Massive donors in interacting binaries: impact of metallicity. (arXiv:2004.00628v1 [astro-ph.SR])

Massive donors in interacting binaries: impact of metallicity. (arXiv:2004.00628v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Klencki_J/0/1/0/all/0/1">Jakub Klencki</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nelemans_G/0/1/0/all/0/1">Gijs Nelemans</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Istrate_A/0/1/0/all/0/1">Alina Istrate</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pols_O/0/1/0/all/0/1">Onno Pols</a>

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.

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.

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