INTERPRETING THE SUBTLE SPECTRAL VARIATIONS OF THE 11.2 AND 12.7 μm POLYCYCLIC AROMATIC HYDROCARBON BANDS

Shannon, M. J.; Stock, D. J.; Peeters, E.

doi:10.3847/0004-637x/824/2/111

Cited by 30 publications

(27 citation statements)

References 55 publications

(115 reference statements)

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“…As a consequence, these methods assign portions of the un-derlying plateau and broad band continuum emission to the overlapping broad wings characteristic of these profiles. On occasion, individual bands are decomposed with Gaussian profiles to probe spectral substructure (e.g., Shannon et al 2016;Peeters et al 2017;Stock & Peeters 2017), including the 7.7 µm complex, but highresolution observations are generally better suited for this approach.…”

Section: Methodsmentioning

confidence: 99%

Examining the Class B to A Shift of the 7.7 μm PAH Band with the NASA Ames PAH IR Spectroscopic Database

Shannon

Boersma

2019

ApJ

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We present insights into the behavior of the astronomical 7.7 µm polycyclic aromatic hydrocarbon (PAH) emission complex as gleaned from analyzing synthesized spectra, utilizing the data and tools from the NASA Ames PAH IR Spectroscopic Database. We specifically study the influence of PAH size, charge, aliphatic content and nitrogen substitution on the profile and peak position of the 7.7 µm feature (λ 7.7 ). The 7.7 µm band is known to vary significantly from object-to-object in astronomical observations, but the origin of these variations remains highly speculative. Our results indicate that PAH size can accommodate the largest shift in λ 7.7 ( 0.4 µm), where relatively small PAHs are consistent with class A spectra (N c ≤ 60) while large PAHs are consistent with red/very red class B spectra. Aliphatic PAHs, of which our sample only contains a few, can produce redshifts typically around 0.15 µm; changes in ionization fraction, depending on the species, produce shifts up to 0.1 µm; and nitrogen substitution has no effect on λ 7.7 . Within the limits of our study, the class B→A transition is best explained with a changing PAH size distribution, with a relatively minor role assigned to aliphatic content and varying charge states. The resulting astronomical picture is that the photochemical evolution of PAHs moving from shielded class C/B environments into exposed ISM-like class A environments may be intrinsically different from the reverse class A→B transition of interstellar PAHs being incorporated into newly-forming star systems.

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Section: Methodsmentioning

confidence: 99%

Examining the Class B to A Shift of the 7.7 μm PAH Band with the NASA Ames PAH IR Spectroscopic Database

Shannon

Boersma

2019

ApJ

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“…Therefore, the systematic trends and characteristics of PAHs as well as the exploitation of the PAH bands as diagnostics of the physical and chemical conditions and processes could be determined in an unbiased manner. Also, the high sensitivity of Spitzer/IRS enabled unprecedented PAH spectral mapping of both Galactic and extragalactic sources [51][52][53][54][55] . Complemented with observations by, e.g., ISO and the Japanese AKARI infrared satellite, Spitzer made lasting contributions and substantially expanded our knowledge about the physical and chemcial properties of PAHs and their important role in astrophysics.…”

Section: The Spitzer Legacy Of Pah Astrophysicsmentioning

confidence: 99%

“…• The high sensitivity of Spitzer/IRS has enabled unprecedented PAH spectral mapping of both Galactic and extragalactic sources, providing valuable information on the spatial variations of the PAH characteristics (e.g., size, charge, structure) and their responses to the changing environments [51][52][53][54][55] .…”

Section: The Spitzer Legacy Of Pah Astrophysicsmentioning

confidence: 99%

Spitzer’s perspective of polycyclic aromatic hydrocarbons in galaxies

2020

Nat Astron

160

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Polycyclic aromatic hydrocarbon (PAH) molecules, as revealed by the distinctive set of emission bands at 3. 3, 6.2, 7.7, 8.6, 11.3 and 12.7 µm characteristic of their vibrational modes, are abundant and widespread throughout the Universe. They are ubiquitously seen in a wide variety of astrophysical regions, ranging from planet-forming disks around young stars to the interstellar medium (ISM) of the Milky Way and external galaxies out to high redshifts at z > ∼ 4. PAHs profoundly influence the thermal budget and chemistry of the ISM by dominating the photoelectric heating of the gas and controlling the ionization balance. Here, we review the current state of knowledge of the astrophysics of PAHs, focusing on their observational characteristics obtained from the Spitzer Space Telescope and their diagnostic power for probing the local physical and chemical conditions and processes. Special attention is paid to the spectral properties of PAHs and their variations revealed by the Infrared Spectrograph (IRS) on board Spitzer across a much broader range of extragalactic environments (e.g., distant galaxies, early-type galaxies, galactic halos, active galactic nuclei, and low-metallicity galaxies) than was previously possible with the Infrared Space Observatory (ISO) or any other telescope facilities. Also highlighted is the relation between the PAH abundance and the galaxy metallicity established for the first time by Spitzer.

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“…Another challenge concerns the lack of correlation between the 11.2 and 12.7 µm bands. For the PAH model the lack of correlation is attributed to the argument that the 12.7 µm band arises from a variable mixture of neutral and charged PAHs (Shannon et al 2016). On the other hand, C 24 exhibits strong 11.2 and 12.7 µm bands, implying their intensities should be strongly correlated.…”

Section: Challenges To the C 24 Carrier Identificationmentioning

confidence: 99%

“…The results are displayed in Figure 8 and the fit parameter values are presented in Table 1. We selected NGC 7023 because it provided a much cleaner background-subtracted 12.7 µm band (Shannon et al 2016) than could be obtained from NGC 7027. An interesting feature of the 12.7 µm band is that it displays a red band head and a blue tail, which means its ΔB must be positive.…”

Section: Spectral Modeling Of the Ngc 7027 127 µM Uir Bandmentioning

confidence: 99%

A Small Fullerene (C₂₄) may be the Carrier of the 11.2 μm Unidentified Infrared Band

Bernstein

Shroll

Lynch³

et al. 2017

ApJ

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We analyze the 11.2 µm unidentified infrared band (UIR) spectrum from NGC 7027 and identify a small fullerene (C 24 ) as a plausible carrier. The blurring effects of lifetime and vibrational anharmonicity broadening obscure the narrower, intrinsic spectral profiles of the UIR band carriers. We use a spectral deconvolution algorithm to remove the blurring, in order to retrieve the intrinsic profile of the UIR band. The shape of the intrinsic profile, a sharp blue peak and an extended red tail, suggests that the UIR band originates from a molecular vibration-rotation band with a blue band head. The fractional area of the band-head feature indicates a spheroidal molecule, implying a non-polar molecule and precluding rotational emission. Its rotational temperature should be well approximated by that measured for non-polar molecular hydrogen, ~825 K for NGC 7027. Using this temperature, and the inferred spherical symmetry, we perform a spectral fit to the intrinsic profile that results in a rotational constant implying C 24 as the carrier. We show that the spectroscopic parameters derived for NGC 7027 are consistent with the 11.2 µm UIR bands observed for other objects. We present density functional theory (DFT) calculations for the frequencies and infrared intensities of C 24 . The DFT results are used to predict a spectral energy distribution (SED) originating from absorption of a 5 eV photon, and characterized by an effective vibrational temperature of 930 K. The C 24 SED is consistent with the entire UIR spectrum and is the dominant contributor to the 11.2 and 12.7 µm bands.

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INTERPRETING THE SUBTLE SPECTRAL VARIATIONS OF THE 11.2 AND 12.7 μm POLYCYCLIC AROMATIC HYDROCARBON BANDS

Cited by 30 publications

References 55 publications

Examining the Class B to A Shift of the 7.7 μm PAH Band with the NASA Ames PAH IR Spectroscopic Database

Examining the Class B to A Shift of the 7.7 μm PAH Band with the NASA Ames PAH IR Spectroscopic Database

Spitzer’s perspective of polycyclic aromatic hydrocarbons in galaxies

A Small Fullerene (C₂₄) may be the Carrier of the 11.2 μm Unidentified Infrared Band

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INTERPRETING THE SUBTLE SPECTRAL VARIATIONS OF THE 11.2 AND 12.7 μm POLYCYCLIC AROMATIC HYDROCARBON BANDS

Cited by 30 publications

References 55 publications

Examining the Class B to A Shift of the 7.7 μm PAH Band with the NASA Ames PAH IR Spectroscopic Database

Examining the Class B to A Shift of the 7.7 μm PAH Band with the NASA Ames PAH IR Spectroscopic Database

Spitzer’s perspective of polycyclic aromatic hydrocarbons in galaxies

A Small Fullerene (C24) may be the Carrier of the 11.2 μm Unidentified Infrared Band

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A Small Fullerene (C₂₄) may be the Carrier of the 11.2 μm Unidentified Infrared Band