THE CARRIERS OF THE INTERSTELLAR UNIDENTIFIED INFRARED EMISSION FEATURES: CONSTRAINTS FROM THE INTERSTELLAR C-H STRETCHING FEATURES AT 3.2-3.5 μm

Journal Article · Sun Oct 20 00:00:00 EDT 2013 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/776/2/110· OSTI ID:22270794

Yang, X. J.; Zhong, J. X. ^[1]; Glaser, R. ^[2]; Li, Aigen ^[3]

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1. Department of Physics, Xiangtan University, 411105 Xiangtan, Hunan Province (China)
2. Department of Chemistry, University of Missouri, Columbia, MO 65211 (United States)
3. Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211 (United States)

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 recently been ascribed to mixed aromatic/aliphatic organic nanoparticles. More recently, an upper limit of <9% was placed on the aliphatic fraction (i.e., the fraction of carbon atoms in aliphatic form) of the UIE carriers based on the observed intensities of the 3.4 μm and 3.3 μm emission features by attributing them to aliphatic and aromatic C-H stretching modes, respectively, and assuming A{sub 3.4}/A{sub 3.3} ≈ 0.68 derived from a small set of aliphatic and aromatic compounds, where A{sub 3.4} and A{sub 3.3} are, respectively, the band strengths of the 3.4 μm aliphatic and 3.3 μm aromatic C-H bonds. To improve the estimate of the aliphatic fraction of the UIE carriers, here we analyze 35 UIE sources exhibiting both the 3.3 μm and 3.4 μm C-H features and determine I{sub 3.4}/I{sub 3.3}, the ratio of the power emitted from the 3.4 μm feature to that from the 3.3 μm feature. We derive the median ratio to be (I{sub 3.4}/I{sub 3.3}) ≈ 0.12. We employ density functional theory to compute A{sub 3.4}/A{sub 3.3} for a range of methyl-substituted PAHs. The resulting A{sub 3.4}/A{sub 3.3} ratio well exceeds ∼1.4, with an average ratio of A{sub 3.4}/A{sub 3.3} ≈ 1.76. By attributing the 3.4 μm feature exclusively to aliphatic C-H stretch (i.e., neglecting anharmonicity and superhydrogenation), we derive the fraction of C atoms in aliphatic form from I{sub 3.4}/I{sub 3.3} ≈ 0.12 and A{sub 3.4}/A{sub 3.3} ≈ 1.76 to be ∼2%. We therefore conclude that the UIE emitters are predominantly aromatic.

• OSTI ID: 22270794
• Journal Information: Astrophysical Journal, Vol. 776, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
77 NANOSCIENCE AND NANOTECHNOLOGY
ASTRONOMY
ASTROPHYSICS
ATOMS
CARBON
CHEMICAL BONDS
COSMIC DUST
DENSITY FUNCTIONAL METHOD
LIMITING VALUES
MOLECULES
NANOSTRUCTURES
PARTICLES
POLYCYCLIC AROMATIC HYDROCARBONS