The profiles of the aromatic infrared bands explained with molecular carriers

Abstract

Abstract. Using a compilation of the photophysical properties of molecules such as coronene, C24H12, we have constructed a numerical model to calculate the IR emission spectrum of a population of polycyclic aromatic hydrocarbon (PAH) species subjected to UV photons. The model enables us to calculate not only the energy emitted in the IR bands but also the detailed band profiles. The calculated spectrum for a PAH size distribution following N −3.5 C with NCmin = 30 was found to provide a good match to the Aromatic Infrared Bands (AIBs) observed in the planetary nebula IRAS 21282+5050. The match is especially good for the intensities and for the profiles of the 6.2 and 11.3 µm bands. This work provides a coherent picture of the AIB problem supporting the PAH molecular hypothesis. It further underlines the poor spectral diversity contained in the AIB spectrum putting new constraints on the nature of the emitting population.

Testing the “strong” PAHs hypothesis

“…Pech et al 2002). Figure 5, for example, shows the expected infrared spectrum of C 32 H + 14 in the radiation field of the Red Rectangle, in the wavelength range between about 5 and 9 µm, this time assuming educated guesses for the bandwidths, following Pech et al (2002).…”

“…Verstraete et al 2001;Pech et al 2002) but just arbitrarily assumed sensible values for their widths, with the aim to emphasize the effect of isotopic substitutions alone on the resulting overall profile.…”

“…Pech et al 2002). Their most natural interpretation is in terms of free gas phase polycyclic aromatic hydrocarbons (PAHs) which are thought to be ubiquitous in the interstellar medium thanks to their high photostability and the fact that they are carbonbased (Léger & Puget 1984;Allamandola et al 1985).…”

Laboratory infrared spectroscopy of PAHs

“…The effects of vibrational anharmonicity (Barker et al 1987;Pirali et al 2009) are naturally incorporated into the observed emission bands. This is clearly an advantage over spectra obtained using absorption spectroscopy since for a true comparison to interstellar emission spectra, one would have to convolute the bands using an emission model including the vibrational anharmonicities (Cook & Saykally 1998;Pech et al 2002), which are however largely unknown. However, the experiments are extremely challenging, particularly for ionic PAHs.…”

High Excitation ISM and Gas

“…These spectral variations have been attributed to a range of different physical or chemical characteristics of the carriers, including charge state, anharmonicity, different subcomponents with variable strength, family of related species with varying composition, substituted/complexed PAHs, isotope variations, clustering, molecular structure, molecular composition etc. (Verstraete et al, 1996;Joblin et al, 2000;Verstraete et al, 2001;Hony et al, 2001;Pech et al, 2002;Peeters et al, 2002a;Wada et al, 2003;Song et al, 2004;Bregman & Temi, 2004;van Diedenhoven et al, 2004). In particular, the variations in the 11.2/6.2 and 3.3/6.2 µm ratios likely reflect variations in the average ionization state of the emitting PAHs.…”