Spatial distribution and interpretation of the 3.3 \mum PAH emission band of the Red Rectangle

TL;DR

This study maps the spatial distribution of the 3.3 μm PAH emission in the Red Rectangle nebula, supporting the existence of two spectroscopic components linked to different PAH molecular sites, and compares observations with laboratory spectra.

Contribution

It provides the first spatially resolved analysis distinguishing two components of the 3.3 μm PAH emission and links them to specific molecular structures on PAHs.

Findings

Two distinct spatial distributions for 3.3 μm components.

Identification of 'bay' and 'non-bay' PAH sites as carriers.

Support for two classes of 3.3 μm band carriers.

Abstract

The spatial distribution of 3.3 \mum PAH and associated emission in the 3.3" x 6.0" inner region of the Red Rectangle nebula has been determined using the UIST imager-spectrometer at the United Kingdom Infrared Telescope (UKIRT). Interpretation of the 3.3 \mum feature as comprising two spectroscopic components centred at 3.30 \mum and 3.28 \mum, as put forward by Song et al. (2003, 2007), is supported by these data which reveal that they have different spatial distributions. It is deduced that there are two classes of 3.3 \mum band carrier with a peak wavelength separation of ~0.02 \mum. From comparison of the 3.3 \mum observations with laboratory and theoretical spectra for a range of PAH molecules it is proposed that the 3.28 \mum and 3.30 \mum components arise from 'bay' and 'non-bay' hydrogen sites, respectively, on the periphery of small neutral PAHs. Observational data are also obtained for L-band continuum emission and for the Pfund \epsilon hydrogen recombination line.