Some diffuse interstellar bands related to interstellar C2 molecules

We have investigated the correlations between the equivalent widths of 21 selected diffuse interstellar bands (DIBs) and the corresponding interstellar column densities N(C-2), N(CN), and N(CH), toward 53 stars with color excesses 0.11less than or equal to E(B-V)less than or equal to1.99. The observational data were derived primarily from echelle spectra acquired at R=38,000 as part of our extensive, continuing survey of the bands. All but six of the 53 final spectra show signal-to-noise ratios greater than or equal to800 at 5780 Angstrom. The principal result presented here is that seven of the 21 bands prove to be examples of the C-2 DIBs, a class of weak, narrow bands whose normalized equivalent widths W-lambda(X)/W-lambda(lambda6196) are well correlated specifically with N(C-2)/E(B-V) via power laws. In contrast, the similarly normalized equivalent widths of the 14 other, well-known DIBs analyzed here are uncorrelated, or weakly anticorrelated, with N(C-2)/E(B-V), to within the observational uncertainties. Thus, the polyatomic molecule(s) presumed to cause these seven C-2 DIBs may bear a direct chemical relation to C-2 that is not shared by the polyatomic molecules putatively responsible for the other 14 bands. The C-2 DIBs also show positive correlations with N(CN)/E(B-V) and N(CH)/E(B-V) in our particular sample of light paths, although generally with shallower slopes in the case of N(CN) and with greater scatter in the case of N(CH). Eleven additional C-2 DIBs are also identified but are not analyzed here. Among the 18 C-2 DIBs identified, four apparently have not been previously detected. The lambda4963 band is generally the strongest of the 18 C-2 DIBs, while the lambda4734 band shows the most sensitive correlation with N(C-2).