Red Rectangle Emission Bands

Abstract

The Red Rectangle is a bipolar nebula with a remarkable spectrum in which emission is excited by the light of the central star HD 44179. The emission spectrum contains lines of Na, CaII, CH⁺ and C₂, extended red emission (ERE) and infrared emission bands. Most striking, however, is the presence of strong unidentified optical emission bands near 5800, 5853, 6378 and 6615 Å (Schmidt et al. 1980, Warren-Smith et al 1981), the most prominent of which appear to arise from the same carriers as a set of relatively narrow diffuse interstellar absorption bands (λλ5797, 5849, 6376/9 and 6614) (Scarrott et al. 1992); these diffuse bands have been classified as a family by Krelowski and Walker (1986, 1987). New high signal-to-noise spectra recorded with the Anglo-Australian telescope confirm the link between the Red Rectangle emission and diffuse absorption bands; it is found that the peak wavelengths and widths of the Red Rectangle bands alter smoothly from the hot region near the star to the cooler outer regions of the nebula and extrapolate to the corresponding diffuse band characteristics. The results provide strong evidence that the carriers of these particular diffuse bands are free molecules.