The role of mass transfer efficiency in stability criteria: A Study with SEVN

The fate of binary systems is largely determined by whether mass transfer proceeds stably or leads to a dynamical disruption. A standard approach to evaluate this stability compares how the donor star’s radius and its Roche lobe respond to mass loss. In reality, most population synthesis codes, including those based on Hurley’s prescriptions, assume fully efficient mass transfer, meaning that all the transferred mass is accreted by the companion, and no mass or angular momentum is lost from the system. In this work, we improve this prescription by introducing a new one that incorporates non-conservative mass transfer—allowing for mass and angular momentum loss—to calculate the Roche lobe’s response more realistically. This leads to a more physically consistent criterion for determining the stability of Roche lobe overflow. We implement this improved stability criterion in the SEVN population synthesis code and explore its impact on two distinct types of systems: blue straggler stars and binary black holes.