Relation between variability, mass loss and binarity in Asymptotic Giant Branch stars

This work investigates the physical conditions leading to the development of Long Secondary Periods (LSPs) in Asymptotic Giant Branch (AGB) stars. These objects, characterized by low surface gravity, high luminosity, and large radii, undergo significant mass loss via winds driven by a combination of pulsation and radiative acceleration of dust grains. The connection between pulsation and mass-loss is explored through the transition from OGLE Small Amplitude Red Giants (OSARGs) to Semi-Regular Variables (SRVs), two types of variable AGB stars, which is marked by a discontinuity between sequences B and C′ in the Period-Luminosity (PL) diagram. This transition correlates with the onset of LSPs and a sudden increase in mass-loss rates. Studies from Goldberg et al. (2024); Soszyński et al. (2021) suggest an important role of binary interaction in shaping these phenomena. In this thesis, I suggest that a slow wind regime could facilitate the interaction between the dusty AGB wind and the binary companion, thus developing the LSP phenomenon. Hydrodynamical simulations from Chen et al. (2020) and Saladino, M. I. et al. (2019) demonstrate that slow wind regimes promote the formation of circumbinary disks and enhance mass transfer efficiency, especially when the ratio of wind velocity to orbital velocity is low. Furthermore, Winters et al. (2000) identify a slow-wind regime driven primarily by mechanical pulsations in the absence of dust, that matches the low mass-loss rates observed prior to the OSARG–SRV transition. Together, these findings indicate that slow winds may create favorable conditions for binary interactions and the onset of LSP variability.