Formation of black holes in the pair-instability mass gap:
  Hydrodynamical simulation of a massive star collision
  The detection of GW190521 by the LIGO-Virgo collaboration proved the
existence of black holes in the theoretically predicted pair-instability gap of
their mass spectrum. Some recent studies suggest that such massive black holes
could be produced by the collision of an evolved star with a carbon-oxygen core
and a main sequence star. Such a post-coalescence star could end its life
avoiding the pair-instability regime and with a direct collapse of its very
massive envelope. It is still not clear, however, how the collision shapes the
structure of the newly produced star and how much mass is actually lost in the
impact. We investigated this issue by means of hydrodynamical simulations with
the smoothed particle hydrodynamics code StarSmasher, finding that the
collision can remove up to 12% of the initial mass of the colliding stars. This
is a non-negligible percentage of the initial mass and could affect the further
evolution of the stellar remnant, particularly in terms of the final mass of a
possibly forming black hole, if the core avoids the pair-instability regime. We
also found that the main sequence star can plunge down to the outer boundary of
the carbon-oxygen core of the primary, changing the inner chemical composition
of the remnant. The collision expels the outer layers of the primary, leaving a
remnant with an helium-enriched envelope (reaching He fractions of about 0.4 at
the surface). These more complex abundance profiles can be directly used in
stellar evolution simulations of the collision product.
