Testing the “strong” PAHs hypothesis

I. Profile invariance of electronic transitions of interstellar PAH cations

¹ INAF - Osservatorio Astronomico di Cagliari - AstroChemistry Group, Strada n. 54, Loc. Poggio dei Pini, 09012 Capoterra (CA), Italy
² Dipartimento di Fisica, Università degli Studi di Cagliari, S.P. Monserrato-Sestu Km 0.7, 09042 Cagliari, Italy
³ Space Telescope-European Coordinating Facility, ESA, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Munchen, Germany

Corresponding authors: G. Malloci, gmalloci@ca.astro.it gmulas@ca.astro.it pbenvenu@eso.org

Received: 15 April 2003
Accepted: 5 August 2003

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 (C₃₂H). 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.