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(C₂), N(CN), and N(CH), toward 53 stars with color excesses 0.11 ≤ E(B-V) ≤ 1.99. 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 ≥800 at 5780 Å. 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_λ(X)/W_λ (λ6196) are well correlated specifically with N(C₂)/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(C₂)/E(B-V), to within the observational uncertainties. Thus, the polyatomic molecule(s) presumed to cause these seven C₂ DIBs may bear a direct chemical relation to C₂ 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 C₂ DIBs are also identified but are not analyzed here. Among the 18 C₂ DIBs identified, four apparently have not been previously detected. The λ4963 band is generally the strongest of the 18 C₂ DIBs, while the λ4734 band shows the most sensitive correlation with N(C₂).