Getting started: How a supersonic stellar wind is initiated from a hydrostatic surface

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Proceedings of the International Astronomical Union
Most of a star’s mass is bound in a hydrostatic equilibrium in which pressure balances gravity. But if at some near-surface layer additional outward forces overcome gravity, this can transition to a supersonic, outflowing wind, with the sonic point, where the outward force cancels gravity, marking the division between hydrostatic atmosphere and wind outflow. This talk will review general issues with such transonic initiation of a stellar wind outflow, and how this helps set the wind mass loss rate. The main discussion contrasts the flow initiation in four prominent classes of steady-state winds: (1) the pressure-driven coronal wind of the sun and other cool stars; (2) line-driven winds from OB stars; (3) a two-stage initiation model for the much denser winds from Wolf-Rayet (WR) stars; and (4) the slow “overflow” mass loss from highly evolved giant stars. A follow on discussion briefly reviews eruptive mass loss, with particular focus on the giant eruption of η Carinae.
This article was originally published in Proceedings of the International Astronomical Union. The version of record is available at: © The Author(s), 2022. Published by Cambridge University Press on behalf of International Astronomical Union
Sun: solar wind, stars: early-type, stars: mass loss, stars: Wolf-Rayet, stars: AGB
Owocki, Stan. “Getting Started: How a Supersonic Stellar Wind Is Initiated from a Hydrostatic Surface.” Proceedings of the International Astronomical Union 16, no. S366 (2020): 1–14. doi:10.1017/S1743921322001089.