On the Intrinsic Rest Wavelengths of Diffuse Interstellar Bands

The rest wavelengths of diffuse interstellar bands (DIBs) are parameters of fundamental importance owing to the lack of unambiguous identification for these mysterious features. Usually the wavelengths of DIBs are estimated using known interstellar atomic or molecular lines serving to shift the spectrum to the rest wavelengths velocity scale. However, the latter may not share in fact the same parts of a cloud which the carriers of diffuse bands occupy. Here we argue that the narrowest known diffuse interstellar band 6196 Å is the best reference feature for building the ‘‘interstellar’’ wavelength scale. Also, we offer the geometrical center of gravity (the effective wavelength) of the diffuse bands as a rest wavelength measurer for these generally asymmetric features. We assessed the magnitude of variation of the center of gravity of the diffuse bands at 5780, 5797, 6284, and 7224 Å measured in 41 lines of sight in the broad range of interstellar reddening (\(E_{B-V}\) varies in the range of 0.13–1.06 stellar magnitudes) with the lack of evident Doppler-split in the profiles of interstellar atomic/molecular lines. We demonstrated that diffuse bands show gradual broadening of their profile widths, accompanied with the red-shift of the center of gravity, i.e., the red wing of the profiles is their most variable part. To estimate the width of the diffuse bands, we offer to apply an ‘‘effective width’’ parameter \(W_{\textrm{eff}}\), which is the relation of the equivalent width (\(EW\)) to the depth of the feature. In contrast to habitual half width at half maximum (\(FWHM\)), the parameter originally introduced by us in 2008 (Galazutdinov, LoCurto, Krełowski) is not sensitive to the profile shape irregularities. \(W_{\textrm{eff}}\) provides lower measurement uncertainties than the \(FWHM\) does. The gradual increase of \(W_{\textrm{eff}}\), accompanied with the red-shift of the center of gravity of the profile, may suggest populating of higher transitions of the P branch of the bands of molecules, assuming the latter are DIB carriers. It is also shown, that diffuse bands are broader although more shallow in the harsh conditions of \(\sigma\)-clouds, where atomic and molecular lines are weakened and/or totally absent. The difference of the effective width of DIBs in \(\zeta\) and \(\sigma\) clouds is discussed as well.