Studies of diffuse interstellar bands V. Pairwise correlations of eight strong DIBs and neutral hydrogen, molecular hydrogen, and color excess

We establish correlations between equivalent widths of eight diffuse interstellar bands (DIBs), and examine their correlations with atomic hydrogen, molecular hydrogen, and E_B-V. The DIBs are centered at λλ 5780.5, 6204.5, 6283.8, 6196.0, 6613.6, 5705.1, 5797.1, and 5487.7, in decreasing order of Pearson's correlation coefficient with N(H) (here defined as the column density of neutral hydrogen), ranging from 0.96 to 0.82. We find the equivalent width (EW) of λ5780.5 is better correlated with column densities of H than with E_B-V or H₂, confirming earlier results based on smaller data sets. We show that the same is true for six of the seven other DIBs presented here. Despite this similarity, the eight strong DIBs chosen are not correlated well enough with each other to suggest they come from the same carrier.We further conclude that these eight DIBs are more likely to be associated with H than with H₂, and hence are not preferentially located in the densest, most UV shielded parts of interstellar clouds. We suggest that they arise from different molecules found in diffuse H regions with very little H₂ (molecular fraction f < 0.01). Of the 133 stars with available data in our study, there are three with significantly weaker λ5780.5 than our mean H-λ5780.5 relationship, all of which are in regions of high radiation fields, as previously noted by Herbig. The correlations will be useful in deriving interstellar parameters when direct methods are not available. For instance, with care, the value of N(H) can be derived from Wλ(5780.5).