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

TL;DR

This paper reviews the mechanisms initiating supersonic stellar winds from hydrostatic surfaces, comparing different wind types and their roles in stellar mass loss processes.

Contribution

It provides a comparative analysis of transonic wind initiation in various stellar classes, enhancing understanding of mass loss mechanisms.

Findings

Different wind initiation mechanisms are characterized for various stellar types.

The transition from hydrostatic to wind outflow is crucial for determining mass loss rates.

Eruptive mass loss events like eta Carinae are briefly discussed.

Abstract

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 eta Carinae.