A general model for the identification of specific PAHs in the far-IR

Mulas, G.; Malloci, Giuliano; Joblin, C.; Toublanc, D.

doi:10.1051/0004-6361:20054276

Cited by 67 publications

(98 citation statements)

References 71 publications

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“…The best scale factor for the 15−20 μm region was found to be 0.956. This scale factor is very similar to the value of 0.958 for the mid-IR and also very similar to that found for the far-IR (Mulas et al 2006;Mattioda et al 2009). Therefore, we adopt a scaling factor of 0.958 for all calculated PAH bands.…”

Section: Synthetic Pah Spectrasupporting

confidence: 88%

“…Laboratory work and quantum-chemical calculations pave the way for identifying the carriers of 15−20 μm emission features and understanding the governing astrophysical conditions (experimental: Mattioda et al 2009, and references therein; theoretical: Bauschlicher et al 2009;Mulas et al 2006 and references therein). The 15−20 μm wavelength region -characteristic for C−C−C bending modes -has received in the past much less attention than the shorter wavelength C−C and C−H stretching and bending modes in laboratory and computational studies.…”

Section: Introductionmentioning

confidence: 99%

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The 15–20 μm PAH emission features: probes of individual PAHs?

Boersma¹,

Bauschlicher

Allamandola

et al. 2010

A&A

124

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Context. Spectral features between about 15−20 μm are commonly associated with polycyclic aromatic hydrocarbons (PAHs). With the NASA Spitzer Space Telescope these features are reported routinely, and as such, warrant a deeper molecular explanation. Aims. We aim to determine the characteristics of the group of carriers of the plateau and the distinct sub-features at 15.8, 16.4, 17.4, 17.8 and 18.9 μm and to draw astronomical implications from these spectra. Methods. We analyse and interpret the spectra of 15 different sources using the NASA Ames PAH IR spectroscopic database. Results. The bands within the 15−20 μm region show large variations. Except for the 16.4 μm band, there is also no connection, both in band strength and feature classification, with the mid-IR PAH bands. Of the PAH spectra considered, only those from species containing pendent rings show one "common" characteristic: a band near the astronomical 16.4 μm position. However, coupling with the carbon skeleton's core influences its precise position in the spectrum. Compact PAHs in the size range 50−130 carbon atoms, consistently show a strong band near the astronomical 17.4 μm band position. Conclusions. The 15−20 μm region is the transition zone from PAH nearest neighbour modes to full-skeleton modes. We conclude that a few individual PAHs dominate the astronomical PAH family when clear features are prominent. In the few cases of a broad plateau, the PAH family would be far richer. Although PAHs containing pendent rings showed promise explaining the astronomical 16.4 μm band, coupling with the skeleton core and the inherent strong quartet mode expected around 13.5 μm, make it a less viable candidate. The number of large PAHs in the database becomes a limitation in studying the emission between 15−20 μm and longward. Computation of larger PAH spectra should therefore be stimulated, especially for understanding the forthcoming far-IR data expected from Herschel, SOFIA and ALMA.

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Section: Synthetic Pah Spectrasupporting

confidence: 88%

Section: Introductionmentioning

confidence: 99%

The 15–20 μm PAH emission features: probes of individual PAHs?

Boersma¹,

Bauschlicher

Allamandola

et al. 2010

A&A

124

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“…The extrapolation of bulk graphite data, in the derivation of the interstellar PAH properties at FIR-cm wavelengths, therefore seemingly requires molecules that are larger than those assumed in current interstellar PAH models. However, current PAH models do include long wavelength bands, such as those that have been investigated experimentally (e.g., Pirali et al 2006) and theoretically (e.g., Mulas et al 2006;Pirali et al 2006) in order to determine their FIR spectral properties and to explain the anomalous microwave emission (Ysard & Verstraete 2010). However, the band widths for 2-4 aromatic ring PAHs are rather narrow, i.e., FWHM 30 cm −1 , at FIR wavelengths (in the 10-700 cm −1 region, as experimentally-measured by Pirali et al 2006).…”

Section: Comparison Of Nm-sized A-c(:h) Particles With Pahsmentioning

confidence: 99%

Variations on a theme – the evolution of hydrocarbon solids

Jones

2012

A&A

102

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Context. The properties of hydrogenated amorphous carbon (a-C:H) dust evolve in response to the local radiation field in the interstellar medium (ISM) and the evolution of these properties is particularly dependent upon the particle size. Aims. A model for finite-sized, low-temperature amorphous hydrocarbon particles, based on the microphysical properties of random and defected networks of carbon and hydrogen atoms, with surfaces passivated by hydrogen atoms, has been developed. Methods. The eRCN/DG and the optEC (s) models have been combined, adapted and extended into a new optEC (s) (a) model that is used to calculate the optical properties of hydrocarbon grain materials down into the sub-nanometre size regime, where the particles contain only a few tens of carbon atoms. Results. The optEC (s) (a) model predicts a continuity in properties from large to small (sub-nm) carbonaceous grains. Tabulated data of the size-dependent optical constants (from EUV to cm wavelengths) for a-C:H (nano-)particles as a function of the bulk material band gap [E g (bulk)], or equivalently the hydrogen content, are provided. The effective band gap [E g (eff.)] is found to be significantly larger than E g (bulk) for hydrogen-poor a-C(:H) nano-particles and their predicted long-wavelength (λ > 30 μm) optical properties differ from those derived for interstellar polycyclic aromatic hydrocarbons (PAHs). Conclusions. The optEC (s) (a) model is used to investigate the size-dependent structural and spectral evolution of a-C(:H) materials under ISM conditions, including: the IR-FUV extinction, the 217 nm bump and the infrared emission bands. The model makes several predictions that can be tested against observations.

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“…According to our models, the PAH abundance threshold required to affect the metal and electron abundance determination in the UV shielded gas is [PAH] > 10 −8 . Herschel observations might allow the identification of specific PAH carriers through their far-IR skeletal modes (Joblin et al 2002;Mulas et al 2006), thus providing clues to their composition and abundance variations in different environments.…”

Section: The Pah Abundance In Uv Shielded Gasmentioning

confidence: 99%

The ionization fraction gradient across the Horsehead edge: an archetype for molecular clouds

Goicoechea¹,

Pety²,

Gérin³

et al. 2009

A&A

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Context. The ionization fraction (i.e., the electron abundance) plays a key role in the chemistry and dynamics of molecular clouds. Aims. We study the H 13 CO + , DCO + and HOC + line emission towards the Horsehead, from the shielded core to the UV irradiated cloud edge, i.e., the photodissociation region (PDR), as a template to investigate the ionization fraction gradient in molecular clouds. Methods. We analyze an IRAM Plateau de Bure Interferometer map of the H 13 CO + J = 1-0 line at a 6.8 × 4.7 resolution, complemented with IRAM-30 m H 13 CO + and DCO + higher-J line maps and new HOC + and CO + observations. We compare self-consistently the observed spatial distribution and line intensities with detailed depth-dependent predictions of a PDR model coupled with a nonlocal radiative transfer calculation. The chemical network includes deuterated species, 13 C fractionation reactions and HCO + /HOC + isomerization reactions. The role of neutral and charged PAHs in the cloud chemistry and ionization balance is investigated. Results. The detection of the HOC + reactive ion towards the Horsehead PDR proves the high ionization fraction of the outer UV irradiated regions, where we derive a low [HCO + ]/[HOC + ] 75-200 abundance ratio. In the absence of PAHs, we reproduce the observations with gas-phase metal abundances, [Fe+Mg+...], lower than 4 × 10 −9 (with respect to H), and a cosmic-ray ionization rate of ζ = (5 ± 3) × 10 −17 s −1 . The inclusion of PAHs modifies the ionization fraction gradient and increases the required metal abundance. Conclusions. The ionization fraction in the Horsehead edge follows a steep gradient, with a scale length of ∼0.05 pc (or ∼25 ), from [e − ] 10 −4 (or n e ∼ 1-5 cm −3 ) in the PDR to a few times ∼10 −9 in the core. PAH − anions play a role in the charge balance of the cold and neutral gas if substantial amounts of free PAHs are present ([PAH] > 10 −8 ).

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A general model for the identification of specific PAHs in the far-IR

Cited by 67 publications

References 71 publications

The 15–20 μm PAH emission features: probes of individual PAHs?

The 15–20 μm PAH emission features: probes of individual PAHs?

Variations on a theme – the evolution of hydrocarbon solids

The ionization fraction gradient across the Horsehead edge: an archetype for molecular clouds

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