s-process Enriched Evolved Binaries in the Galaxy and the Magellanic Clouds

Abstract: Post-asymptotic giant branch stars (post-AGB) in binary systems, with typical orbital periods between ~100 to ~1000 days, result from a poorly understood interaction that terminates their precursory AGB phase. The majority of these binaries display a photospheric anomaly called 'chemical depletion', thought to arise from an interaction between the circumbinary disc and the post-AGB star, leading to the reaccretion of pure gas onto the star, devoid of refractory elements due to dust formation. In this paper, we focus on a subset of chemically peculiar binary post-AGBs in the Galaxy and the Magellanic Clouds (MCs) whose high-resolution optical spectroscopic study revealed a carbon and s-process enrichment, contrary to the commonly observed photospheric chemical depletion. Using spectral energy distribution (SED) fitting and period-luminosity-colour (PLC) relation methods, we determine the luminosity of the targets ($2700-8300L_{\odot}$), which enables confirmation of their evolutionary phase and estimation of initial masses (as a function of metallicity) ($1-2.5M_{\odot}$). In conjunction with predictions from dedicated ATON stellar evolutionary models, our results indicate a predominant intrinsic enrichment of carbon and s-process elements in our binary post-AGB targets. We qualitatively rule out extrinsic enrichment and inherited s-process enrichment from the host galaxy as plausible explanations for the observed overabundances. Our chemically peculiar subset of intrinsic carbon and s-process enriched binary post-AGBs also hints at potential variation in the efficiency of chemical depletion between stars with C-rich and O-rich circumbinary disc chemistries. However, critical observational studies of circumbinary disc chemistry are necessary to address gaps in our current understanding of disc-binary interactions inducing chemical depletion in binary post-AGB systems.