Abundances and rotational temperatures of the C-2 interstellar molecule towards six reddened early-type stars

Using high-resolution (similar to 85 000) and high signal-to-noise ratio (similar to 200) optical spectra acquired with the European Southern Observatory Ultraviolet and Visual Echelle Spectrograph, we have determined the interstellar column densities of C-2 for six Galactic lines of sight with E(B-V) ranging from 0.33 to 1.03. For our purposes, we identified and measured absorption lines belonging to the (1, 0), (2, 0) and (3, 0) Phillips bands A(1)Pi(u)-X-1 Sigma(+)(g). We report on the identification of a few lines of the C-2 (4, 0) Phillips system towards HD 147889. The curve-of-growth method is applied to the equivalent widths to determine the column densities of the individual rotational levels of C-2. The excitation temperature is extracted from the rotational diagrams. The physical parameters of the intervening molecular clouds (e. g. gas kinetic temperatures and densities of collision partners) were estimated by comparison with the theoretical model of excitation of C-2.