Silicate condensation in Mira variables

TL;DR

This study models silicate dust formation in Mira variables, suggesting that SiO molecule clustering could initiate dust nucleation, leading to stellar mass loss driven by radiation pressure.

Contribution

It introduces a simplified pulsation model combined with dust nucleation calculations based on experimental data, proposing a new mechanism for silicate dust formation in Mira stars.

Findings

Dust formation occurs at low temperatures during gas parcel ascent.

Radiation pressure on dust exceeds gravity, driving stellar outflows.

Silicate dust nucleation by SiO clustering is a plausible mechanism.

Abstract

We study whether the condensation of silicate dust in Mira envelopes could be caused by cluster formation by the abundant SiO molecules. For a simplified model of the pulsational motions of matter in the the outer layers of a Mira variable which is guided by a numerical model for Mira pulsations, the equations of dust nucleation and growth are solved in the co-moving frame of a fixed mass element. It is assumed that seed particles form by clustering of SiO molecules. The calculation of the nucleation rate is based on the experimental data of Nuth and Donn (1982). The quantity of dust formed is calculated by a moment method and the calculation of radiation pressure on the dusty gas is based on a dirty silicate model. Dust nucleation occurs in the model at the upper culmination of the trajectory of a gas parcel where it stays for a considerable time at low temperatures while subsequent dust growth occurs during the descending part of the motion and continues after the next shock reversed motion. It is found that sufficient dust forms that radiation pressure exceeds gravitational pull of the stars such that the mass element is finally driven out of the star. Nucleation of dust particles by clustering of the abundant SiO molecules could be the mechanism that triggers silicate dust formation in Miras.