Atmospheric Heating and Wind Acceleration: Results for Cool Evolved Stars based on Proposed Processes

TL;DR

This paper reviews the processes of chromospheric heating and wind acceleration in cool evolved stars, discussing observational data, theoretical models, and recent advances involving acoustic and MHD waves.

Contribution

It provides a comprehensive review of current models and observational requirements for understanding chromospheric heating and stellar wind formation in evolved stars.

Findings

Acoustic and MHD waves can explain plasma heating and wind acceleration.

Significant progress has been made in modeling chromospheric processes.

Challenges remain in fully understanding wind origins in evolved stars.

Abstract

A chromosphere is a universal attribute of stars of spectral type later than ~F5. Evolved (K and M) giants and supergiants (including the zeta Aurigae binaries) show extended and highly turbulent chromospheres, which develop into slow massive winds. The associated continuous mass loss has a significant impact on stellar evolution, and thence on the chemical evolution of galaxies. Yet despite the fundamental importance of those winds in astrophysics, the question of their origin(s) remains unsolved. What sources heat a chromosphere? What is the role of the chromosphere in the formation of stellar winds? This chapter provides a review of the observational requirements and theoretical approaches for modeling chromospheric heating and the acceleration of winds in single cool, evolved stars and in eclipsing binary stars, including physical models that have recently been proposed. It describes the successes that have been achieved so far by invoking acoustic and MHD waves to provide a physical description of plasma heating and wind acceleration, and discusses the challenges that still remain.