STUDIES OF DIFFUSE INTERSTELLAR BANDS V. PAIRWISE CORRELATIONS OF EIGHT STRONG DIBs AND NEUTRAL HYDROGEN, MOLECULAR HYDROGEN, AND COLOR EXCESS

Abstract

We establish correlations between equivalent widths of eight diffuse\ninterstellar bands (DIBs), and examine their correlations with atomic hydrogen,\nmolecular hydrogen, and EB-V . The DIBs are centered at \\lambda\\lambda 5780.5,\n6204.5, 6283.8, 6196.0, 6613.6, 5705.1, 5797.1, and 5487.7, in decreasing order\nof Pearson\\^as correlation coefficient with N(H) (here defined as the column\ndensity of neutral hydrogen), ranging from 0.96 to 0.82. We find the equivalent\nwidth of \\lambda 5780.5 is better correlated with column densities of H than\nwith E(B-V) or H2, confirming earlier results based on smaller datasets. We\nshow the same is true for six of the seven other DIBs presented here. Despite\nthis similarity, the eight strong DIBs chosen are not well enough correlated\nwith each other to suggest they come from the same carrier. We further conclude\nthat these eight DIBs are more likely to be associated with H than with H2, and\nhence are not preferentially located in the densest, most UV shielded parts of\ninterstellar clouds. We suggest they arise from different molecules found in\ndiffuse H regions with very little H (molecular fraction f<0.01). Of the 133\nstars with available data in our study, there are three with significantly\nweaker \\lambda 5780.5 than our mean H-5780.5 relationship, all of which are in\nregions of high radiation fields, as previously noted by Herbig. The\ncorrelations will be useful in deriving interstellar parameters when direct\nmethods are not available. For instance, with care, the value of N(H) can be\nderived from W{\\lambda}(5780.5).\n