On the intrinsic rest wavelengths of diffuse interstellar bands

Abstract

The rest wavelengths of DIBs are of fundamental importance owing to the lack of unambiguous identification for these mysterious features. Usually the wavelengths of DIBs are estimated using known interstellar 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 carriers of DIBs occupy. Here we argue that the narrowest known DIB 6196 A is the best reference feature. Also, we offer the geometrical center of gravity (the effective wavelength) of diffuse bands as a rest wavelength measurer for these generally asymmetric features. The exception is DIB 6196: its symmetrical (bottom) part must be used as a reference for the interstellar wavelength scale, and its center of gravity serves to study the variability of the feature visible at the upper part of its profile. We assessed the magnitude of variation of the center of gravity of DIBs at 5780, 5797, 6284 and 7224 A measured in 41 objects in the range of E{B-V} between 0.13 and 1.06 mag. with the lack of Doppler-split in profiles of interstellar atomic/molecular lines. To estimate the width of diffuse bands we offer to apply a parameter "effective width" W_eff which is the relation of the equivalent width to the depth of the feature. In contrast to habitual FWHM, W_eff is not sensitive to the profile shape irregularities. W_eff provides lower uncertainties of the measurements than the FWHM does. The gradual increase of W_eff, accompanied with the red-shift of the center of gravity of the profile, may suggest populating of higher transitions of 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. The difference of the effective width of DIBs in zeta and sigma clouds is discussed as well.