  The detection of the binary black hole merger GW190521, with primary black
hole mass $85^{+21}_{-14}$ ${\rm M}_{\odot}$, proved the existence of black
holes in the theoretically predicted pair-instability gap ($\sim60-120 \, {\rm
M}_{\odot}$) 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 {\sc
StarSmasher}, finding that a head-on 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. We
also found that the main sequence star can plunge down to the outer boundary of
the 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.
