Modeling the interstellar aromatic infrared bands with co-added spectra of PAHs

Physics Department, DDU Gorakhpur University, Gorakhpur 273009, India e-mail: shantanu_r@hotmail.com

Received: 22 October 2006
Accepted: 21 March 2008

Abstract

Aims. The observed variations in profiles of the interstellar aromatic infrared bands correlate with the object type and are indicative of PAH populations existing in different sources. Spectroscopic studies of PAHs can provide tools for interpreting the variations accompanying the AIBs. As the observed spectra results from a mix of possible species in the region, we try to model this composite spectra by co-adding emissions from PAHs in different size groups.

Methods. Theoretical IR data of PAHs having 10 to 96 carbon atoms is used to obtain emission spectra. The models are taken in size groups made up of small, medium, and large PAHs.

Results. The models show a good profile match with observations for the 7.7  complex having sub-features at 7.6 and 7.8 . The 7.6  sub-feature dominates in the spectra of medium-sized PAH cations, matching observations from UV-rich interstellar environments. The 7.8  component is more intense in the spectra of large PAH cations (model III), correlating with observations from benign astrophysical regions. A possible interpretation of the observations of out-of-plane bend modes and the weak outliers on the blue side of the intense 11.2  band is proposed. The models provide pointers to possible PAH populations in different regions.