Massive stars have strong stellar winds that direct their evolution through the upper HR diagram and determine the black hole (BH) 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, including Gravitational Wave (GW) 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 (WR) populations, BH masses, and GW events dependent on host galaxy Z. * On top of smooth mass-loss behaviour, there are several *transitions* in the HR 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 80-100 Msun a high Gamma mass-loss transition implies that hydrogen-rich very massive stars (VMS) have higher mass-loss rates than commonly considered. * Finally, low-mass helium (He) stars no longer appear as WR stars, but as optically-thin stripped-He stars. These He-stars, in addition to VMS, are 2 newly identified stellar sources of ionising radiation that could play a key role in local star formation as well as at high-redshift.
- Pub Date:
- September 2021
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - High Energy Astrophysical Phenomena
- Preprint of invited review article to appear in the Annual Review of Astronomy and Astrophysics (2022) - 47 pages, 13 figures