Dynamic atmospheres and winds of cool luminous giants, I. Al$_2$O$_3$ and silicate dust in the close vicinity of M-type AGB stars

TL;DR

This paper introduces advanced dynamic models of M-type AGB star atmospheres that incorporate detailed dust formation processes, revealing how Al$_2$O$_3$ and silicate dust influence stellar winds and observable properties.

Contribution

The study develops a new generation of dynamic atmosphere and wind models that include time-dependent dust grain growth and evaporation for Al$_2$O$_3$ and silicates, improving understanding of mass loss mechanisms.

Findings

Al$_2$O$_3$$ grains condense close to the star, prior to silicates.

Radiation pressure on Al$_2$O$_3$ alone is insufficient to drive winds.

Core-mantle grains enhance mass loss rates and match observed color variations.

Abstract

High spatial resolution techniques have given valuable insights into the mass loss mechanism of AGB stars, which presumably involves a combination of atmospheric levitation by pulsation-induced shock waves and radiation pressure on dust. Observations indicate that Al$_2$O$_3$ condenses at distances of about 2 stellar radii or less, prior to the formation of silicates. Al$_2$O$_3$ grains are therefore prime candidates for producing the scattered light observed in the close vicinity of several M-type AGB stars, and they may be seed particles for the condensation of silicates at lower temperatures. We have constructed a new generation of Dynamic Atmosphere & Radiation-driven Wind models based on Implicit Numerics (DARWIN), including a time-dependent treatment of grain growth & evaporation for both Al$_2$O$_3$ and Fe-free silicates (Mg$_2$SiO$_4$). The equations describing these dust species are solved in the framework of a frequency-dependent radiation-hydrodynamical model for the atmosphere & wind structure, taking pulsation-induced shock waves and periodic luminosity variations into account. Condensation of Al$_2$O$_3$ at the close distances and in the high concentrations implied by observations requires high transparency of the grains in the visual and near-IR region to avoid destruction by radiative heating. For solar abundances, radiation pressure due to Al$_2$O$_3$ is too low to drive a wind. Nevertheless, this dust species may have indirect effects on mass loss. The formation of composite grains with an Al$_2$O$_3$ core and a silicate mantle can give grain growth a head start, increasing both mass loss rates and wind velocities. Furthermore, our experimental core-mantle grain models lead to variations of visual and near-IR colors during a pulsation cycle which are in excellent agreement with observations.