Speaker
Description
The Pair Instability (PI) boundary was previously thought to be robustly determined by theory as starting at $~50 M_\odot$. Recent Gravitational Wave observations such as GW190521 suggested otherwise, igniting a flurry of theories and predictions, which has had significant follow-on implications for stellar populations in Population II and III environments. Stars above the PI boundary are expected to undergo one or more pulsations before collapse to a black hole, or be fully disrupted by a PISN, enriching the local environment. Stars below the boundary are expected to collapse to black holes without a supernova, thus not enriching the local environment in their final fates and only through wind mass loss, which is expected to be very small at low metallicity. Thus the exact location and progenitor stars of BHs/PI forms an important question in chemical evolution in low metallicity galaxies. By changing stellar evolution parameters such as mass, metallicity, rotation, overshooting, and semiconvection, we have explored the previously theorised PI mass gap with stellar evolution models. We also include mechanical mass loss, which provides an enhancement to the mass loss rates, especially for low metallicity stars. From this we find that the maximum black hole mass could be as high as $90 M_\odot$ under specific conditions, but that with more physically moderate assumptions, the Pop II and Pop III stars could fill the PI gap with black holes in low metallicity environments.
