The dust composition of the OH/IR superwind

Robin Lombaert & B.L. de Vries

Onsala Space Observatory, Sweden

OH/IR stars are believed to be the final stage on the asymptotic giant branch (AGB) of intermediate main-sequence mass (5-8 solar masses) stars before they evolve into the post-AGB and planetary-nebula stages. AGB stars lose most of their stellar envelope in the form of a gaseous and dusty stellar wind. This wind eventually grows to such high mass-loss rates that the central star becomes entirely enshrouded by a dense, dusty superwind. The dust is primarily composed of amorphous and crystalline silicates, which can be either Mg- or Fe-rich. We show new results that constrain the dust composition of the OH/IR star OH30.1-0.7, with four main conclusions: a fraction of Fe-rich olivines is needed, pyroxenes have to be included, no indication of alumina is found, and modelling suggests the need for micron-sized grains. The detailed modelling of the dust component of the OH/IR superwind can contribute to constraining the size of the superwind, a subject under strong debate recently.

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