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

By E. Peeters, S. Hony, C. V. Kerckhoven, A. Tielens, L. Allamandola, D. Hudgins, C. Bauschlicher. 2002

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IR spectroscopy provides a valuable tool for the characterisation and identification of interstellar molecular species. Here, we present 6-9 mum 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 mum feature shifts from 6.22 to 6.3 mum and clearly shows profile variations. 2) The 7.7 mum complex is comprised of at least two subpeaks peaking at 7.6 and one longwards of 7.7 mum. In some cases the main peak can apparently shift up to 8 mum. Two sources do not exhibit a 7.7 mum complex but instead show a broad emission feature at 8.22 mum. 3) The 8.6 mum 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 mum 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 mum feature peaking at similar to6.22 mum exhibit a 7.7 mum complex dominated by the 7.6 mum component. In contrast, sources with a 6.2 mum profile peaking longwards of 6.24 mum show a 7.7 mum complex with a dominant peak longwards of 7.7 mum and a 8.6 mum feature shifted toward the red. Furthermore, the observed 6-9 mum 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 mum profile and peak position to the combined effect of a PAH family and anharmonicity with pure PAHs representing the 6.3 mum component and substituted/complexed PAHs representing the 6.2 mum component. The 7.6 mum component is well reproduced by both pure and substituted/complexed PAHs but the 7.8 mum component remains an enigma. In addition, the exact identification of the 8.22 mum 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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/51ce57f155b6479777ca19af38e11ee0797adb8e

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Citations in the corpus

Variations of the Mid-IR Aromatic Features inside and among Galaxies (2008, background): “PAH VARIATIONS INSIDE AND AMONG GALAXIES 311 1999), and the Orion Bar (Cesarsky et al. 2000), and the ISO SWS spectra (2.5Y45 m) of several compact H ii regions published by Peeters et al. (2002a).”
Analysis of the emission of very small dust particles from Spitzer spectro-imagery data using blind signal separation methods (2007, background): “Thus, the observed variations of the mid-IR interstellar spectra (Peeters et al. 2002; Werner et al. 2004; Bregman & Temi 2005) have to be explained as the evolution of a mixture of PAHs and VSGs under the eff ct of UV flux.”
THE NASA AMES POLYCYCLIC AROMATIC HYDROCARBON INFRARED SPECTROSCOPIC DATABASE: THE COMPUTED SPECTRA (2010, unknown): “…the physical conditions in the emitting regions and the nature of the PAH carriers (e.g., Joblin et al. 1994; Li & Draine 2001; Hony et al. 2001; Peeters et al. 2002; van Diedenhoven et al. 2004; Mulas et al. 2006; Bauschlicher et al. 2008, 2009; Galliano et al. 2008; Boersma et al. 2009, 2010;…”
The Infrared Spectra of Very Large, Compact, Highly Symmetric, Polycyclic Aromatic Hydrocarbons (PAHs) (2008, background): “…spectra of many objects revealed that the PAH features vary between different classes of object and spatially within extended objects (e.g., Peeters et al. 2002a; vanDiedenhoven et al. 2004; Joblin et al. 2000; Bregman & Temi 2005; Sloan et al. 2005; Compiègne et al. 2007; Joblin et al.…”
C2D Spitzer-IRS spectra of disks around T Tauri stars. II. PAH emission features (2006, background): “The 7.7 and 8.6 µm features are generally found to be well correlated with the 6.2 µm feature (Peeters et al. 2002) and their absence is thus puzzling in our high sensitivity spectra.”
Infrared Emission of Normal Galaxies from 2.5 to 12 Micron: Infrared Space Observatory Spectra, Near-Infrared Continuum, and Mid-Infrared Emission Features (2003, background): “…in theMid-Infrared AFEs Some variations have been observed among Galactic sources in the profiles of the AFEs (e.g., Roelfsema et al. 1996; Peeters et al. 2002) and the ratios of one feature relative to another (e.g., Joblin et al. 1996; Lu 1998; Vermeij et al. 2002), but the physical…”
The Mid-Infrared Spectrum of Star-forming Galaxies: Global Properties of Polycyclic Aromatic Hydrocarbon Emission (2006, methodology): “…extracting PAH strength involves selecting narrow continuum regions free from line or feature emission, forming linear or cubic spline fits through these continuum points, and integrating the spectrum above this pseudo-continuum (e.g. Uchida et al. 2000; Peeters et al. 2002; Brandl et al. 2006).”
The Mid-Infrared Properties of Starburst Galaxies from Spitzer-IRS Spectroscopy (2006, background): “Following Peeters et al. (2002) we included one point close to 8.2 m to separate the contribution from both features.”
Laboratory Infrared Spectroscopy of Cationic Polycyclic Aromatic Hydrocarbon Molecules (2003, methodology): “…galactic nuclei, and starburst galaxies (Verstraete et al. 1996, 2001; Moutou et al. 1999; Joblin et al. 2000; Genzel et al. 1998; Henning et al. 1998; Boulanger et al. 1998; Mattila et al. 1996; Mattila, Lehtinen, & Lemke 1999; van Kerckhoven et al. 2000; Hony et al. 2001; Peeters et al. 2002).”
THE INFRARED SPECTRA OF VERY LARGE IRREGULAR POLYCYCLIC AROMATIC HYDROCARBONS (PAHs): OBSERVATIONAL PROBES OF ASTRONOMICAL PAH GEOMETRY, SIZE, AND CHARGE (2009, methodology): “…10 compares the emission spectra from the H ii region IRAS 23133+6050, the post-AGB star, the Red Rectangle, and the [WC10] PN He2-113 as studied by Peeters et al. (2002a) to three different collections of PAH spectra: the average spectrum produced by the large irregular PAHs computed here, the…”
Polycyclic aromatic hydrocarbon processing in interstellar shocks (2009, background): “The possible presence of N-atoms deeply in the C-skeleton of PAHs in the diffuse medium (but not in C-dust birth sites such as C-rich planetary nebulae) has been inferred from the peak position of the 6.2µm band (Peeters et al. 2002).”
GLIMPSE. I. An SIRTF Legacy Project to Map the Inner Galaxy (2003, background): “…Regions (PDRs): The near/mid IR spectrum of photo-dissociation regions at the surface of molecular clouds is dominated by emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 µm, probably arising from polycyclic aromatic hydrocarbon (PAH) molecules (Peeters et al. 2002 and references therein).”
Polycyclic Aromatic Hydrocarbons as a Tracer of Star Formation? (2004, methodology): “For the compact H ii regions, the FIR flux is derived by fitting a modified blackbody to the IRAS PSC fluxes (Peeters et al. 2002b).”
The Profiles of the 3-12 Micron Polycyclic Aromatic Hydrocarbon Features (2004, methodology): “Further data processing consisted of bad data removal and rebinning with a constant resolution, as described in Peeters et al. (2002b).”
The Behavior of the Aromatic Features in M101 H II Regions: Evidence for Dust Processing (2008, background): “…models using PAHs as the carriers of the aromatic features predict the strength of the 6.22, 7.70, and 8.61 µm features to strengthen relative to the 11.3 and 12.7 µm features as the ionization level increases or to weaken as the smaller PAHs are removed (Bakes et al. 2001; Peeters et al. 2002).”
Variations in the Peak Position of the 6.2 μm Interstellar Emission Feature: A Tracer of N in the Interstellar Polycyclic Aromatic Hydrocarbon Population (2005, methodology): “As the effect of multiple ionization on the position of the dominant 6.2 m feature was not explicitly examined by Peeters et al. (2002), we revisit those earlier results here.”
Spectroscopy of polycyclic aromatic hydrocarbons and very small grains in photodissociation regions (2005, methodology): “The shift of 0.1 µm reported by Peeters et al. (2002) for the 6.2 µm band is not evidenced by our analysis.”