Spatial variation of UIR band intensity ratios in the Red Rectangle

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Strong infrared emission bands (3.3, 6.2, 7.7, 8.6 and 11.3 µm) appear in carbon-rich objects. These are thought to be due to carbon-carbon and carbon-hydrogen vibrational modes, however they await definite identification in order to fully understand the nature of interstellar and circumstellar media. This study uses the spectral variation within one object with the advantage that there is a relatively small variation in physical conditions, rather than those of widely different sources. We observed mid-infrared spectra of the Red Rectangle proto-planetary nebula as a function of the distance from the centre along the SE-NW whisker. The emission bands at 7.7, 8.6, 11.3 and 12.7 µm appear and the spatial evolution of these emissions has been recorded from the on-star position towards the outer nebula. The data were recorded at UKIRT using MICHELLE at a resolving power of 200. The spatial variation of the intensity ratio of the 8.6 µm to 11.3 µm features is presented and decreases moving away from the central illuminating star. We compare the observed intensity ratio of the Red Rectangle spatially with the ionisation model dependent on UV strength. It showed more ionised PAHs (Polycyclic Aromatic Hydrocarbons) are expected to exist near the centre than the outer nebula. We suggest that there is another possibility to explain the intensity variation of the 8.6 µm to the 11.3 µm feature; dehydrogenated PAHs in addition to ionised ones. Model spectra of dehydrogenated PAHs have been calculated using density functional theory using the commercial package, QChem 2.0, and this shows enhancement of the intensity of the C-C stretching modes in the range 6 - 9 µm relative to the C-H out-of-plane bending modes (~11 - 14 µm). The strengthening of the features in the range 6 - 9 µm region could be due to dehydrogenated PAHs in addition to ionised species.

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