Non-equilibrium chemistry and dust formation in AGB stars as probed by SiO line emission

TL;DR

This study uses high-resolution observations and radiative transfer modeling to analyze SiO line emission in AGB stars, revealing steep chemical gradients and non-equilibrium processes affecting dust formation.

Contribution

It provides detailed insights into the radial SiO abundance distribution and highlights the role of non-equilibrium chemistry in carbon stars, advancing understanding of circumstellar chemistry.

Findings

Steep SiO abundance gradients in AGB star envelopes

SiO freezes onto dust grains at ~10^15 cm

Non-equilibrium chemistry explains high SiO in carbon stars

Abstract

We have performed high spatial resolution observations of SiO line emission for a sample of 11 AGB stars using the ATCA, VLA and SMA interferometers. Detailed radiative transfer modelling suggests that there are steep chemical gradients of SiO in their circumstellar envelopes. The emerging picture is one where the radial SiO abundance distribution starts at an initial high abundance, in the case of M-stars consistent with LTE chemistry, that drastically decreases at a radius of ~1E15 cm. This is consistent with a scenario where SiO freezes out onto dust grains. The region of the wind with low abundance is much more extended, typically ~1E16 cm, and limited by photodissociation. The surpisingly high SiO abundances found in carbon stars requires non-equilibrium chemical processes.