The Ionising Flux of Very Massive Stars

"Now that JWST has been launched, the Early Universe is finally opening up. What has already transpired is that we do not understand star formation at high redshift. Furthermore, with the emergence of gravitational waves, surprisingly heavy black holes have been revealed, even in previous ""forbidden"" mass zones. What is crucial to make progress is a massive-star framework that works Locally and can be applied to the distant Universe -- allowing to test my hypothesis that very massive stars (VMS) re-ionised it. Today, such a framework does not exist. One of the reasons is that historically observations of massive stars have been rare, but with the emergence of large spectroscopic datasets, such as XShootU (PI: Vink), the physics dominating massive stars can finally be revealed. Moreover, I have recently made theoretical breakthroughs in wind mass loss (that sets the ionising flux), as well as a wind envelope interaction mechanism underlying luminous blue variables and supernova progenitors. In ION-FLUX, I propose a fundamentally new approach that combines my unique expertise in observations as well as stellar evolutionary, atmospheric, and wind theory. We have now implemented my mass-loss kink to evolutionary models, revealing that VMS do not evolve horizontally in the Hertzsprung-Russell diagram as canonical OB-type stars do, but vertically. Moreover, VMS are shown to completely ""evaporate"" owing to their strong metallicity-dependent winds, providing a preference for black-hole formation over pair instability supernovae, at least Locally. In parallel, Helium II emission from these VMS has now been included in population synthesis models, confirming that it is the VMS that are the likely culprits for this puzzling emission in star-forming galaxies. My innovative approach of forward - backwards modelling -- simultaneously benchmarking my models to new spectroscopic survey data -will culminate in a framework that correctly links star formation Near and Far."