Condensation of MgS in outflows from carbon stars

TL;DR

This paper investigates the formation of MgS dust in the outflows of carbon stars, proposing a condensation model based on thermochemical calculations and grain growth kinetics, with implications for observed infrared features.

Contribution

It introduces a new model suggesting MgS forms via precipitation on SiC grains, explaining observed spectral features in carbon star dust shells.

Findings

MgS likely forms on SiC grains in stellar outflows.

Core-mantle grains with MgS reproduce the 30 μm feature.

Secondary spectral peaks suggest MgS coatings on SiC.

Abstract

The basic mechanism responsible for the widespread condensation of MgS in the outflows from carbon rich stars on the tip of the AGB is discussed with the aim of developing a condensation model that can be applied in model calculations of dust formation in stellar winds. The different possibilities how MgS may be formed in the chemical environment of outflows from carbon stars are explored by some thermochemical calculations and by a detailed analysis of the growth kinetics of grains in stellar winds. The optical properties of core-mantle grains with a MgS mantle are calculated to demonstrate that such grains reproduce the structure of the observed 30 $\mu$m feature. These considerations are complemented by model calculations of circumstellar dust shells around carbon stars. It is argued that MgS is formed via precipitation on silicon carbide grains. This formation mechanism explains some of the basic observed features of MgS condensation in dust shells around carbon stars. A weak secondary peak at about 33 ... 36 $\mu$m is shown to exist in certain cases if MgS forms a coating on SiC.