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2021-09-27, 15:00: Dr. Erin Higgins (Armagh Observatory & Planetarium)

Dr. Erin Higgins

Armagh Observatory & Planetarium, UK

The evolution of massive stars as black hole progenitors

Abstract:  The evolution of massive stars is poorly constrained, even on the main sequence. Dominant processes such as rotation and mass loss have various influences on this evolution based on the relevant mass range. The most massive stars (> 40M) are dominated by stellar winds, while intermediate masses (20…30M) are influenced by their rotation rates. I will introduce a novel tool called the ‘Mass-Luminosity Plane’ as an alternative to the Hertzsprung-Russel Diagram to constrain the evolution of massive stars with regards to these key processes. Our analysis of the M-L plane has shown multiple constraints including the exclusion of extreme factors of mass-loss rate due to the gradient in the M-L plane, as well as the possibility of a widened main sequence due to increased convective mixing by overshooting. We reveal the necessity of rotational mixing in reproducing observed surface nitrogen enrichments and positions in the M-L plane. We have followed this study with an analysis of core He-burning as a function of time spent as a red supergiant or blue supergiant for a range of metallicities. We find that this is heavily influenced by the efficiency of semiconvective mixing and indirectly driven by stellar winds due to their effect on the envelope structure. We find that very efficient semiconvection is required in order to reproduce the most luminous red supergiants observed. These findings have consequences for the Humphreys-Davidson limit as well as the blue-to-red supergiant ratio at various metallicities. I will introduce new grids of models for Wolf-Rayet stars as black hole progenitors at varied metallicity which compare the most utilised mass-loss prescriptions in current stellar evolution theory, impacting the final mass estimates of black holes, as well as the metallicity limit for observing Pair Instability Supernovae. Finally, I will provide a method of forming a first generation 85 solar mass black hole, as recently detected in the event GW190521, from single star physics.