The Profiles of the 3–12 Micron Polycyclic Aromatic Hydrocarbon Features

Abstract

We present spectra of the 3.3 m and 11.2 m 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 m feature peaking at $3.290 m, while only very few show an asymmetric 3.3 m feature peaking at a slightly longer wavelength. The profiles of the 11.2 m feature are distinctly asymmetric. The majority of the sources has a 11.2 m feature peaking between 11.20 and 11.24 m, with a very steep blue rise and a low tail-to-top ratio. A few sources show a 11.2 m feature with a peak position of $11.25 m, 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 m feature profiles and peak positions. Correlations between these classes and those based on the 6-9 m features (Peeters et al.) are found. In particular, sources with the most common profiles in the 6-9 m region also show the most common 3.3 and 11.2 m 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 m 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 m 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 m bands) versus the CC modes (6.2, 7.7, and 8.6 m bands) is striking: the peak wavelengths of the features attributed to CC modes vary by $15-80 cm À1 , while for the CH modes the variations are $4-6.5 cm À1 . We summarize existing laboratory data and theoretical calculations of the modes emitting in the 3-12 m region of PAH molecules and complexes. In contrast to the 6.2 and 7.7 m components, which are attributed to PAH cations, the 3.3 m 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 m feature remains an enigma, while those of the 11.2 m 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.

Nanostructuration of carbonaceous dust as seen through the positions of the 6.2 and 7.7μm AIBs

“…Table 1 summarizes the log of the observations. This covers the three main classes of the observed astrophysical AIB spectra (Peeters et al 2002;van Diedenhoven et al 2004), the classes A, B and C. Spectral characteristics of the bands were extracted by fitting the 6.2 and 7.7 micron bands with a set of three Gaussian bands, one for each of the 6.2, 7.7, and 8.6 micron AIBs, added by a local continuum baseline (Fig. 3).…”

PRESSURE EFFECTS IN POLYCYCLIC AROMATIC NITROGENATED HETEROCYCLES (PANHs): DIAGNOSTIC QUALITIES AND COSMOBAROMETRY POTENTIAL

“…broad emission plateaus at 6-9 μm, 11-14 μm, and 15-19 μm are recognized as part of the ISM (Tielens 2008), and the minor peaks produced under pressure could contribute to these features while explaining the known variation between astronomical regions (van Diedenhoven et al 2004). …”

Simulation of organic interstellar dust in the laboratory

“…Key information on the composition of organic interstellar material can be obtained from infrared spectra of many circumstellar, interstellar and extra-galactic sources as these are dominated by emission from PAHs (Allamandola et al 1989, Peeters et al 2002, 2004, van Diedenhoven et al 2004, Sloan et al 2005 The strongest emission features (historically called the unidentified infrared (UIR) bands) occur at 3. 3, 3.4, 6.2, ∼7.7, ∼8.6, 11.3 and 12.7 µm and have been known for many years, but recent observations indicate that there are numerous other weaker bands (Werner et al 2004, Sloan et al 2005.…”

“…3, 3.4, 6.2, ∼7.7, ∼8.6, 11.3 and 12.7 µm and have been known for many years, but recent observations indicate that there are numerous other weaker bands (Werner et al 2004, Sloan et al 2005. Observations have shown that there are three basic types of emission spectra as indicated by the relative intensity and characteristic profile of features in the 3.3/3.4, 6-9 and 11-11.4 µm wavelength range (Geballe 1997, Tokunaga 1996, Peeters et al 2002, van Diedenhoven et al 2004. Adopting the notation of Peeters, most sources can be classified as type A, with a pronounced feature at 6.19-6.23 µm (1616-1605 cm −1 ) together with bands at ∼7.6/7.8 µm (1316 / 1280 cm −1 ) and ∼8.6 µm (1163 cm −1 ).…”

“…However, the composition of the compounds responsible for these bands is still uncertain despite numerous theoretical and laboratory studies (Scott et al 1997, Hudgins et al 1999, 2004, van Diedenhoven et al 2004). This difficulty arises because the dominant observational bands at 6.2 and 7.7 µm do not appear at these wavelengths in laboratory or theoretical emission spectra of known PAH molecules.…”