Evolution of Massive Main-sequence Stars in Rapid Population Synthesis. I. Framework and Implementation

Stars spend most of their lifetime on the main sequence (MS), where hydrogen burning establishes the internal chemical structure that governs the subsequent evolution. In massive stars, mass loss through winds and binary interactions can significantly modify this structure during the MS. We present a new MS evolution framework suitable for rapid binary population synthesis, implemented in the COMPAS code. Building on the semianalytical model of Shikauchi et al., our framework captures the evolution of the convective core on the MS under arbitrary mass-loss or mass-gain histories, including a treatment for stellar rejuvenation and MS mergers. This new framework yields more massive helium cores at terminal-age MS, more compact radii in stripped MS stars, and systematically higher black hole masses than commonly used prescriptions. By providing a more realistic treatment of MS evolution, this framework improves the physical consistency of massive stars and binary evolution in rapid population synthesis.