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.

Observations and identifications of the Merrill–Sanford bands of the silicon dicarbide molecule are reported for a range of N and J stars. The data were recorded at 0.8−1.6 Å resolution between 4100 and 5500 Å. A full description of the assignments and spectral features in the stars is given. The spectra are characterized by absorption from the ground vibrational level (0, 0, 0) and from higher‐lying vibrational levels involving one quantum of excitation in ν1, and up to two quanta in ν2 and ν3, giving rise to numerous hot‐bands. The first results of rotational contour calculations are presented, and are consistent with SiC2 lying in the upper atmosphere of the star with a rotational temperature substantially lower than that of the photosphere.

No abstract

The optical spectrum of the carbon star IRAS 12311−3509 is dominated by the Merrill–Sanford emission bands of SiC2, by absorption and emission in the Swan system of C2, and by resonance emission lines of neutral metals. The infrared energy distribution is flat from 1 to 60 μm. These observations are interpreted as arising from a star with a cool dusty disc which is edge‐on to the observer and obscures direct starlight. The infrared continuum is caused predominantly by absorption of stellar light by dust in the disc and re‐emission at longer wavelengths. The optical stellar spectrum is seen by reflection off dusty material which lies out of the plane of the disc, and the molecular and atomic emission arises in the same geometry through resonance fluorescence. The object has similarities to the J‐silicate stars, but may have a carbon‐rich rather than oxygen‐rich disc. A full spectroscopic assignment and discussion of the SiC2 bands and their intensities are given. Modelling of the rotational contours of the band yields a rotational temperature of 250 K, indicating very cool gas.

Although observed in the blue-green spectra of some carbon stars as long ago as 1926, the Merrill-Sanford bands were not identified as arising from S1C2 until 1956. In 1984 it was recognized that the molecule is not linear like C3, but has an unusual T-shape. Following recent laboratory work, extensive S1C2 absorption spectra of a number of stars, including W Pic, RV TrA, and T Mus, have been assigned for the first time. Hot bands involving lowfrequency vibrations are generally very strong in typical N-type spectra but were greatly weakened in T Mus for a time in 1994, indicating that the bands were then formed in a much cooler region than those typical of carbon-star photospheres. Of particular interest is the spectrum of IRAS 12311-3509, where S1C2 bands appear in emission, possibly indicating an edge-on disk.These results are discussed more fully in ApJ 471, L107, 1996. available at https://www.cambridge.org/core/terms. https://doi