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 has 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 hydrodynamical simulation of the collision between a core helium burning star (~58 M_⊙) and a main-sequence star (~42 M_⊙). The hydrodynamical simulation allows us to take into account the mass lost during the collision (~12 M_⊙) 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 supergiant star. We estimate a total mass-loss of about 1 M_⊙ during the post-collision evolution, due to stellar winds and shocks induced by neutrino emission in a failed supernova. The final BH mass is ≈87 M_⊙. Therefore, we confirm that the collision scenario is a suitable formation channel to populate the PI mass gap.