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

The biconical Red Rectangle nebula exhibits very strong unidentified infrar ed (UIR) emission bands, a subset of the optical diffuse interstellar bands (in emission) and extended red emission (ERE). A key question is the exten t to which the carriers of these spectroscopic signatures may be related. I n a new study of the 3.3-mu m emission, CGS 4 spectra were recorded at UKIR T, which give information on the spatial distribution of the 3.3-mu m carri er in the nebula and on the width, peak wavelength and profile of the featu re as a function of offset from the central star, HD 44179. Both Type I (la mbda(0) similar to 3.289 mu m, full width at half-maximum FWHM similar to 0 .042 mu m) and Type 2 (lambda(0) similar to 3.296 mu m, FWHM similar to 0.0 20 mu m) 3.3-mu m 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 I feature appears in the nebula 5 arcsec south of the central star, wh ereas the 3.3-mu m band at 5 arcsec north appears to be a blend of Type 1 a nd Type 2. We find that there is no significant correlation between the int ensity of the 3.3-mu m 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-mu m band and this subs et of diffuse bands. Such a link could arise, for example, if these diffuse band carriers were formed by chemical erosion or photodissociation of poly cyclic aromatic hydrocarbon material.