The Carriers of the "Unidentified" Infrared Emission Features: Clues from Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups

Abstract

The "unidentified" infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 $\mu$m are ubiquitously seen in various astrophysical regions. The UIE features are characteristic of the stretching and bending vibrations of aromatic hydrocarbons. The 3.3 $\mu$m feature resulting from aromatic C--H stretches is often accompanied by a weaker feature at 3.4 $\mu$m often attributed to aliphatic C--H stretches. The ratio of the observed intensity of the 3.3 $\mu$m aromatic C--H feature ($I_{3.3}$) to that of the 3.4 $\mu$m aliphatic C--H feature ($I_{3.4}$) allows one to estimate the aliphatic fraction (i.e. $N_{\mathrm{C},\mathrm{aliph}}/N_{\mathrm{C},\mathrm{arom}}$, the number of C atoms in aliphatic units to that in aromatic rings) of the UIE carriers, provided the intrinsic oscillator strengths of the 3.3 $\mu$m aromatic C--H stretch ($A_{3.3}$) and the 3.4 $\mu$m aliphatic C--H stretch ($A_{3.4}$) are known. In this article we summarize the computational results on $A_{3.3}$ and $A_{3.4}$ and their implications for the aromaticity and aliphaticity of the UIE carriers. We use density functional theory and second-order perturbation theory to derive $A_{3.3}$ and $A_{3.4}$ from the infrared vibrational spectra of seven PAHs with various aliphatic substituents (e.g., methyl-, dimethyl-, ethyl-, propyl-, butyl-PAHs, and PAHs with unsaturated alkyl-chains). The mean band strengths of the aromatic ($A_{3.3}$) and aliphatic ($A_{3.4}$) C--H stretches are derived and then employed to estimate the aliphatic fraction of the UIE carriers by comparing $A_{3.4}$/$A_{3.3}$ with $I_{3.4}$/$I_{3.3}$. We conclude that the UIE emitters are predominantly aromatic, as revealed by the observationally-derived ratio <$I_{3.4}$/$I_{3.3}$> ~ 0.12 and the computationally-derived ratio <$A_{3.4}$/$A_{3.3}$> ~ 1.76 which suggest an upper limit of $N_{\mathrm{C},\mathrm{aliph}}/N_{\mathrm{C},\mathrm{arom}}$ ~ 0.02 for the aliphatic fraction of the UIE carriers.

arXiv comment67 pages, 18 figures, 8 tables; invited article accepted for publication in "New Astronomy Review"; a considerable fraction of this article is concerned with the computational techniques and results, readers who are mainly interested in astrophysics may wish to only read "Introduction", and "Astrophysical Implications"

References in corpus (27)

• Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era
• Infrared Emission from Interstellar Dust. II. The Diffuse Interstellar Medium
• The Mid-Infrared Spectrum of Star-Forming Galaxies: Global Properties of PAH Emission
• Infrared Emission from Interstellar Dust. I. Stochastic Heating of Small Grains
• Photoelectric Emission from Interstellar Dust: Grain Charging and Gas Heating
• PAHs as a tracer of star formation?
• C${}_{60}$ in Reflection Nebulae
• The Composition and Distribution of Dust Along the Line of Sight Towards the Galactic Center
• ALMA Imaging of Gas and Dust in a Galaxy Protocluster at Redshift 5.3: [CII] Emission in "Typical" Galaxies and Dusty Starbursts ~1 Billion Years after the Big Bang
• Do the Infrared Emission Features Need Ultraviolet Excitation? The PAH Model in UV-Poor Reflection Nebulae
• Unidentified Infrared Emission bands: PAHs or MAONs?
• Carbon-rich dust past the asymptotic giant branch: aliphatics, aromatics, and fullerenes in the Magellanic Clouds
• The Carriers of the Interstellar Unidentified Infrared Emission Features: Aromatic or Aliphatic?
• Electronic spectroscopy of medium-sized polycyclic aromatic hydrocarbons: Implications for the carriers of the 2175 Å UV bump
• The role of the charge state of PAHs in ultraviolet extinction
• Small PAHs in the Red Rectangle
• The Carriers of the Interstellar Unidentified Infrared Emission Features: Constraints from the Interstellar C-H Stretching Features at 3.2-3.5 Micrometers
• The abundances of hydrocarbon functional groups in the interstellar medium inferred from laboratory spectra of hydrogenated and methylated polycyclic aromatic hydrocarbons
• Blue Luminescence and the Presence of Small PAHs in the ISM
• AKARI near-infrared spectroscopy of the aromatic and aliphatic hydrocarbon emission features in the galactic superwind of M 82
• Far- and mid-infrared spectroscopy of complex organic matter of astrochemical interest: coal, heavy petroleum fractions, and asphaltenes
• The C--H Stretching Features at 3.2--3.5 Micrometer of Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups
• Observational studies on the Near-Infrared Unidentified Emission Bands in Galactic HII regions
• Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups: Intensity Scaling for the C-H Stretching Modes and Astrophysical Implications
• On the Aliphatic versus Aromatic Content of the Carriers of the "Unidentified" Infrared Emission Features
• The Central Region of the Barred Spiral Galaxy NGC1097 Probed by AKARI Near-Infrared Spectroscopy
• On the relevance of polyynyl-substituted PAHs to astrophysics

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• Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: The C--D Band Strengths of Multi-Deuterated Species
• Fragmentation and isomerization of polycyclic aromatic hydrocarbons in the interstellar medium: coronene as a case study
• Interstellar dehydrogenated PAH anions: vibrational spectra
• The 3.3 $\mu$m Infrared Emission Feature: Observational and Laboratory Constraints on Its Carrier
• PDRs4All XVI. Tracing aromatic infrared band characteristics in photodissociation region spectra with PAHFIT in the JWST era
• Evolution of grain size distribution with enhanced abundance of small carbonaceous grains in galactic environments
• Threshold dissociation of the 1-ethynylpyrene cation at internal energies relevant to H I regions
• Polycyclic aromatic hydrocarbons and the ionized gas
• Laboratory measurements of stretching band strengths of deuterated Quenched Carbonaceous Composites (D-QCC)
• SPHEREx: Aromatics, Aliphatics and PAH Size across the Iris Nebula
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