Photospheric Chemical Depletion in Post-AGB/Post-RGB Binaries with Second-Generation Protoplanetary Discs

Abstract

The origin and evolution of chemical elements in the Universe are governed not only by nucleosynthesis processes in stars, but also by mechanisms that alter observed photospheric compositions. Among these, chemical depletion (underabundance of refractory elements in stellar photospheres) presents a key puzzle in understanding the full chemical lifecycle. This PhD thesis explores the role of disc-binary interaction in shaping chemical abundances in evolved low- and intermediate-mass binary stars, focusing on systems that have undergone the red giant branch (RGB) or asymptotic giant branch (AGB) phase. In this thesis, we investigate binary systems containing post-asymptotic giant branch (post-AGB, $L_{\rm post-AGB}\,\gtrsim\,2\,500\,L_\odot$) and post-red giant branch (post-RGB, $L_{\rm post-RGB}\,\lesssim\,2\,500\,L_\odot$) binaries as key tracers of AGB/RGB nucleosynthesis. Although the effects of these interactions remain poorly understood, they are known to drive photospheric chemical depletion. This depletion closely resembles that observed in young planet-hosting stars with protoplanetary discs. Combined with other structural and dynamical similarities in disc properties, this suggests a potential link to second-generation planet formation in post-AGB/RGB binaries with circumbinary discs. Although direct imaging of such planets is not feasible, studying signatures such as photospheric depletion provides an indirect means of exploring their possible presence within these systems.