Search

We present spectra of the 3.3 mum and 11.2 mum polycyclic aromatic hydrocarbon (PAH) features of a large number of stellar sources, planetary nebulae, reflection nebulae, H II regions, and galaxies, obtained with Infrared Space Observatory Short Wavelength Spectrometer. Clear variations are present in the profiles of these features. Most of the sources show a symmetric 3.3 mum feature peaking at similar to3.290 mum, while only very few show an asymmetric 3.3 mum feature peaking at a slightly longer wavelength. The profiles of the 11.2 mum feature are distinctly asymmetric. The majority of the sources has a 11.2 mum feature peaking between 11.20 and 11.24 mum, with a very steep blue rise and a low tail-to-top ratio. A few sources show a 11.2 mum feature with a peak position of similar to11.25 mum, a less steep blue rise, and a high tail-to-top ratio. The sources are classified independently on the basis of the 3.3 and 11.2 mum feature profiles and peak positions. Correlations between these classes and those based on the 6-9 mum features (Peeters et al.) are found. In particular, sources with the most common profiles in the 6-9 mum region also show the most common 3.3 and 11.2 mum feature profiles. However, the uncommon profiles do not correlate with each other. Also, these classifications depend on the type of object. In general, H II regions, nonisolated Herbig AeBe stars and young stellar objects show the same profiles for all 3-12 mum features. Many planetary nebulae and post-asymptotic giant branch stars show uncommon feature profiles. The three galaxies in our sample show the same profiles as the H II regions for all but the 11.2 mum feature, being similar to that of evolved stars. The observed pronounced contrast in the spectral variations for the CH modes (3.3 and 11.2 mum bands) versus the CC modes (6.2, 7.7, and 8.6 mum bands) is striking: the peak wavelengths of the features attributed to CC modes vary by similar to15-80 cm(-1), while for the CH modes the variations are similar to4-6.5 cm(-1). We summarize existing laboratory data and theoretical calculations of the modes emitting in the 3-12 mum region of PAH molecules and complexes. In contrast to the 6.2 and 7.7 mum components, which are attributed to PAH cations, the 3.3 mum feature appears to originate in neutral and/or negatively charged PAHs. We attribute the variations in peak position and profile of these IR emission features to the composition of the PAH family. The variations in FWHM of the 3.3 mum feature remains an enigma, while those of the 11.2 mum can be explained by anharmonicity and molecular structure. The possible origin of the observed contrast in profile variations between the CH modes and the CC modes is highlighted.

We present spectra of the 3.3 mum and 11.2 mum polycyclic aromatic hydrocarbon (PAH) features of a large number of stellar sources, planetary nebulae, reflection nebulae, H II regions, and galaxies, obtained with Infrared Space Observatory Short Wavelength Spectrometer. Clear variations are present in the profiles of these features. Most of the sources show a symmetric 3.3 mum feature peaking at similar to3.290 mum, while only very few show an asymmetric 3.3 mum feature peaking at a slightly longer wavelength. The profiles of the 11.2 mum feature are distinctly asymmetric. The majority of the sources has a 11.2 mum feature peaking between 11.20 and 11.24 mum, with a very steep blue rise and a low tail-to-top ratio. A few sources show a 11.2 mum feature with a peak position of similar to11.25 mum, a less steep blue rise, and a high tail-to-top ratio. The sources are classified independently on the basis of the 3.3 and 11.2 mum feature profiles and peak positions. Correlations between these classes and those based on the 6-9 mum features (Peeters et al.) are found. In particular, sources with the most common profiles in the 6-9 mum region also show the most common 3.3 and 11.2 mum feature profiles. However, the uncommon profiles do not correlate with each other. Also, these classifications depend on the type of object. In general, H II regions, nonisolated Herbig AeBe stars and young stellar objects show the same profiles for all 3-12 mum features. Many planetary nebulae and post-asymptotic giant branch stars show uncommon feature profiles. The three galaxies in our sample show the same profiles as the H II regions for all but the 11.2 mum feature, being similar to that of evolved stars. The observed pronounced contrast in the spectral variations for the CH modes (3.3 and 11.2 mum bands) versus the CC modes (6.2, 7.7, and 8.6 mum bands) is striking: the peak wavelengths of the features attributed to CC modes vary by similar to15-80 cm(-1), while for the CH modes the variations are similar to4-6.5 cm(-1). We summarize existing laboratory data and theoretical calculations of the modes emitting in the 3-12 mum region of PAH molecules and complexes. In contrast to the 6.2 and 7.7 mum components, which are attributed to PAH cations, the 3.3 mum feature appears to originate in neutral and/or negatively charged PAHs. We attribute the variations in peak position and profile of these IR emission features to the composition of the PAH family. The variations in FWHM of the 3.3 mum feature remains an enigma, while those of the 11.2 mum can be explained by anharmonicity and molecular structure. The possible origin of the observed contrast in profile variations between the CH modes and the CC modes is highlighted.

We report infrared spectral evidence of deuterated interstellar polycyclic aromatic hydrocarbons (PAHs). Two bands are detected in the infrared emission from the ionization bar in Orion at 4.4 and 4.65 mum. The one at 4.65 mum is present at the 4.4 sigma level, while the one at 4.4 mum is more tentative, with a sigma equal to 1.9. An emission band at 4.65 mum is also detected in the infrared emission from M17 by Verstraete and coworkers at the 4.4 sigma level. These wavelengths are highly characteristic of C-D stretching modes of deuterated PAHs and fall in a featureless region of the interstellar spectrum. The IR spectroscopic properties of deuterated PAHs are summarized, and an estimate of the deuterium fractionation between the aromatic and aliphatic carbons associated with the PAHs is given. High deuterium fractionation is implied; this is discussed in terms of interstellar PAH fractionation mechanisms.

The gas-phase infrared spectrum of the coronene cation in the 700-1700 cm(-1) range is presented. The spectrum is obtained via multiphoton dissociation spectroscopy of ionic coronene stored in a quadrupole ion trap using the intense and narrowband infrared radiation of a free electron laser. The spectrum shows main absorption peaks at 849, 1327, and 1533 cm(-1) along with some weak and barely resolved features, in good agreement with density functional calculations if the effects of vibrational anharmonicity are accounted for. Relative line intensities show remarkable differences with respect to matrix isolation data. The novel experimental technique applied here leads in a natural way to an absorption spectrum of highly excited species. Hence, measured absorption spectra can be compared rather directly to interstellar emission spectra, negating to some extent the need for detailed model calculations.

The gas-phase infrared spectrum of the coronene cation in the 700-1700 cm(-1) range is presented. The spectrum is obtained via multiphoton dissociation spectroscopy of ionic coronene stored in a quadrupole ion trap using the intense and narrowband infrared radiation of a free electron laser. The spectrum shows main absorption peaks at 849, 1327, and 1533 cm(-1) along with some weak and barely resolved features, in good agreement with density functional calculations if the effects of vibrational anharmonicity are accounted for. Relative line intensities show remarkable differences with respect to matrix isolation data. The novel experimental technique applied here leads in a natural way to an absorption spectrum of highly excited species. Hence, measured absorption spectra can be compared rather directly to interstellar emission spectra, negating to some extent the need for detailed model calculations.

Astronomical mid-IR spectra show two minor PAH features at 5.25 and 5.7 $\mu$m (1905 and 1754 cm$^{\rm - 1}$) that hitherto have been little studied, but contain information about the astronomical PAH population that complements that of the major emission bands. Here we report a study involving both laboratory and theoretical analysis of the fundamentals of PAH spectroscopy that produce features in this region and use these to analyze the astronomical spectra. The ISO SWS spectra of fifteen objects showing these PAH features were considered for this study, of which four have sufficient S/N between 5 and 6 $\mu$m to allow for an in-depth analysis. All four astronomical spectra show similar peak positions and profiles. The 5.25 $\mu$m feature is peaked and asymmetric, while the 5.7 $\mu$m feature is broader and flatter. Detailed analysis of the laboratory spectra and quantum chemical calculations show that the astronomical 5.25 and 5.7 $\mu$m bands are a blend of combination, difference and overtone bands primarily involving CH stretching and CH in-plane and CH out-of-plane bending fundamental vibrations. The experimental and computational spectra show that, of all the hydrogen adjacency classes possible on PAHs, solo and duo hydrogens consistently produce prominent bands at the observed positions whereas quartet hydrogens do not. In all, this a study supports the picture that astronomical PAHs are large with compact, regular structures. From the coupling with primarily strong CH out-of-plane bending modes one might surmise that the 5.25 and 5.7 $\mu$m bands track the neutral PAH population. However, theory suggests the role of charge in these astronomical bands might also be important., Comment: Accepted ApJ, 40 pages in pre-print, 14 figures, two online

We report infrared spectral evidence of deuterated interstellar polycyclic aromatic hydrocarbons (PAHs). Two bands are detected in the infrared emission from the ionization bar in Orion at 4.4 and 4.65 mum. The one at 4.65 mum is present at the 4.4 sigma level, while the one at 4.4 mum is more tentative, with a sigma equal to 1.9. An emission band at 4.65 mum is also detected in the infrared emission from M17 by Verstraete and coworkers at the 4.4 sigma level. These wavelengths are highly characteristic of C-D stretching modes of deuterated PAHs and fall in a featureless region of the interstellar spectrum. The IR spectroscopic properties of deuterated PAHs are summarized, and an estimate of the deuterium fractionation between the aromatic and aliphatic carbons associated with the PAHs is given. High deuterium fractionation is implied; this is discussed in terms of interstellar PAH fractionation mechanisms.

The gas-phase infrared spectrum of the coronene cation in the 700-1700 cm(-1) range is presented. The spectrum is obtained via multiphoton dissociation spectroscopy of ionic coronene stored in a quadrupole ion trap using the intense and narrowband infrared radiation of a free electron laser. The spectrum shows main absorption peaks at 849, 1327, and 1533 cm(-1) along with some weak and barely resolved features, in good agreement with density functional calculations if the effects of vibrational anharmonicity are accounted for. Relative line intensities show remarkable differences with respect to matrix isolation data. The novel experimental technique applied here leads in a natural way to an absorption spectrum of highly excited species. Hence, measured absorption spectra can be compared rather directly to interstellar emission spectra, negating to some extent the need for detailed model calculations.