~\rm{M}_\odot$, but fail to explain the [hs/ls] ratio. 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 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 (abridged for arXiv).