http://arxiv.org/abs/2004.00628
Metallicity is known to significantly affect the radial expansion of a massive star: the lower the metallicity the more compact the star, especially during its post-MS evolution. We study the impact of this effect in the context of binary evolution. Using the stellar-evolution code MESA, we compute evolutionary tracks of stars at different metallicities, exploring variations of factors known to affect the radial expansion (eg. semiconvection, overshooting, rotation). We find observational support for evolution in which already at metallicity $0.2Z_{\odot}$ massive stars stay relatively compact during the Hertzprung-Gap phase (HG) and most of their expansion happens during core-helium burning (CHeB). Consequently, we show that metallicity has a strong influence on the type of mass transfer evolution in binary systems. At solar metallicity a case-B mass transfer is initiated shortly after the end of MS and a giant donor is almost always a rapidly-expanding HG star. At lower metallicity the parameter space for mass transfer from a more evolved CHeB star increases dramatically. This means that envelope stripping and formation of helium stars in low metallicity environments happens later during the evolution of the donor, implying a much shorter duration of the Wolf-Rayet phase (even by an order of magnitude) and higher final core masses. This metallicity effect is independent of the impact of metallicity-dependent stellar winds. At very low metallicities a significant fraction of massive stars engage in their first episode of mass transfer very late into their evolution, when they already have a well developed CO core. The remaining lifetime ($< 10^4$ yr) is unlikely to be enough to strip the entire H-rich envelope. We also briefly discuss the extremely small parameter space for mass transfer from massive convective-envelope donors in the context of binary black hole merger formation.
J. Klencki, G. Nelemans, A. Istrate, et. al.
Fri, 3 Apr 20
46/62
Comments: 15 pages, 8 figures (+ 4 pages, 4 fig. appendix), accepted for publication in A&A