Nuclear timescale mass transfer in models of supergiant and ultra-luminous X-ray binaries

Understand

We investigate how the proximity of supergiant donor stars to the Eddington-limit, and their advanced evolutionary stage, may influence the evolution of massive and ultra-luminous X-ray binaries with supergiant donor stars (SGXBs and ULXs).

• We construct models of massive stars with different internal hydrogen/helium gradients and different hydrogen-rich envelope masses, and expose them to slow mass loss to probe the response of the stellar radius.
• In addition, we compute the corresponding Roche-lobe overflow mass-transfer evolution with our detailed binary stellar evolution code, approximating the compact objects as point masses.
• We find that a hydrogen/helium gradient in the layers beneath the surface, as it is likely present in the well-studied donor stars of observed SGBXs, can enable nuclear timescale mass-transfer in SGXBs with a BH or a NS accretor, even for mass ratios in excess of 20.