Tracing chemical depletion in evolved binaries hosting second-generation transition discs.

Abstract

The mechanisms responsible for chemical depletion across diverse astrophysical environments are not yet fully understood. In this paper, we investigate chemical depletion in post-AGB/post-RGB binary stars hosting second-generation transition discs using high-resolution optical spectra from HERMES/Mercator and UVES/VLT. We performed a detailed chemical abundance analysis of six post-AGB/post-RGB stars and six post-AGB/post-RGB candidates with transition discs in the Galaxy and in the Large Magellanic Cloud. The atmospheric parameters and elemental abundances were obtained through 1D LTE analysis of chemical elements from C to Eu, and 1D NLTE corrections were incorporated for elements from C to Fe. Our results confirmed that depletion efficiency, traced by the [S/Ti] abundance ratio, is higher in post-AGB/post-RGB binaries with transition discs compared to the overall sample of post-AGB/post-RGB binaries. We also examined correlations between derived abundances and binary system parameters (astrometric, photometric, orbital, pulsational). Additionally, we compared the depletion patterns in our sample to those observed in young stars with transition discs and in the interstellar medium. We confirmed that the depletion is significantly stronger in post-AGB/post-RGB binaries with transition discs than in young stars with transition discs. Furthermore, we found that [X/Zn] abundance ratio trends of volatile and refractory elements in post-AGB/post-RGB binaries with transition discs generally resemble similar trends in the interstellar medium (except for trends of [Si/Zn] and [Mg/Zn] ratios). These findings, although based on a limited sample, provide indirect constraints for depletion mechanism in circumbinary discs around post-AGB/post-RGB stars.