Formation and Photodestruction of Pahs

Allain, T.; Sedlmayr, E.; Leach, Sydney

doi:10.1007/978-94-011-0147-9_73

Cited by 133 publications

(209 citation statements)

References 2 publications

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“…High G 0 /n conditions causing low values of the [C ii] to FIR ratio would not produce a correspondingly low 6.2 lm feature flux, unless a significant fraction of the grains responsible for the 6.2 lm feature are destroyed by these conditions. However, laboratory studies indicate that PAH sizes larger than 50 carbon atoms probably dominate the emission in this band , while theoretical studies indicate that it is difficult to destroy PAHs with more than 50 carbon atoms in the regimes of G 0 /n derived here (Allain, Leach, & Sedlmayr 1996a, 1996b). According to , it would appear that grains up to at least 100 Å in size would have to be destroyed in order to lower the heating by a factor of 10.…”

Section: No 2 2003mentioning

confidence: 71%

The [Cii] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies Revisited

Luhman

Satyapal

Fischer

et al. 2003

ApJ

211

350

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We present a study of the [C ii] 157.74 lm fine-structure line in a sample of 15 ultraluminous infrared (IR) galaxies (IR luminosity L IR k10 12 L ; ULIRGs) using the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). We confirm the observed order of magnitude deficit (compared to normal and starburst galaxies) in the strength of the [C ii] line relative to the far-infrared (FIR) dust continuum emission found in our initial report, but here with a sample that is twice as large. This result suggests that the deficit is a general phenomenon affecting 4 out of 5 ULIRGs. We present an analysis using observations of generally acknowledged photodissociation region (PDR) tracers ([C ii], [O i] 63 and 145 lm, and FIR continuum emission), which suggests that a high ultraviolet flux G 0 incident on a moderate density n PDR could explain the deficit. However, comparisons with other ULIRG observations, including CO (1-0), [C i] (1-0), and 6.2 lm polycyclic aromatic hydrocarbon (PAH) emission, suggest that high G 0 =n PDRs alone cannot produce a self-consistent solution that is compatible with all of the observations. We propose that non-PDR contributions to the FIR continuum can explain the apparent [C ii] deficiency. Here, unusually high G 0 and/ or n physical conditions in ULIRGs as compared to those in normal and starburst galaxies are not required to explain the [C ii] deficit. Dust-bounded photoionization regions, which generate much of the FIR emission but do not contribute significant [C ii] emission, offer one possible physical origin for this additional non-PDR component. Such environments may also contribute to the observed suppression of FIR fine-structure emission from ionized gas and PAHs, as well as the warmer FIR colors found in ULIRGs. The implications for observations at higher redshifts are also revisited.

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Section: No 2 2003mentioning

confidence: 71%

The [Cii] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies Revisited

Luhman

Satyapal

Fischer

et al. 2003

ApJ

211

350

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“…In this sense, we use coronene as a molecule representative of all the PAHs in the PDRs studied in this work. It should be noted that this is a rough assumption, since it is widely believed that small PAHs (N C 40) cannot survive in strong radiation fields, which would exclude the existence of the coronene molecule in PDR environments (Allain et al 1996), and that compact PAHs are more stable than extended molecules (Jochims et al 1994;Ekern et al 1997). However, other studies show that small PAHs can survive strong radiation fields, but in a very dehydrogenated state (Montillaud et al 2013).…”

Section: Pahsmentioning

confidence: 97%

“…σ UV (E) is the photoabsorption cross section (Malloci et al 2007) and N(E) is the intensity of the Habing interstellar radiation field in photons cm −2 s −1 eV −1 (Draine 1978). If the molecule is not ionized but excited, it can dissipate the excitation energy by either IR emission, loss of an H atom, or loss of an H 2 molecule (Allain et al 1996). To calculate the rates for these processes, we use the RRKM method as described in Le Page et al (2001), which gives the rate of bond dissociation as a function of excitation energy, bond energy and vibrational degrees of freedom in the molecule.…”

Section: Photodesorptionmentioning

confidence: 99%

H₂formation on PAHs in photodissociation regions: a high-temperature pathway to molecular hydrogen

Boschman

Cazaux

Spaans

et al. 2015

A&A

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Aims. Molecular hydrogen is the most abundant molecule in the Universe. It is thought that a large portion of H 2 forms by association of hydrogen atoms to polycyclic aromatic hydrocarbons (PAHs). We model the influence of PAHs on total H 2 formation rates in photodissociation regions (PDRs) and assess the effect of these formation rates on the total cloud structure. Methods. We set up a chemical kinetic model at steady state in a PDR environment and included radiative transfer to calculate the chemistry at different depths in the PDR. This model includes known dust grain chemistry for the formation of H 2 and a H 2 formation mechanism on PAHs. Since H 2 formation on PAHs is impeded by thermal barriers, this pathway is only efficient at higher temperatures (T > 200 K). At these temperatures the conventional route of H 2 formation via H atoms physisorbed on dust grains is no longer feasible, so the PAH mechanism enlarges the region where H 2 formation is possible. Results. We find that PAHs have a significant influence on the structure of PDRs. The extinction at which the transition from atomic to molecular hydrogen occurs strongly depends on the presence of PAHs, especially for PDRs with a strong external radiation field. A sharp spatial transition between fully dehydrogenated PAHs on the outside of the cloud and normally hydrogenated PAHs on the inside is found. As a proof of concept, we use coronene to show that H 2 forms very efficiently on PAHs, and that this process can reproduce the high H 2 formation rates derived in several PDRs.

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“…Allain et al 1996); -an hollow spherical shell and its limb brightening, but it is difficult to explain the off-centering with this model. Any proposed model must be able to explain the origin of the intrinsic eccentricity and off-centering of the emission ring A natural explanation for this could be the presence of a massive perturber on an eccentric orbit.…”

Section: Interpretation: Disk Structure and The Nature Of The Emittinmentioning

confidence: 99%

An emission ring at 20.5 $\mathsf{\mu}$m around the HAEBE star AB Aurigæ: Unveiling the disk structure

Pantin

Bouwman

Lagage

2005

A&A

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Abstract. Isolated Herbig Ae/Be stars are believed to be a class of objects at an intermediate stage between young stellar objects surrounded by massive, optically thick, gaseous and dusty disks and Vega like stars surrounded by debris disks. The Herbig Ae star AB Aurigae is already known for being surrounded by an intermediate-stage dust disk emitting a fairly large infrared and (sub-)millimetric excess. Until now, its outer disk structure has only been resolved at millimeter wavelengths and at optical and near infrared wavelengths with coronographic imaging. We have obtained 20.5 µm images which show an unexpected elliptical ring-like emission structure in the disk around AB Aurigae at a distance of about 280 AU from the central star. This structure is characterized by a large azimuthal asymmetry in its brightness profile and an off-centered position with respect to the central star. To explain the observations, we propose a simple, purely geometrical model based on an emission ring with an uniform surface brightness, but having an intrinsic eccentricity. Our modeling of this ring-like structure provides valuable constraints on the inclination and the dust composition of the disk. Given the large distance from the central star, only transient heating of very small particles can explain the occurrence of the bright emission ring at mid-infrared wavelengths. Our observations point towards an unexpected geometry of the pre-main-sequence disk. In contrast to the usual sketch of a disk having a constant flaring angle, we argue that the circumstellar disk has a sudden, non uniform increase in the disk thickness. This sudden increase in the disk thickness as inferred by our modeling could be caused by disk instabilities. This suggests, together with the derived intrinsic eccentricity of the emission ring, the presence of a still undetected massive planetary type object, in an orbit at the outer-parts of the disk, disturbing the disk structure through gravitational interaction.

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Formation and Photodestruction of Pahs

Cited by 133 publications

References 2 publications

The [Cii] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies Revisited

The [Cii] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies Revisited

H₂formation on PAHs in photodissociation regions: a high-temperature pathway to molecular hydrogen

An emission ring at 20.5 $\mathsf{\mu}$m around the HAEBE star AB Aurigæ: Unveiling the disk structure

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Formation and Photodestruction of Pahs

Cited by 133 publications

References 2 publications

The [Cii] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies Revisited

The [Cii] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies Revisited

H2formation on PAHs in photodissociation regions: a high-temperature pathway to molecular hydrogen

An emission ring at 20.5 $\mathsf{\mu}$m around the HAEBE star AB Aurigæ: Unveiling the disk structure

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H₂formation on PAHs in photodissociation regions: a high-temperature pathway to molecular hydrogen