15min:

J. A. THORBURN, L. M. HOBBS, T. OKA, D. E. WELTY AND D. G. YORK, Department of Astronomy and Astrophysics, the University of Chicago, Chicago, IL, 60637; B. J. MCCALL, Department of Astronomy, University of California at Berkeley, Berkeley, CA 94720; S. D. FRIEDMAN AND P. SONNENTRUCKER, Space Telescope Science Institute, Baltimore, MD 21218; T. P. SNOW, Department of Astronomy, University of Colorado, Boulder CO 80309.

Our survey of diffuse interstellar bands (DIBs) toward 53 stars with color excesses 0.11 \leq E(B-V) \leq 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 0.50. They are strongest toward HD 204827, the star whose sightline contains by far the highest C₂ and C₃ column densities, 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 0.54, mostly \geq 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 19.1 cm^-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 2 suggest that they are spin-orbit split levels of linear molecules.