The rich 6 to 9 $\vec\mu$m spectrum of interstellar PAHs

Abstract

Abstract. IR spectroscopy provides a valuable tool for the characterisation and identification of interstellar molecular species.Here, we present 6-9 µm spectra of a sample of reflection nebulae, HII regions, YSOs, evolved stars and galaxies that show strong unidentified infrared bands, obtained with the SWS spectrograph on board ISO. The IR emission features in this wavelength region show pronounced variations. 1) The 6.2 µm feature shifts from 6.22 to 6.3 µm and clearly shows profile variations.2) The 7.7 µm complex is comprised of at least two subpeaks peaking at 7.6 and one longwards of 7.7 µm. In some cases the main peak can apparently shift up to 8 µm. Two sources do not exhibit a 7.7 µm complex but instead show a broad emission feature at 8.22 µm.3) The 8.6 µm feature has a symmetric profile in all sources and some sources exhibit this band at slightly longer wavelengths. For the 6.2, 7.7 and 8.6 µm features, the sources have been classified independently based on their profile and peak position. The classes derived for these features are directly linked with each other. Sources with a 6.2 µm feature peaking at ∼6.22 µm exhibit a 7.7 µm complex dominated by the 7.6 µm component. In contrast, sources with a 6.2 µm profile peaking longwards of 6.24 µm show a 7.7 µm complex with a dominant peak longwards of 7.7 µm and a 8.6 µm feature shifted toward the red. Furthermore, the observed 6-9 µm spectrum depends on the type of object. All ISM-like sources and a few PNe and Post-AGB stars belong to the first group while isolated Herbig AeBe stars, a few Post-AGB stars and most PNe belong to the second group. We summarise existing laboratory data and theoretical quantum chemical calculations of the modes emitting in this wavelength region of PAH molecules. We discuss the variations in peak position and profile in view of the exact nature of the carrier. We attribute the observed 6.2 µm profile and peak position to the combined effect of a PAH family and anharmonicity with pure PAHs representing the 6.3 µm component and substituted/complexed PAHs representing the 6.2 µm component. The 7.6 µm component is well reproduced by both pure and substituted/complexed PAHs but the 7.8 µm component remains an enigma. In addition, the exact identification of the 8.22 µm feature remains unknown. The observed variations in the characteristics of the IR emission bands are linked to the local physical conditions. Possible formation and evolution processes that may influence the interstellar PAH class are highlighted.

Theoretical PAH emission models for aromatic infrared bands

“…The spectral model comprising large PAHs has the 7.7 µm band dominated by the higher wavelength component at 7.8 µm (1285 cm −1 ) with a shoulder at 7.6 µm (1315 cm −1 ). Such profiles conform with observations of relatively benign astrophysical regions classified as B profiles (Peeters et al 2002). Intensity ratios of the two components of the 7.7 µm complex and their wavenumber separation in the three models is presented in Table 1.…”

Simulation of organic interstellar dust in the laboratory

“…It is important to note that only the peaks near 7.6/7.8 µm are diagnostic of the presence of aromatic rings since the C=C vibration in other non-aromatic hydrocarbon molecules can also contribute to the 6.2 µm band. The features in Figure 3 all contain structure arising from individual molecular groups and are, themselves, superimposed on a broad continuum extending from ∼1700 -1100 cm −1 (5.9-9.1 µm) similar to that commonly observed in astronomical spectra (Allamandola et al 1989, Peeters et al 2002. The relative strength of the two main peaks in Figure 3 is found to be variable, but there is no obvious detailed correlation between the relative amplitude of the 6.2 and 386 Walt W. Duley 7.6/7.8 µm peaks and preparation conditions.…”

“…Measurements also indicate that the 6.2 µm feature extends from 6.06-6.45 µm (1650-1550 cm −1 ) implying that the observation of type A, B, or C emission may simply reflect changes in the excitation and/or concentration of a limited number (4-5) of specific molecular structures. There is also evidence for another component at 5.98 µm (1671 cm −1 ) in the type B spectrum (Figure 4) that could correspond to the weak emission feature observed at 2248 6.0 µm in the spectrum of HD44179 and other objects (Peeters et al 2002). Emission at this wavelength is characteristic of non-aromatic C=C groups (Duley 2000), although it could also arise from an overtone and/or combination band involving ring vibrations (Allamandola et al 1989).…”

Laboratory analogues of hydrocarbonated interstellar nanograins

“…Consequently, by tuning the operating conditions of our particle source we can produce specific astrophysically relevant analogues. Spectroscopic studies on the infrared transmission of thin films have allowed us to propose a coherent scenario (Pino et al 2008) to explain the shift in the position of the "6.2-6.3 µm band" among the aromatic emission features observed in various astrophysical environments ranging from "Class A" to "Class C" sources (Peeters et al 2002).…”