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Free keywords:
Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE, Astrophysics, Solar and Stellar Astrophysics, astro-ph.SR
Abstract:
We have devised fitting formulae for evolution tracks of massive stars with
$8 \lesssim M/M_\odot \lesssim 160$ under extreme metal poor (EMP) environments
for $\log (Z/Z_\odot) = -2, -4, -5, -6$, and $-8$, where $M_\odot$ and
$Z_\odot$ are the solar mass and metallicity, respectively. Our fitting
formulae are based on reference stellar models which we have newly obtained by
simulating the time evolutions of EMP stars. Our fitting formulae take into
account stars ending with blue supergiant (BSG) stars, and stars skipping
Hertzsprung gap (HG) phases and blue loops, which are characteristics of
massive EMP stars. In our fitting formulae, stars may remain BSG stars when
they finish their core Helium burning (CHeB) phases. Our fitting formulae are
in good agreement with our stellar evolution models. We can use these fitting
formulae on the SSE, BSE, NBODY4, and NBODY6 codes, which are widely used for
population synthesis calculations and star cluster simulations. These fitting
formulae should be useful to make theoretical templates of binary black holes
formed under EMP environments.
