The observable consequences of main-sequence mass loss for massive stars

Stars spend most of their lifetimes on the main sequence (MS), fusing hydrogen into helium. This phase is particularly important for massive stars, which can lose a significant amount of mass through winds and binary interactions — processes that influence their subsequent evolution and the transients they produce. LSST will observe millions of transients influenced by binary evolution, including core-collapse supernovae and luminous red novae, making accurate modeling essential. Many rapid binary population synthesis codes systematically underestimate the core mass during the MS relative to detailed 1D models. We present a new framework for modeling convective core mass evolution under mass loss and accretion, implemented in the binary population synthesis code COMPAS. Our approach yields higher post-MS core masses and black hole masses, improving agreement with observed populations of high-mass X-ray binaries, and enabling more realistic predictions of LSST-observable transients.