Observations of diffuse interstellar bands attributed to C^-₇

Recent advances in laboratory gas-phase spectroscopy of large molecules and their ions permit a direct comparison between the diffuse interstellar bands (DIBs) and proposed carriers. On the basis of gas-phase data, Tulej et al. recently suggested that five A²Π_u ←-X²Π_g electronic transitions of the linear carbon-chain anion C^-₇ match with DIBs. We have obtained high-resolution visible spectra of four reddened stars (HD 46711, HD 50064, HD 183143, and Cyg OB2 12) to make a detailed comparison with the C^-₇ laboratory data. Our data show that three of the C^-₇ bands (0⁰₀ at 6270.2 Ä, 3¹₀ at 6064.0 Å, and 1²₀ 3¹₀ at 4963.0 Å) are in good agreement with DIBs in wavelength and relative intensity. A fourth band (1¹₀ at 5612.8 Å) also agrees in intensity but is apparently off by 2 Å in wavelength. All other laboratory bands of C^-₇ are not expected to be detectable in astronomical spectra with the current level of sensitivity. The gas-phase spectrum of C^-₇ agrees with the DIBs better than that of any previously proposed molecule. However, the question of whether C^-₇ is a DIB carrier cannot be definitively answered until (1) better laboratory measurements confirm, refute, or explain the wavelength discrepancy for the 1¹₀ band and/or (2) better astronomical spectra reveal the presence or absence of other C₇ bands.