Formation of black holes in the pair-instability mass gap: evolution of
  a post-collision star
  The detection of GW190521 by the LIGO-Virgo collaboration revealed the
existence of black holes (BHs) in the pair-instability (PI) mass gap. Here, we
investigate the formation of BHs in the PI mass gap via star -- star collisions
in young stellar clusters. To avoid PI, the stellar-collision product must have
a relatively small core and a massive envelope. We generate our initial
conditions from the outputs of a hydro-dynamical simulation of the collision
between a core helium burning star ($\sim 58$ M$_\odot$) and a main-sequence
star ($\sim 42$ M$_\odot$). The hydro-dynamical simulation allows us to take
into account the mass lost during the collision ($\sim 12$ M$_\odot$) and to
build the chemical composition profile of the post-collision star. We then
evolve the collision product with the stellar evolution codes PARSEC and MESA.
We find that the post-collision star evolves through all the stellar burning
phases until core collapse, avoiding PI. At the onset of core collapse, the
post-collision product is a blue super-giant star. We estimate a total mass
loss of about 1 M$_\odot$ during the post-collision evolution, due to stellar
winds and shocks induced by neutrino emission in a failed supernova. The final
BH mass is $\approx{87}$ M$_\odot$. Therefore, we confirm that the collision
scenario is a suitable formation channel to populate the PI mass gap.
