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

Abstract

A B S T R A C TThe biconical Red Rectangle nebula exhibits very strong unidenti®ed 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 pro®le 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 de®ned 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 signi®cant correlation between the intensity of the 3.3-mm feature and that of either the unidenti®ed 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.q 1999 RAS, MNRAS 303, 446±454 Figure 6. The 3.3-mm spectra along the north-east and south-west interfaces out to approximately 6 arcsec, with the spectra towards HD 44179 for comparison. The spectra in (b) (right column) are normalized to a common peak intensity in order to allow comparison of the band pro®les.

Bispectrum speckle interferometry of the Red Rectangle: Diffraction-limited near-infrared images reconstructed from Keck telescope speckle data

“…Polarization maps by Perkins et al (1981) suggested that the spikes are produced by dust scattering at dense surfaces of the bicone. Unidentified optical emission bands, extended red emission, and 3.3 µm PAH emission line (of Type I) also originate close to the biconical surfaces along the X spikes (Kerr et al 1999). The spikes are clearly seen in all images, traced along the surface of the biconical outflow cavities to within the distances of 0.…”

“…At all wavelengths we observed continuum radiation from the Red Rectangle with the exception of the filter centered at 3.31 µm, where around 20-30% of the emission received is from strong line emission of polycyclic aromatic hydrocarbons (PAHs) as can be seen from spectra obtained with a 1. 2 aperture pointing at the center of the nebula (Kerr et al 1999). We presume that essentially all of the radiation in the K Brγ filter is continuum, although there may be very little of Br γ emission from the Red Rectangle observed in a 17 aperture, as is seen from spectrophotometry by Russell et al (1977).…”

VLT/UVES and WHT/UES absorption spectroscopy of the circumstellar envelope of IRC + 10° 216 using background stars$^\star$

“…For this object, Schmidt & Witt (1991) have shown that the strong optical emission features attributed to a subset of diffuse band carriers appear only at the bicone interfaces, where carbon-rich material is presumably being eroded by a bipolar plasma flow, and/or by the ultraviolet radiation from the central star. From a study of the spatial distribution and spectral structure of the 3.3 µm UIR feature, Kerr et al (1999) suggested that through this erosion the grains might produce even completely dehydrogenated DIB-emitting molecules, perhaps monocyclic carbon ring molecules (see also Kerr et al 1996). If this scenario were correct, the absence of DIBs in the outer layers of IRC + 10 • 216 could be understood as arising from the fact that its circumstellar material has not been processed at all in a similar way during its formation and ejection history.…”

The Infrared Emission Bands

“…Furthermore, PAH studies within extended objects reveal that the band profiles depend on distance from the illuminating star and hence the local physical conditions (Fig. 5, Kerr et al 1999;Tokunaga et al 1991;Bregman & Temi 2005;Candian et al 2012). The latter is exemplified by the dependence of the band profiles with the UV radiation field, G, and G/n e (with n e the electron density; Bregman & Temi 2005).…”