Dust in AGB stars: transparent or opaque?

TL;DR

This study uses detailed dynamical models to investigate the optical properties of dust in M-type AGB stars, aiming to identify which dust species drive stellar winds by comparing synthetic photometry with observations.

Contribution

It introduces a method to adjust dust opacity properties in models to match observed stellar spectra, helping to identify the dust responsible for driving winds in M-type AGB stars.

Findings

Dust in oxygen-rich AGB stars is likely transparent to radiation.

Model adjustments can reproduce observed spectral energy distributions.

Constraints on dust species responsible for stellar winds are proposed.

Abstract

It is commonly believed that winds of cool giants in their late evolutionary stages are driven by radiative pressure on dust grains, but the actual grain species responsible for driving winds of M-type AGB stars are still a matter of debate (see H\"ofner, this volume). The optical properties of the dust particles responsible for initiating the outflow affect the stellar spectra in two ways: (i) indirectly, through their influence on the dynamical structure of the atmosphere/envelope and the resulting molecular features, and (ii) directly, by changes of the spectral energy distribution due to absorption and scattering on dust grains. The qualitative differences in the energy distributions of C-type and M-type AGB stars in the visual and near-infrared regions suggest that the dust particles in oxygen rich atmospheres are relatively transparent to radiation. By using detailed dynamical models of gas and radiation combined with a simple description for the dust opacity (which can be adjusted to mimic different wavelength dependencies and condensation temperatures) and also by adjusting the fraction of the opacity that is treated as true absorption, we investigate which dust properties produce synthetic photometry consistent with observations. The goal of this study is to narrow down the possible dust species that may be driving the winds in M-type AGB stars.