Observational evidence for composite grains in an AGB outflow

MgS in the extreme carbon star LL Pegasi

¹ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D 2401, 3001 Leuven, Belgium
e-mail: robin.lombaert@ster.kuleuven.be
² Astronomical Institute “Anton Pannekoek”, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
³ Astronomical Institute Utrecht, University Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands
⁴ Astrophysikalisches Institut und Universitäts-Sternwarte, Schillergäßchen 2-3, 07745 Jena, Germany
⁵ Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

Received: 8 June 2012
Accepted: 1 July 2012

Abstract

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