Formation of black holes in the pair-instability mass gap: evolution of a post-collision star

Understand

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 hydro-dynamical simulation of the collision between a core helium burning star ($\sim 58$ M$_\odot$) and a main-sequence star ($\sim 42$ M$_\odot$).