Knowledge Bank

Our survey of diffuse interstellar bands (DIBs) toward 53 stars with color excesses $0.11\le E(B-V)\le 1.99$ has revealed ``the $C2$ DIBs'', a class of about a dozen weak and narrow DIBs whose intensities correlate well with column densities of carbon molecules $C2$ and $C3$ with the correlation coefficient $r=0.85\sim 0.50a$. They are strongest toward HD 204827, the star whose sightline contains by far the highest $C2$ and $C3$ column $densities,b$ and weak or undetectable toward Herbig's classic DIB star HD 183143, although the two stars have comparable color excess of 1.11 and 1.27, respectively. The $C2$ DIBs have high correlation among them $(r=0.94\sim 0.54$, mostly $\ge 0.75$) suggesting that their carriers are several molecules with similar chemical properties that exist abundantly in the diffuse interstellar medium where carbon molecules abound. We note among the $C2$ DIBs 4 pairs of doublet lines with very close spacings of $20.9\sim 19.1cm-1$. High correlations between the components of a doublet suggest that they are due to the same molecule. The magnitude of the splitting and the relative intensities of the doublets of $4\sim 2$ suggest that they are spin-orbit split levels of linear molecules.