Contributed Talk - Splinter HotStars

Thursday, 15 September 2022, 14:05   (SFG 1030 / virtual HotStars)

Nuclear-timescale reverse Algol evolution and hydrogen-rich Wolf-Rayet stars from very massive binaries

Koushik Sen, Norbert Langer, Daniel Pauli, Abel Schootemeijer
Argelander-Institut für Astronomie, Universität Bonn; Max-Planck-Institut für Radioastronomie; Institut für Physik und Astronomie, Universität Potsdam

Algol binary stars are short-period semi-detached systems where the less massive yet Roche-lobe filling donor star transfers mass to its more massive companion on the nuclear timescale, while both stars undergo core hydrogen burning. This picture has been verified in detail, both observationally and through binary evolution models. Here, we find that a large fraction of main-sequence binary models with initial donor masses above 35 Msun may undergo nuclear timescale mass transfer in the opposite direction, where the more massive star donates mass to a less massive companion. This reverse Algol evolution occurs as a consequence of the large convective core mass fraction of very massive stars, and because in very massive binaries mass transfer may be stable even for small initial mass ratios. While still being very hydrogen-rich, the mass donors in reverse Algols have an elevated luminosity-to-mass ratio and may thus develop dense winds with Wolf-Rayet characteristics. When their companion is the less massive binary component, these systems may be hard to identify as binaries. Observations of hydrogen-rich luminous WN stars and of massive semi-detached binaries provide evidence for the reverse Algol binary evolution channel in high mass binaries. We discuss implications for the mass distribution and evolution of high-mass black hole binaries.