2018A&A...616A.143E

Context. An unambiguous identification of the carriers of the diffuse interstellar bands (DIBs) would provide important clues to the life cycle of interstellar matter. The so-called C₂-DIBs are a class of very weak bands that fall in the blue part of the optical spectrum and are associated with high column densities of the C₂ molecule. DIB profile structures constrain potential molecular carriers, but their measurement requires high signal-to-noise, high-resolution spectra and the use of sightlines without Doppler splitting, as typical for a single-cloud situation.
Aims. Spectra from the ESO DiffuseInterstellar Bands Large Exploration Survey (EDIBLES) conducted at the Very Large Telescope (ESO/Paranal) were explored to identify single-cloud and high C₂ column sightlines, extract the corresponding C₂-DIBs and study their strengths and profiles, and to investigate in detail any sub-structures.
Methods. The target selection was made based on profile-fitting of the 3303 and 5895Å NaI doublets and the detection of C₂ lines. The C₂ (2-0) (8750-8849Å) Phillips system was fitted using a physical model of the host cloud. C₂ column densities, temperatures as well as gas densities were derived for each sightline.
Results. Eighteen known C₂-DIBs and eight strong non-C₂ DIBs were extracted towards eight targets, comprising seven single-cloud and one multi-cloud line-of-sights. Correlational studies revealed a tight association of the former group with the C₂ columns, whereas the non-C₂ DIBs are primarily correlated with reddening. We report three new weak diffuse band candidates at 4737.5, 5547.4, and 5769.8Å. We show for the first time that at least 14 C₂-DIBs exhibit spectral sub-structures which are consistent with unresolved rotational branches of molecular carriers. The variability of their peak separations among the bands for a given sightline implies that their carriers are different molecules with quite different sizes. We also illustrate how profiles of the same DIB vary among targets and as a function of physical parameters, and provide tables defining the sub-structures to be compared with future models and experimental results.