Abundances and rotational temperatures of the C2 interstellar molecule
  towards six reddened early-type stars
  Using high-resolution (~85000) and high signal-to-noise ratio (~200) optical
spectra acquired with the European Southern Observatory Ultraviolet and Visual
Echelle Spectrograph, we have determined the interstellar column densities of
C2 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 A1{\Pi}u-X1{\Sigma}+g. We report on the
identification of a few lines of the C2 (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 C2. 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 C2.
