On the Relation between Interstellar Spectral Features and Reddening

It is well known that interstellar spectral features correlate with color excess E(B - V). This suggests that measuring intensities of these features allows one to estimate reddening of stars. The aim of this paper is to check how tight intensities of interstellar diffuse bands (DIBs) are related to the amount of extinction, measured using E(B -V). We have measured equivalent widths of the strongest DIBs (centered at lambda lambda 5780.6 angstrom, 5797.0 angstrom, 6196.0 angstrom, 6379.3 angstrom, 6613.5 angstrom, and 8620 angstrom), as well as of CH (near 4300.3 angstrom) and CH+ (near 4232.5 angstrom) in high resolution, high S/N ratio echelle spectra from several spectrographs. The equivalent widths of the 8620 DIB in noisy spectra were measured using a template which was constructed using the high quality spectrum of BD+40 4220. DIB relations with the color excess in the range 0.1-2.0 mag were examined. Our analysis demonstrates that all the above mentioned interstellar spectral features (except for, perhaps, 6379 DIB) do correlate with E(B - V) relatively tightly (with the Pearson's correlation coefficient of greater than or similar to 0.8). Moreover, the observed scatter is apparently not caused by measurement errors but of physical origin. We present several examples where the strength ratios of a DIB/molecule to E(B - V) are different than the average.