Photospheric chemical depletion in post-AGB/post-RGB binaries with second-generation protoplanetary discs

<p dir="ltr">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_post-AGB ≳ 2 500L_⊙) and post-red giant branch (post-RGB, L_post-RGB ≲ 2 500L⊙) 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. </p><p dir="ltr">This thesis investigates the depletion mechanisms in post-AGB and post-RGB binaries by analysing high-resolution optical and near-infrared spectra across a diverse sample of full, transition, and dust-poor disc systems. By examining these disc types, we explore how disc structure influences photospheric chemical depletion and nucleosynthesis signatures in evolved binaries. Previous abundance studies of these binaries have often been piecemeal, typically assuming local thermodynamic equilibrium (LTE), which does not always hold for hot, metal-poor giants. This thesis presents the first homogeneous abundance analysis of post-AGB and post-RGB binaries, incorporating NLTE corrections and a multiwavelength approach to derive atmospheric parameters, elemental abundances, isotopic ratios, and depletion profiles. The multiwavelength approach, particularly near-infrared spectra, enables the accurate determination of CNO isotopic ratios through molecular bands such as CO, CN, and OH, as well as a comprehensive analysis of elemental abundances. Since existing spectral analysis codes are inadequate for post-AGB and post-RGB binaries, we developed E-iSpec, a specialised tool designed for evolved stars. E-iSpec was applied to conduct abundance analyses of post-AGB and post-RGB binaries in both the Galaxy and the Large Magellanic Cloud, facilitating a systematic investigation of depletion profiles across different metallicity environments. </p><p dir="ltr">We demonstrate that post-RGB binaries exhibit depletion profiles similar to those of post-AGB binaries, yet with a higher onset temperature of depletion. The first carbon isotopic ratios for post-RGB stars were measured, confirming consistency with single-star nucleosynthesis predictions. We further show that depletion efficiency is significantly enhanced in post-AGB and post-RGB binaries with transition discs, with depletion profiles resembling those observed in the ISM and being stronger than depletion profiles in young stars with transition discs. Finally, our analysis of dust-poor discs reveals a bimodal distribution in depletion onset temperature, allowing us to classify these systems into full-like and transition-like dust-poor discs. This suggests a possible evolutionary pathway from full and transition discs to dust-poor discs over time. These findings provide a systematic framework for understanding photospheric depletion in evolved binaries, offering indirect constraints on circumbinary disc evolution and second-generation planet formation. Future work integrating larger statistical samples and advanced modelling will further refine our understanding of these complex systems. </p><p dir="ltr">The thesis is structured as follows: In Chapter 1, we provide an overview of nucleosynthetic enrichment and chemical depletion in the Universe, and introduce photospheric chemical depletion in post-AGB/post-RGB binary systems with second-generation protoplanetary discs. In addition, we outline the motivation for the study and the gaps in understanding that this thesis aims to address. In Chapter 2, we provide an overview of spectral analysis techniques and the spectroscopic data used in this thesis, as well as the development of the E-iSpec code. In Chapter 3, we present an abundance analysis of two dusty post-RGB binaries, SZ Mon and DF Cyg, and compare the derived abundances with predictions from the ATON evolutionary models. In Chapter 4, we investigate photospheric depletion in 12 post-AGB/post-RGB binaries with transition discs and compare the depletion efficiency in these targets with depletion efficiencies reported in the ISM and in the young planet-hosting stars. In Chapter 5, we explore the evolutionary status of these discs and their relation to full and transition discs by analysing 9 post-AGB/post-RGB binaries with dust-poor discs. In Chapter 6, we summarise the key findings of this thesis and outline future prospects in binary evolution, disc chemistry, and planet formation.</p>