Impact of accretion on the evolution and surface chemistry of post-AGB binary stars

Post-asymptotic giant branch (post-AGB) binaries are surrounded by large dusty circumbinary disks, formed from recent mass loss. Interactions, namely accretion, between the binary and disk significantly alter the binary orbit and produce a distinct photospheric feature in the post-AGB star known as chemical depletion. Despite recent efforts, many gaps remain in our knowledge of post- AGB binary evolution. To better understand this fascinating class of objects, we use the open-source code MESA to simulate the evolution of post-AGB stars with accretion of metal-poor gas. We study the formation of diverse depletion patterns for a sample of six post-AGB binary stars, and explore the impact of accretion on post-AGB evolution. Further, we assess whether an extended post- AGB phase may explain the observed lack of strongly-ionised planetary nebulae (PNe) in many post-AGB binaries. We find high accretion rates of ∼10⁻⁷ −10⁻⁶ M_⊙yr⁻¹ necessary to reproduce observed depletion levels within short post-AGB timescales. Given their faster evolution, post- AGB stars with 𝑀_c ≥ 0.60M_⊙ require higher initial accretion rates (≳ 5×10⁻⁷M_⊙yr⁻¹) to become significantly depleted. High disk masses of ∼ 10⁻² M_⊙ are further needed to explain depletion, particularly at lower effective temperatures (< 5000 K). While we successfully model saturated depletion patterns, we are unable to reproduce plateaued patterns, warranting a revision of the disk model. Further, at the high rates required for depletion, accretion significantly impacts post-AGB evolution, extending the post-AGB phase by factors of between two and ten. Despite this, all systems are found capable of forming an ionised PN, suggesting accretion is not responsible for their sparsity amongst post-AGB binaries. We conclude our knowledge of disk-binary interactions is incomplete, with additional unknown mechanisms clearly at play. This thesis makes an important step forward in understanding the formation of depletion patterns and the effects of accretion on post-AGB stars, setting the stage for future work in this dynamic field.