Poster

The Carriers of the Unidentified Infrared Emission Bands: Aromatic or Aliphatic?

¹M.L. Wang, ²Aigen Li, ¹F.Y. Xiang, ¹J.X. Zhong (¹Xiangtan University, ²University of Missouri)

The "unidentified infrared emission" (UIE) bands, a distinct set of spectral features at wavelengths of 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 micrometer, dominate the mid-infrared spectra of most bright astronomical objects. They are ubiquitously seen in the interstellar medium (ISM) of our own galaxy and star-forming galaxies, both near and far, and account for over 10% of their total infrared (IR) luminosity. Although the exact nature of the carriers of these UIE bands remains unknown, they are commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules. The identification of the UIE bands is important as they are a useful probe of the cosmic star-formation history, and their carriers are an essential player in galactic evolution. Very recently, Kwok & Zhang (2011, Nature, 479, 80) hypothesized that the UIE bands arise from coal- or kerogen-like organic nanoparticles. They postulated that these nanoparticles consist of both chain-like aliphatic hydrocarbon units and benzene-ring-like aromatic hydrocarbon units. This hypothesis is interesting and potentially has important implications for our understanding of stellar evolution, interstellar chemistry, and the formation of our solar system. Particularly, if confirmed, this would establish an important link among stars at their late evolutionary stages, the ISM, and the solar system, as the kerogen-like organic matter seen in meteorites has similar chemical structures as those suggested by Kwok & Zhang for the UIE bands seen in the ISM and in circumstellar environments around evolved stars (i.e., planetary nebulae and proto-planetary nebulae).

In this work we estimate the aliphatic fraction of the "UIE" emitters based on the observed strengths of the 3.4, 6.95 and 7.25 micrometer features arising from aliphatic hydrocarbon.

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