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 ban ds (DIBs) and proposed carriers. On the basis of gas-phase data, Tulej et a l. recently suggested that five A(2)Pi(u) <-- X(2)Pi(g) electronic transiti ons of the linear carbon-chain anion C-7(-) match with DIBs. We have obtain ed high-resolution visible spectra of four reddened stars (HD 46711, HD 500 64, 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 (0(0)(0) at 6270.2 Angstrom, 3(0)(1) at 6064.0 Angstrom, and 1(0)(2)3(0)(1) at 4963.0 Angstrom) are in good agreement with DIBs in wavelength and relative intens ity. A fourth band (1(0)(1) at 5612.8 Angstrom) also agrees in intensity bu t is apparently off by 2 Angstrom in wavelength. All other laboratory bands of C-7(-) are not expected to be detectable in astronomical spectra with t he current level of sensitivity. The gas-phase spectrum of C-7(-) agrees wi th the DIBs better than that of any previously proposed molecule. However, the question of whether C-7(-) is a DIB carrier cannot be definitively answ ered until (1) better laboratory measurements confirm, refute, or explain t he wavelength discrepancy for the 1(0)(1) band and/or (2) better astronomic al spectra reveal the presence or absence of other C-7(-) bands.