The carriers of the interstellar unidentified infrared emission features: Aromatic or aliphatic?

Aigen Li, B. T. Draine

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64 Scopus citations

Abstract

The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 μm, commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules, have been recently ascribed to coal- or kerogen-like organic nanoparticles with a mixed aromatic-aliphatic structure. However, we show in this Letter that this hypothesis is inconsistent with observations. We estimate the aliphatic fraction of the UIE carriers based on the observed intensities of the 3.4 μm and 6.85 μm emission features by attributing them exclusively to aliphatic C-H stretch and aliphatic C-H deformation vibrational modes, respectively. We derive the fraction of carbon atoms in aliphatic form to be <15%. We conclude that the UIE emitters are predominantly aromatic, with aliphatic material at most a minor part of the UIE carriers. The PAH model is consistent with astronomical observations and PAHs dominate the strong UIE bands.

Original languageEnglish (US)
Article numberL35
JournalAstrophysical Journal Letters
Volume760
Issue number2
DOIs
StatePublished - Dec 1 2012

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Keywords

  • ISM: lines and bands
  • ISM: molecules
  • dust, extinction

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