Testing the “strong” PAHs hypothesis

II. A quantitative link between DIBs and far-IR emission features

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. Mulas, gmulas@ca.astro.it gmalloci@ca.astro.it. pbenvenu@eso.org.

Received: 15 April 2003
Accepted: 5 August 2003

Abstract

In Paper I [CITE] we proved the profile invariance of the first permitted electronic transition of the typical Polycyclic Aromatic Hydrocarbon cation C₃₂H 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. Contrary to the mid-IR bands, collectively known as “Unidentified Infrared Bands” (UIBs), which do not discriminate specific molecules, the far IR, skeletal bands can be expected to be a fingerprint of each single species. This fact provides a number of independent constraints which must be simultaneously fulfilled for a successful PAH identification. Our approach thus offers a powerful criterion for the identification of specific PAHs, both in the presently available ISO data and in those of the forthcoming SIRTF and Herschel missions. As an interesting by-product, we quantitatively evaluate the impact of isotopic substitutions ( and ) on the resulting infrared emission bands.