Observations of the 3.3- m UIR band in the Red Rectangle: relation to unidentified optical emission

Abstract

The biconical Red Rectangle nebula exhibits very strong unidentified infrared (UIR) emission bands, a subset of the optical diffuse interstellar bands (in emission) and extended red emission (ERE). A key question is the extent to which the carriers of these spectroscopic signatures may be related. In a new study of the 3.3-μm emission, CGS 4 spectra were recorded at UKIRT, which give information on the spatial distribution of the 3.3-μm carrier in the nebula and on the width, peak wavelength and profile of the feature as a function of offset from the central star, HD 44179. Both Type 1 (λ0∼ 3.289 μm, full width at half-maximum FWHM ∼ 0.042 μ m) and Type 2 (λ0∼ 3.296 μm, FWHM ∼ 0.020 μm) 3.3-μm features, as defined by Tokunaga et al., are found within the nebula. Type 2 is seen predominantly towards the central star, at the bicone interfaces and east and west of the star in the nebula. The broader Type 1 feature appears in the nebula 5 arcsec south of the central star, whereas the 3.3-μm band at 5 arcsec north appears to be a blend of Type 1 and Type 2. We find that there is no significant correlation between the intensity of the 3.3-μm feature and that of either the unidentified optical (diffuse) emission bands or ERE. This result suggests that there is at most an indirect link between the carrier(s) of the 3.3-μm band and this subset of diffuse bands. Such a link could arise, for example, if these diffuse band carriers were formed by chemical erosion or photodissociation of polycyclic aromatic hydrocarbon material.