The carrier of the "30" micron emission feature in evolved stars. A simple model using magnesium sulfide

TL;DR

This study analyzes the 30 micron emission feature in evolved stars, modeling it with magnesium sulfide dust grains to understand its variations and implications for stellar evolution and dust survival.

Contribution

The paper introduces a simple MgS dust grain model to explain the variability of the 30 micron feature across different evolved star types.

Findings

MgS grains explain the 30 micron feature variations.

Cooling of MgS grains accounts for peak shifts during stellar evolution.

MgS dust likely survives into the interstellar medium.

Abstract

We present 2-45 micron spectra of a large sample of carbon-rich evolved stars in order to study the ``30'' micron feature. We find the ``30'' micron feature in sources in a wide range of sources: low mass loss carbon stars, extreme carbon-stars, post-AGB objects and planetary nebulae. We extract the profiles from the sources by using a simple systematic approach to model the continuum. We find large variations in the wavelength and width of the extracted profiles of the ``30'' micron feature. We modelled the whole range of profiles in a simple way by using magnesium sulfide (MgS) dust grains with a MgS grain temperature different from the continuum temperature. The systematic change in peak positions can be explained by cooling of MgS grains as the star evolves off the AGB. In several sources we find that a residual emission excess at ~26 micron can also be fitted using MgS grains but with a different grains shape distribution. The profiles of the ``30'' micron feature in planetary nebulae are narrower than our simple MgS model predicts. We discuss the possible reasons for this difference. We find a sample of warm carbon-stars with very cold MgS grains. We discuss possible causes for this phenomenon. We find no evidence for rapid destruction of MgS during the planetary nebula phase and conclude that the MgS may survive to be incorporated in the ISM.