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. Furthermore, wind strength dictates the atmospheric structure that sets the ionizing flux. Finally, 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 comoving frame computations, revealing a rich multivariate behavior of the mass-loss rate [Formula: see text] in terms of M, L, Eddington Γ, Teff, and chemical composition Z. Concerning the latter, [Formula: see text] 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 behavior, there are several transitions in the Hertzsprung–Russell diagram, involving bistability jumps around Fe recombination temperatures, leading to quasi-stationary episodic, and not necessarily eruptive, luminous blue variable and pre-SN mass loss. ▪ Furthermore, there are kinks. At 100 [Formula: see text] a high Γ 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 ionizing radiation that could play a key role in local star formation as well as at high redshift. Expected final online publication date for the Annual Review of Astronomy and Astrophysics Volume 60 is August 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.