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 unidentied 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-mm emission, CGS 4 spectra were recorded at UKIRT, which give information on the spatial distribution of the 3.3-mm carrier in the nebula and on the width, peak wavelength and prole of the feature as a function of offset from the central star, HD 44179. Both Type 1 (l 0 , 3:289 mm, full width at half-maximum FWHM , 0.042 mm) and Type 2 (l 0 , 3:296 mm, FWHM , 0.020 mm) 3.3-mm features, as dened 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 3.3mm band at 5 arcsec north appears to be a blend of Type 1 and Type 2. We nd that there is no signicant correlation between the intensity of the 3.3-mm feature and that of either the unidentied 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-mm 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.