Theory and Diagnostics of Hot Star Mass Loss

Massive stars have strong stellar winds that direct their evolution through the upper Hertzsprung-Russell diagram and determine the black hole mass function. Secondly, wind strength dictates the atmospheric structure that sets the ionising flux. Thirdly, the wind directly intervenes with the stellar envelope structure, which is decisive for both single star and binary evolution, affecting predictions for gravitational wave events. Key findings of current hot-star research include: * The traditional line-driven wind theory is being updated with Monte Carlo and co-moving frame computations, revealing a rich multi-variate behaviour of the mass-loss rate dM/dt in terms of M, L, Eddington Gamma, Teff , and chemical composition Z. Concerning the latter, dM/dt is shown to depend on the iron (Fe) opacity, making Wolf-Rayet populations, and gravitational wave events dependent on host galaxy Z. * On top of smooth mass-loss behaviour, there are several transitions in the Hertzsprung-Russell diagram, involving bi-stability jumps around Fe recombination temperatures, leading to quasi-stationary episodic, and not necessarily eruptive, Luminous Blue Variable and pre-SN mass loss. * Moreover, there are kinks. At 100 Solar Masses, a high Gamma mass-loss transition implies that hydrogen-rich very massive stars have higher mass-loss rates than commonly considered. At the other end of the mass spectrum, low-mass stripped helium stars no longer appear as Wolf-Rayet stars, but as optically-thin stars. These stripped stars, in addition to very massive stars, are two newly identified sources of ionising radiation that could play a key role in local star formation as well as at high-redshift.Comment: Invited Annual Review of Astronomy and Astrophysics (ARA&A 2022) - 49 pages, 13 figures (now Link to Review in Advance first posted online on May 13, 2022