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

Abstract

The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer, 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 and 6.85 micrometer 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.

10 pages, 2 figures, accepted for publication in ApJ Letters

References in corpus (5)

• Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era
• The Diffuse Interstellar Bands: A Major Problem in Astronomical Spectroscopy
• 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
• Far-Infrared Spectroscopy of Cationic Polycyclic Aromatic Hydrocarbons: Zero Kinetic Energy Photoelectron Spectroscopy of Pentacene Vaporized from Laser Desorption

Cited by in corpus (60)

• The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point
• Observational Constraints on the Physical Properties of Interstellar Dust in the Post-Planck Era
• Unidentified Infrared Emission bands: PAHs or MAONs?
• From the far-ultraviolet to the far-infrared -- galaxy emission at $0\le z\le 10$ in the Shark semi-analytic model
• Mixed aliphatic and aromatic composition of evaporating very small grains in NGC 7023 revealed by the 3.4/3.3 $\mu$m ratio
• Polycyclic Aromatic Hydrocarbon in Protoplanetary Disks around Herbig Ae/Be and T Tauri Stars
• The sequence to hydrogenate coronene cations: A journey guided by magic numbers
• High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons in the 3 micron region: role of hydrogenation and alkylation
• Formation History of Polycyclic Aromatic Hydrocarbons in Galaxies
• The abundances of hydrocarbon functional groups in the interstellar medium inferred from laboratory spectra of hydrogenated and methylated polycyclic aromatic hydrocarbons
• The Carriers of the Interstellar Unidentified Infrared Emission Features: Constraints from the Interstellar C-H Stretching Features at 3.2-3.5 Micrometers
• The Carriers of the "Unidentified" Infrared Emission Features: Clues from Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups
• The chemistry of cosmic dust analogues from C, C${}_2$, and C${}_2$H${}_2$ in C-rich circumstellar envelopes
• Hydrogenation of PAH cations: a first step towards H2 formation
• The Mystery of Unidentified Infrared Emission Bands
• Self-consistent modelling of aromatic dust species and extinction curves in galaxy evolution
• Machine-learning prediction of infrared spectra of interstellar polycyclic aromatic hydrocarbons
• On the viability of the PAH model as an explanation of the unidentified infrared emission features
• Deuterated polycyclic aromatic hydrocarbons: Revisited
• The Role of Acetylene in the Chemical Evolution of Carbon Complexity
• Examining the class B-to-A shift of the 7.7 $\mu$m PAH band with the NASA Ames PAH IR Spectroscopic Database
• Graphite Revisited
• Random mixtures of polycyclic aromatic hydrocarbon spectra match interstellar infrared emission
• 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
• H2 formation via the UV photo-processing of a-C:H nano-particles
• Superhydrogenated Polycyclic Aromatic Hydrocarbon Molecules: Vibrational Spectra in the Infrared
• DFT study on interstellar PAH molecules with aliphatic side groups
• Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups: Intensity Scaling for the C-H Stretching Modes and Astrophysical Implications
• Chemical Abundances and Dust in the Halo Planetary Nebula K648 in M15: Its Origin and Evolution based on an Analysis of Multiwavelength Data
• Rotational spectroscopy of interstellar PAHs
• Buckyball-metal complexes as promising carriers of astronomical unidentified infrared emission bands
• On the Aliphatic versus Aromatic Content of the Carriers of the "Unidentified" Infrared Emission Features
• VUV spectroscopy of carbon dust analogs: contribution to interstellar extinction
• Aliphatic hydrocarbon content of the interstellar dust
• Modelling the evolution of PAH abundance in galaxies
• Search for PAHs in the Perseus molecular cloud with the Green Bank Telescope
• Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: The C--D Band Strengths of Mono-Deuterated Species
• Dust and Polycyclic Aromatic Hydrocarbon in the Pre-Transitional Disk around HD 169142
• Dust and Polycyclic Aromatic Hydrocarbon in the HD 34700 Debris Disk
• PDRs4All. XII. FUV-driven formation of hydrocarbon radicals and their relation with PAHs
• Complex Organic and Inorganic Compounds in Shells of Lithium-rich K Giant Stars
• Orion Bar as a window to the evolution of PAHs
• On the Relation between the Mysterious 21 Micrometer Emission Feature of Post-Asymptotic Giant Branch Stars and Their Mass Loss Rates
• Efficiency of the top-down PAH-to-fullerene conversion in UV irradiated environments
• The well correlated DIBs at $\lambda \lambda$ 6196, 6614 Å and their possible companions
• On the carbonaceous carriers of IR plateau and continuum emission
• GTC/CanariCam mid-IR imaging of the fullerene-rich Planetary Nebula IC 418: searching for the spatial distribution of fullerene-like molecules
• Spectral energy distributions of dust and PAHs based on the evolution of grain size distribution in galaxies
• Restructuring and destruction of hydrocarbon dust in the interstellar medium
• Evolution of grain size distribution with enhanced abundance of small carbonaceous grains in galactic environments
• A systematic study of ULIRGs using near-infrared absorption bands reveals a strong UV environment in their star-forming regions
• Carbon 'fluffy' aggregates produced by helium - hydrocarbon high pressure plasmas as analogs to interstellar dust
• Dust in the Local Group
• In situ characterization of volatile and refractory hydrocarbons produced by UV photolysis of interstellar C${}_2$H${}_2$ ice
• SPHEREx: Aromatics, Aliphatics and PAH Size across the Iris Nebula
• Molecular studies of Planetary Nebulae
• ReveaLLAGN 1: JWST Emission-Line Spectra Reveal Low-Luminosity AGN with UV-Deficient SEDs and Warm Molecular Gas
• A systematic study on the properties of aromatic and aliphatic hydrocarbon dust in active galactic nuclei with AKARI near-infrared spectroscopy
• Predicting Resolved Dust Attenuation from Local Galaxy Properties Using MaNGA