THE STELLAR FOUNTAIN OF YOUTH: CORE REJUVENATION IN VERY MASSIVE STARS

Abstract

Binary systems with a sufficiently small orbital separation undergo stable mass transfer when the donor overflows its Roche lobe. In response, the accretor gains mass which causes the growth of the convective core, consequently refueling of the burning region and elongation of the lifespan in a process known as ``core rejuvenation''. Although dependent on assumptions about convective boundary mixing, core rejuvenation has been shown in accretors up to initial masses $30\,M_\odot$. However, more massive stars approach constant lifetimes and contain a steeper hydrogen gradient, which may inhibit core rejuvenation. We use computational models with the 1D stellar evolution code Modules for Experiments in Stellar Astrophysics (\textsc{MESA}) to (1) determine the radial expansion of very massive stars during main sequence, (2) estimate typical mass transfer rates, and (3) study whether rejuvenation occurs for initial accretor masses exceeding $30\,M_\odot$. We find that when asing convective boundary mixing informed by the width of the main sequence in the 30 Doradus region, case A mass transfer dominates for donor masses $ \gtrsim 75\,M_\odot$. We also find that core rejuvenation occurs in stars of each mass, metallicity, and accretion rate tested. These results could significantly impact the rates of all post mass transfer binaries, from Wolf–Rayet-O-type binaries, to X-ray binaries and gravitational wave progenitors.