SCYON Abstract

Received on May 20 2010

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

AuthorsIain McDonald (1), Gregory C. Sloan (2), Albert A. Zijlstra (1), Noriyuki Matsunaga (3), Mikako Matsuura (4,5), Kathleen E. Kraemer (6), Jeronimo Bernard-Salas (2), and Andrew J. Markwick (1)
Affiliation(1) Jodrell Bank Centre for Astrophysics, Alan Turing Building, Manchester, M13 9PL, UK
(2) Cornell University, Astronomy Department, Ithaca, NY 14853-6801
(3) Institute of Astronomy, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015
(4) UCL-Institute of Origins, Astrophysics Group, Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT
(5) UCL-Institute of Origins, Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT
(6) Air Force Research Laboratory, Space Vehicles Directorate, Hanscom AFB, MA 01731
Accepted byAstrophysical Journal
Contactmcdonald@jb.man.ac.uk
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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.