The Aromatic Infrared Bands as seen by ISO-SWS: Probing the PAH model

By L. Verstraete, C. Pech, C. Moutou, K. Sellgren, C. Wright, M. Giard, A. Leger, R. Timmermann, S. Drapatz. 2001

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We discuss the Aromatic Infrared Band (AIB) proles observed by ISO-SWS towards a number of bright interstellar regions where dense molecular gas is illuminated by stellar radiation. Our sample spans a broad range of excitation conditions (exciting radiation elds with eective temperature, Te, ranging from 23 000 to 45 000 K). The SWS spectra are decomposed coherently in our sample into Lorentz proles and a broadband continuum. We nd that the individual proles of the main AIBs at 3.3, 6.2, 8.6 and 11.3 m are well represented with at most two Lorentzians. The 7.7 m-AIB has a more complex shape and requires at least three Lorentz proles. Furthermore, we show that the positions and widths of these AIBs are remarkably stable (within a few cm 1 ) conrming, at higher spectral resolution, the results from ISOCAM-CVF and ISOPHOT-S. This spectral decomposition with a small number of Lorentz proles implicitly assumes that most of the observed bandwidth arises from a few, large carriers. Boulanger et al. (1998b) recently proposed that the AIBs are the intrinsic proles of resonances in small carbon clusters. This interpretation can be tested by comparing the AIB prole parameters (band position and width) given in this work to laboratory data on relevant species when it becomes available. Taking advantage of our decomposition, we extract the proles of individual AIBs from the data and compare them to a state-of-the-art model of Polycyclic Aromatic Hydrocarbon (PAH) cation emission. In this model, the position and width of the AIBs are rather explained by a redshift and a broadening of the PAH vibrational bands as the temperature of the molecule increases (Joblin et al. 1995). In this context, the present similarity of the AIB proles requires that the PAH temperature distribution remains roughly the same whatever the radiation eld hardness. Deriving the temperature distribution of interstellar PAHs, we show that its hot tail, which controls the AIB spectrum, sensitively depends on Nmin (the number of C-atoms in the smallest PAH) and Te. Comparing the observed proles of the individual AIBs to our model results, we can match all the AIB proles (except the 8.6 m-AIB prole) if Nmin is increased with Te. This increase is naturally explained in a picture where small PAHs are more eciently photodissociated in harsher radiation elds. The observed 8.6 m-prole, both intensity and width, is not explained by our model. We then discuss our results in the broader context of ISO observations of fainter interstellar regions where PAHs are expected to be in neutral form.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/15e5c8d18dd77b1095b5d455f8b85bc44e569186

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Citations in the corpus

The Infrared Spectra of Very Large, Compact, Highly Symmetric, Polycyclic Aromatic Hydrocarbons (PAHs) (2008, background): “For most objects in which the PAH features are present, their spectroscopic signature dominates themid-IR emission (e.g., Uchida et al. 2000; Verstraete et al. 2001; Peeters et al. 2002b, 2004; Onaka 2004; Sloan et al. 2005; Brandl et al. 2006).”
Are PAHs precursors of small hydrocarbons in photo-dissociation regions? The Horsehead case (2005, background): “…of carbon chains suggests a last hypothesis: the fragmentation of PAHs due to the intense far UV–radiation could seed the interstellar medium with a variety of carbon clusters, chains and rings (Scott et al. 1997; Verstraete et al. 2001; Le Page et al. 2003; Joblin 2003, and references therein).”
ISM properties in low-metallicity environments I. mid-infrared spectra of dwarf galaxies (2005, background): “Due to their very small nature, these particles undergo thermal fluctuations of a few hundreds of degrees K upon single photon absorption (e.g. Sellgren et al. 1990; Verstraete et al. 2001).”
Laboratory Infrared Spectroscopy of Cationic Polycyclic Aromatic Hydrocarbon Molecules (2003, background): “…studies have shown that these effects can be substantial for PAHs (Joblin et al. 1995; Oomens et al. 2001b), and the observed profiles of some interstellar IR emission features indicate that these effects are important for interstellar emission spectra (Verstraete et al. 2001; Pech et al. 2002).”
The global dust SED: tracing the nature and evolution of dust with DustEM (2010, methodology): “As in the case of silicates (see her - after in the Appendix), an empirical approach can be followed where the ratios and profiles of the IR bands are derived from observations (Verstraete et al. 2001; Li & Draine 2001a).”
The rich 6 to 9 vec mu m spectrum of interstellar PAHs (2002, methodology): “For example, Verstraete et al. (2001) fit the total region with several Lorentzian profiles and Van Kerckhoven et al. (2002, in prep.) fit the 7.7 µm complex with 4 Gaussians peaking at 7.5, 7.6, 7.8 and 8.0 µm.”
The Profiles of the 3-12 Micron Polycyclic Aromatic Hydrocarbon Features (2004, background): “Compelling evidence suggests that the features are due to a complex mixture of ionized and neutral, possibly substituted/complexed, PAHs of many different sizes (Schutte et al. 1993; Bakes & Tielens 1994; Boulanger et al. 1998; Bakes et al. 2001; Hony et al. 2001, hereafter HVP01; Verstraete et al. ”
ISO Spectroscopy of Gas and Dust: From Molecular Clouds to Protoplanetary Disks (2004, background): “The smaller PAHs may be destroyed by intense UV radiation inside the H II region (Verstraete et al. 2001), although observational evidence is still controversial (Abergel et al. 2003).”
The Mid-Infrared Spectrum of Star-forming Galaxies: Global Properties of Polycyclic Aromatic Hydrocarbon Emission (2006, methodology): “Related fitting approaches have been used with ISO spectra (Boulanger et al. 1998; Verstraete et al. 2001).”
Polycyclic Aromatic Hydrocarbons as a Tracer of Star Formation? (2004, background): “…are very similar, the individual sources display great diversity in their spectral properties (for this sample see Roelfsema et al. 1996; Verstraete et al. 1996, 2001; Hony et al. 2001; Peeters et al. 2002a, 2002b; Martı́n-Hernández et al. 2002; Vermeij et al. 2002; van Diedenhoven et…”
Spectroscopy of polycyclic aromatic hydrocarbons and very small grains in photodissociation regions (2005, methodology): “Such a decomposition is useful to study the variations of the relative band strengths and of the band-to-continuum ratio within nebulae (Verstraete et al. 2001; Abergel et al. 2002).”
A simple model to interpret the ultraviolet, optical and infrared emission from galaxies (2008, methodology): “We extend it to include the 3.3 µm PAH emission feature using the Lorentzian profile parametrised by Verstraete et al. (2001), FLorentzν = f0[1 + (x − x0) 2/σ2]−1, where x = 1/λ is the wavenumber, x0 = 3039.1 cm −1 the central wavenumber of the feature and σ = 19.4 cm−1 half the FWHM.”
Some empirical estimates of the H 2 formation rate in photon-dominated regions (2003, background): “…the physical conditions typical of the PDRs, the rate of thermal fluctuationsτ−1abs (∼1.6 × 10−9 × N(C) × χ s−1 with N(C) the number of carbon atom in the grain, Verstraete et al. 2001) is comparable to or larger than the accretion rate of H atomsτ−1acc: for a grain with a radius of 1.5 nm (N(C) '…”
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): “…medium (ISM) of our Galaxy: they are seen in H ii and photodisassociation regions (PDRs; e.g., Césarsky et al. 1996; Roelfsema et al. 1996; Verstraete et al. 1996, 2001), planetary nebulae and circumstellar regions (e.g., Beintema et al. 1996), reflection nebulae (e.g., Boulanger et al.…”
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, background): “…pattern of band positions, intensities, and profiles observed in the interstellar IR emission spectra as well as the variations in those characteristics (e.g., Langhoff 1996; Allamandola et al. 1999; Verstraete et al. 2001; Bakes et al. 2001, Draine & Li 2001; Li & Draine 2001; Pech et al. 2002).”
THE INFRARED SPECTRA OF VERY LARGE IRREGULAR POLYCYCLIC AROMATIC HYDROCARBONS (PAHs): OBSERVATIONAL PROBES OF ASTRONOMICAL PAH GEOMETRY, SIZE, AND CHARGE (2009, background): “A number of plausible suggestions have been made to account for the wing, including different PAHs with slightly shifted solo bands, and hot band emission shifted due to anharmonicity (Barker et al. 1987; Pech et al. 2002; Verstraete et al. 2001).”
The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point (2013, methodology): “For the CH bond heat capacity, CV(CH), and for the aromatic/olefinic C atom heat capacity, CV(CC)sp2 , we adopt the Draine & Li (2001) approach used to determine the heat capacities of free-flying PAHs in the ISM.”
Variations of the Mid-IR Aromatic Features inside and among Galaxies (2008, methodology): “Their absorption efficiency has beenmodeled using astrophysical observations, laboratorymeasurements, and quantum theory (in particularDésert et al. 1990; Joblin et al. 1992; Verstraete et al. 2001; Li & Draine 2001; Mattioda et al. 2005a, 2005b; Draine & Li 2007; Malloci et al. 2007).”