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

The broad 30 μ m feature in carbon stars is commonly attributed to MgS dust particles. However, reproducing the 30 μ 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 μ 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 λ < 10 μ 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 μ 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 μ m feature when it is part of a composite grain population that has optical properties representative of an ensemble of particle shapes.