Journal Article

A polycrystalline graphite model for the 2175 Å interstellar extinction band

Robert J. Papoular and Renaud Papoular

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 394, issue 4, pages 2175-2181
Published in print April 2009 | ISSN: 0035-8711
Published online April 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.14484.x
A polycrystalline graphite model for the 2175 Å interstellar extinction band

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A random, hydrogen-free, assembly of microscopic sp2 carbon chips, forming a macroscopically homogeneous and isotropic solid, is proposed as a model carrier for the ultraviolet (UV) interstellar extinction band. The validity of this model is based on the calculation of the Bruggeman average dielectric function of a mixture of the known parallel and perpendicular dielectric functions of graphite. The π absorption feature of Rayleigh-sized spheres of this mixture falls near 4.6 μm−1 (2175 Å), but its width is 1.5 μm−1, somewhat larger than the astronomically observed average, 1 μm−1. This is confirmed by measurements of the reflectance of an industrial material, polycrystalline graphite. A better fit to the interstellar feature position and width is obtained with a hypothetical material, having the same dielectric functions as natural graphite, except for less extended wings of the π resonance. Physically, this could result from changes in the electronic band structure due to previous thermal histories. In this model, the Frölich feature central wavelength depends only on the π resonance frequency, while its width depends only on the damping constant of the same resonance. This explains the range of observed feature widths at constant feature wavelength.

Keywords: astrochemistry-molecular processes; dust, extinction

Journal Article.  5225 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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