Testing the “strong” PAHs hypothesis

Abstract

Abstract. The so-called "strong" Polycyclic Aromatic Hydrocarbons (PAHs) hypothesis postulates that isolated PAHs, which are thought to be the carriers of the Unidentified Infrared Bands, ought to be also responsible for a large number of Diffuse Interstellar Bands (DIBs). In this framework, the spectral profile of such DIBs should be due to unresolved rotational structure of vibronic absorption bands, the rotation of the molecule being by and large governed by the interaction with the interstellar radiation field. In this paper we quantitatively test the above hypothesis against the observational constraint of DIBs profile invariance, by using Monte-Carlo methods to model the photophysics of a prototypical interstellar PAH, namely the ovalene cation (C 32 H + 14 ). Our results show that the predicted rotational band profiles are remarkably insensitive to both the ambient conditions and the assumed values of some poorly known parameters. The present model therefore offers a quantitative link between any given PAH and the observed DIB profiles, providing a valuable tool for molecular identification.

Testing the “strong” PAHs hypothesis

“…Salama 1999 and references therein). Indeed, as shown by Rouan et al (1992) and quantitatively studied in Mulas (1998) and Malloci et al (2003), infrared emission is supposed to be the dominant mechanism driving the rotation of an isolated PAH in the ISM, hence suggesting a direct link between DIB spectral profiles and IR emission bands.…”

“…The complete list of the 56 IR-active modes (symmetries B 1u , B 2u and B 3u ), the corresponding symmetries, the vibrational type (CC stretch, CH bend etc.) and the absolute intensities of the corresponding 0−1 transitions of C 32 H 14 and C 32 H 14 + can be found in Malloci (2003). The vibrational frequencies obtained through quantumchemical calculations at the B3LYP/4-31G level of theory are well known to slightly overestimate the real values for this class of molecules, and are usually scaled with an empirical factor of ∼0.958, which accounts for anharmonicity (Langhoff 1996;Bauschlicher & Langhoff 1997;Hudgins et al 2001) and brings them into near coincidence with experimental data.…”

“…In Paper I (Malloci et al 2003) we proved the profile invariance of the first permitted electronic transition of the typical Polycyclic Aromatic Hydrocarbon cation C 32 H 14 + as a first necessary check for the "strong" PAHs hypothesis. In this paper we derive a quantitative relation between the intensities of the former band, which ought to be observable in absorption in the visible range, and those of the far-IR bands, which are predicted by the PAH model to be simultaneously present in emission.…”

“…However, these single isotopic substitutions produce a negligible effect on the overall predicted band intensities. The lengthy, complete list of the estimated P in every IR-active band, for all the radiation fields considered and each of the inequivalent single isotopic susbtitutions is presented in Malloci (2003), and it is not reproduced here.…”

“…In Malloci et al (2003), henceforth Paper I, we modelled the photophysics of a prototypical medium sized PAH cation, namely C 32 H 14 + , and showed that its vibronic bands can easily match the observational constraint of spectral profile stability of DIBs in a wide variety of interstellar environments. Here we used the same model to obtain its expected IR emission spectra in the same environments, providing a quantitative estimate of the ratio of the equivalent width of the modelled "DIB" and the intensities of the infrared emission bands which ought to be simultaneously produced by the same molecules.…”