Observational evidence for composite grains in an AGB outflow: MgS in the extreme carbon star LL Peg

TL;DR

This study models the 30 μm feature in the extreme carbon star LL Peg, demonstrating that MgS in composite grains can explain observations without sulfur abundance issues, supporting MgS as its carrier.

Contribution

It shows that MgS dust in composite grains can reproduce the 30 μm feature in LL Peg, resolving previous sulfur abundance discrepancies.

Findings

MgS in composite grains explains the 30 μm feature.

Modeling is insensitive to MgS optical properties below 10 μm.

MgS abundance aligns with solar sulfur levels.

Abstract

The broad 30 \mu m feature in carbon stars is commonly attributed to MgS dust particles. However, reproducing the 30 \mu m feature with homogeneous MgS grains would require much more sulfur relative to the solar abundance. Direct gas-phase condensation of MgS occurs at a low efficiency. Precipitation of MgS on SiC precursor grains provides a more efficient formation mechanism, such that the assumption of homogeneous MgS grains may not be correct. Using a Monte Carlo-based radiative transfer code, we aim to model the 30 \mu m feature of the extreme carbon star LL Peg with MgS dust particles. We find that for LL Peg this modeling is insensitive to the unknown MgS optical properties at \lambda < 10 \mu m. When MgS is allowed to be in thermal contact with amorphous carbon and SiC, the amount of MgS required to reproduce the strength of 30 \mu m feature agrees with the solar abundance of sulfur, thereby resolving the reported MgS mass problem. We conclude that MgS is a valid candidate to be the carrier of the 30 \mu m feature when it is part of a composite grain population that has optical properties representative of an ensemble of particle shapes.