Evolution of the 3.3- m emission feature in the Red Rectangle

Abstract

Medium-resolution infrared spectroscopic observations of the biconical Red Rectangle nebula in the region of the 3.3-μm unidentified infrared (UIR) emission band are reported. The data were recorded at UKIRT using CGS4 and have allowed the peak wavelength, width, profile and intensity of the 3.3-μm feature to be investigated as a function of offset from the central star HD 44179. Analysis of the profile of the feature along the north-western bicone interface shows an evolution from Type 2 to Type 1 in the classification of Tokunaga et al. The 3.3-μm band recorded on-star shows a close fit to a Lorentzian profile, possibly suggesting a single class of carriers. Subtraction of the Lorentzian fit to the on-star spectrum from the spectrum at each offset reveals a new ‘3.28’-μm emission feature that grows in intensity relative to the main 3.3-μm band as a function of distance from the central star. The 3.28-μm emission band at large offset appears to correspond well in wavelength and FWHM with an absorption feature seen towards the Galactic Centre.