Polycyclic Aromatic Hydrocarbons and the Extinction Curve

Mulas, G.; Malloci, Giuliano; Joblin, C.; Cecchi‐Pestellini, C.

doi:10.1051/eas/1146034

Cited by 4 publications

(3 citation statements)

References 62 publications

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“…PAH cross sections vary by large factors from one molecule species to the next, and they depend strongly on their hydrogenation coverage and charge state. Still in C PAH one notices a spectral trend: molecules have a cut-off at low (optical) frequencies, that depends on the PAH ionisation degree (Schutte et al 1993;Salama et al 1996), a local maximum near the 2175 Å extinction bump (Verstraete & Léger 1992;Mulas et al 2011), and a steep rise in the far UV Zonca et al 2011).…”

Section: Cross Sections Of Pahsmentioning

confidence: 99%

Dust in the diffuse interstellar medium

Siebenmorgen

Вощинников

Bagnulo

2014

A&A

112

123

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We present a model for the diffuse interstellar dust that explains the observed wavelength-dependence of extinction, emission, and the linear and circular polarisation of light. The model is set up with a small number of parameters. It consists of a mixture of amorphous carbon and silicate grains with sizes from the molecular domain of 0.5 up to about 500 nm. Dust grains with radii larger than 6 nm are spheroids. Spheroidal dust particles have a factor 1.5-3 greater absorption cross section in the far-infrared than spherical grains of the same volume do. Mass estimates derived from submillimetre observations that ignore this effect are overestimated by the same amount. In the presence of a magnetic field, spheroids may be partly aligned and polarise light. We find that polarisation spectra help to determine the upper particle radius of the otherwise rather unconstrained dust size distribution. Stochastically heated small grains of graphite, silicates, and polycyclic aromatic hydrocarbons (PAHs) are included. We tabulate parameters for PAH emission bands in various environments. They show a trend with the hardness of the radiation field that can be explained by the ionisation state or hydrogenation coverage of the molecules. For each dust component its relative weight is specified so that absolute element abundances are not direct input parameters. The model is compared to the average properties of the Milky Way, which seem to represent dust in the solar neighbourhood. It is then applied to specific sight lines towards four particular stars, with one of them located in the reflection nebula NGC 2023. For these sight lines, we present ultra-high signal-to-noise linear and circular spectro-polarimetric observations obtained with FORS at the VLT. Using prolate rather than oblate grains gives a better fit to observed spectra; the axial ratio of the spheroids is typically two and aligned silicates are the dominant contributors to the polarisation.

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Section: Cross Sections Of Pahsmentioning

confidence: 99%

Dust in the diffuse interstellar medium

Siebenmorgen

Вощинников

Bagnulo

2014

A&A

112

123

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“…Polycyclic aromatic hydrocarbons (PAHs) hold significant promise in explaining various chemical phenomena, but they also can give rise to significant complexity in their chemical analysis. PAHs are significant products of fossil fuel combustion, − and they are believed to be of great importance in astrochemistry. − These remarkably stable structures have been hypothesized to exist in environments as varied as the circumstellar envelopes of dying, carbon-rich stars to the soot of simple fires on Earth . Many PAHs are known carcinogens and environmental pollutants largely due to their remarkable stability. , However, their characteristics also bode well for the discovery of new physical and chemical phenomena.…”

Section: Introductionmentioning

confidence: 99%

Electronically Excited States of Anisotropically Extended Singly-Deprotonated PAH Anions

et al. 2015

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Polycyclic aromatic hydrocarbons (PAHs) play a significant role in the chemistry of the interstellar medium (ISM) as well as in hydrocarbon combustion. These molecules can have high levels of diversity with the inclusion of heteroatoms and the addition or removal of hydrogens to form charged or radical species. There is an abundance of data on the cationic forms of these molecules, but there have been many fewer studies on the anionic species. The present study focuses on the anionic forms of deprotonated PAHs. It has been shown in previous work that PAHs containing nitrogen heteroatoms (PANHs) have the ability to form valence excited states giving anions electronic absorption features. This work analyzes how the isoelectronic pure PAHs behave under similar structural constructions. Singly deprotonated forms of benzene, naphthalene, anthracene, and tetracene classes are examined. None of the neutral-radicals possess dipole moments large enough to support dipole-bound excited states in their corresponding closed-shell anions. Even though the PANH anion derivatives support valence excited states for three-ringed structures, it is not until four-ringed structures of the pure PAH anion derivatives that valence excited states are exhibited. However, anisotropically extended PAHs larger than tetracene will likely exhibit valence excited states. The relative energies for the anion isomers are very small for all of the systems in this study.

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“…PAH cross sections vary by large factors from one molecule species to the next and they depend strongly on their hydrogenation coverage and charge state. Still in C PAH one notices a spectral trend: molecules have a cut-off at low (optical) frequencies that depends on the PAH ionisation degree (Schutte et al 1993, Salama et al 1996, a local maximum near the 2175 Å extinction bump (Verstraete & Léger 1992;Mulas et al 2011), and a steep rise in the far UV (Malloci et al 2011;Zonca et al 2011).…”

Section: Cross Sections Of Pahsmentioning

confidence: 99%

Dust in the diffuse interstellar medium: Extinction, emission, linear and circular polarisation

Siebenmorgen,

Voshchinnikov,

Bagnulo

2013

Preprint

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We present a model for the diffuse interstellar dust that explains the observed wavelength-dependence of extinction, emission, linear and circular polarisation of light. The model is set-up with a small number of parameters. It consists of a mixture of amorphous carbon and silicate grains with sizes from the molecular domain of 0.5 up to about 500 nm. Dust grains with radii larger than 6 nm are spheroids. Spheroidal dust particles have a factor 1.5 -3 larger absorption cross section in the far IR than spherical grains of the same volume. Mass estimates derived from submillimeter observations that ignore this effect are overestimated by the same amount. In the presence of a magnetic field, spheroids may be partly aligned and polarise light. We find that polarisation spectra help to determine the upper particle radius of the otherwise rather unconstrained dust size distribution. Stochastically heated small grains of graphite, silicates and polycyclic aromatic hydrocarbons (PAHs) are included. We tabulate parameters for PAH emission bands in various environments. They show a trend with the hardness of the radiation field that can be explained by the ionisation state or hydrogenation coverage of the molecules. For each dust component its relative weight is specified, so that absolute element abundances are not direct input parameters. The model is confronted with the average properties of the Milky Way, which seems to represent dust in the solar neighbourhood. It is then applied to specific sight lines towards four particular stars one of them is located in the reflection nebula NGC 2023. For these sight lines, we present ultra-high signal-to-noise linear and circular spectro-polarimetric observations obtained with FORS at the VLT. Using prolate rather than oblate grains gives a better fit to observed spectra; the axial ratio of the spheroids is typically two and aligned silicates are the dominant contributor to the polarisation.

show abstract

Polycyclic Aromatic Hydrocarbons and the Extinction Curve

Cited by 4 publications

References 62 publications

Dust in the diffuse interstellar medium

Dust in the diffuse interstellar medium

Electronically Excited States of Anisotropically Extended Singly-Deprotonated PAH Anions

Dust in the diffuse interstellar medium: Extinction, emission, linear and circular polarisation

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