Starlight and Sandstorms: Mass Loss Mechanisms on the AGB

TL;DR

This paper reviews the mechanisms driving mass loss in AGB stars, emphasizing dust-driven winds, and discusses recent models and discrepancies for different stellar types, highlighting the role of dust composition and scattering.

Contribution

It provides a comprehensive overview of current models for AGB star winds, including recent developments and unresolved issues related to dust properties and wind driving mechanisms.

Findings

Carbon star models align well with observations.

Silicate dust absorption may be insufficient for C/O<1 stars.

Fe-free silicate grains could drive winds via scattering, affecting reddening.

Abstract

There are strong observational indications that the dense slow winds of cool luminous AGB stars are driven by radiative pressure on dust grains which form in the extended atmospheres resulting from pulsation-induced shocks. For carbon stars, detailed models of outflows driven by amorphous carbon grains show good agreement with observations. Some still existing discrepancies may be due to a simplified treatment of cooling in shocks, drift of the grains relative to the gas, or effects of giant convection cells or dust-induced pattern formation. For stars with C/O < 1, recent models indicate that absorption by silicate dust is probably insuffcient to drive their winds. A possible alternative is scattering by Fe-free silicate grains with radii of a few tenths of a micron. In this scenario one should expect less circumstellar reddening for M- and S-type AGB stars than for C-stars with comparable stellar parameters and mass loss rates.