Rusty old stars: a source of the missing interstellar iron?

TL;DR

This study presents evidence that metallic iron grains are present around oxygen-rich AGB stars, suggesting iron's significant role in dust formation and the interstellar medium, especially at low metallicities.

Contribution

It provides observational evidence of metallic iron grains around AGB stars and explores their implications for dust formation and interstellar medium enrichment.

Findings

Iron dust production is widespread around AGB stars.

Iron may be the sole dust component in some cases.

Iron influences stellar outflows and chemistry.

Abstract

Iron, the Universe's most abundant refractory element, is highly depleted in both circumstellar and interstellar environments, meaning it exists in solid form. The nature of this solid is unknown. In this Letter, we provide evidence that metallic iron grains are present around oxygen-rich AGB stars, where it is observationally manifest as a featureless mid-infrared excess. This identification is made using Spitzer Space Telescope observations of evolved globular cluster stars, where iron dust production appears ubiquitous and in some cases can be modelled as the only observed dust product. In this context, FeO is examined as the likely carrier for the 20-micron feature observed in some of these stars. Metallic iron appears to be an important part of the dust condensation sequence at low metallicity, and subsequently plays an influential role in the interstellar medium. We explore the stellar metallicities and luminosities at which iron formation is observed, and how the presence of iron affects the outflow and its chemistry. The conditions under which iron can provide sufficient opacity to drive a wind remain unclear.