The evolution of accretor stars in binary systems due to accretion of increasingly helium-rich material

ABSTRACT The recent discovery of examples of intermediate-mass helium stars has offered new insights into interacting binaries. These observations will allow significant improvements in our understanding of helium stars. However, in the creation of these stars, their companions may accrete a significant amount of helium-rich stellar material. This leads to stars with unusual composition profiles – stars with helium-rich cores, hydrogen-rich lower envelopes, and a helium-rich outer envelope. Thus, the mean molecular weight reaches a minimum in the middle of the star rather than continuously decreasing outwards in mass. To demonstrate this structure, we present Cambridge stars model calculations of an example interacting binary system where helium-rich material is transferred, and compare it to one where the composition of the accreted mass is fixed to the companion’s surface composition. We show that the helium-rich material leads to the accretor being 0.15 dex hotter and 0.1 dex more luminous than models where the composition is not helium-rich. If we allow for thermohaline mixing, we find that the luminosity difference remains, but the accretor is now only 0.07 dex hotter. We use a simple bpass v2.2 population model to estimate that helium-rich mass transfer occurs in 23 per cent of massive binaries that undergo mass transfer. This suggests that this is a common process. This binary process has implications for the discrepancy between spectroscopic and gravitational masses of stars and may answer some open questions in stellar astrophysics.