A B S T R A C TWe have observed the Red Rectangle nebula with the Multi-Object Spectrograph on the WIYN telescope. Moderate-resolution spectra (Dl ¼ 0.4 Å ) in the region of 5800 Å were obtained in 3-arcsec apertures at over 50 positions in the nebula. Accurate and precise wavelength calibrations were obtained against a thorium-argon lamp and the sodium lines in the sky and nebula. The peak position and full width at half-maximum of the 5800-Å Red Rectangle band (RRB) were measured to beyond 15 arcsec from the star. The shortest wavelength of the band is found to be 5799.10^0.15 Å in the rest frame of the nebula. None of the emission bands has intensity coincident with the wavelength of the diffuse interstellar band (DIB) at 5797.11^0.05 Å . The 2-Å offset cannot be explained by an instrumental, spectroscopic or photophysical effect. The hypothesis that the same molecule may be the carrier of the RRB and the DIB is contradicted by these observations. As a further test of the hypothesis, absorption has been sought that would be due to a potential DIB carrier in the nebula. Tentative evidence for absorption is found in the RRB spectra taken within 9 arcsec of the star; but any absorption has a peak position essentially coincident in wavelength with the band maximum of the emission band.

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A B S T R A C TWe have calculated synthetic spectra of perpendicular and parallel rovibronic bands of cumulene carbene molecules of the form C n H 2 . The perpendicular bands are consistent with a regularly spaced group of diffuse interstellar bands (DIBs) near 6850 A Ê . Parallel bands calculated for these molecular structures are consistent with the intrinsic profile of the associated 6614-A Ê DIB. Both types of bands are expected for an electronic transition that these species should have at those energies. We could not determine if the molecule was charged or if an atom other than carbon terminated the chain-end. Constraints due to molecular geometry and temperature place the chain length at 7±15 carbons to fit the 6850-A Ê group and 9±13 carbons to fit the 6614-A Ê DIB.