The EDIBLES survey
III. C2-DIBs and their profiles
GEPI, Observatoire de Paris, PSL University, CNRS,
Place Jules Janssen,
2 ACRI-ST, 260 route du Pin Montard, 06904 Sophia Antipolis, France
3 Department of Physics and Astronomy and Centre for Planetary Science and Exploration (CPSX), The University of Western Ontario, London, ON N6A 3K7, Canada
4 SETI Institute, 189 Bernardo Avenue, Suite 100, Mountain View, CA 94043, USA
5 NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
6 Department of Physics, Catholic University of America, Washington, DC 20064, USA
7 European Southern Observatory, Alonso de Cordova 3107 Vitacura, Santiago, Chile
8 School of Astronomy, Institute for Research in Fundamental Sciences, 19395-5531 Tehran, Iran
9 School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, UK
10 Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
Accepted: 28 May 2018
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 Diffuse Interstellar 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 5895 Å Na I doublets and the detection of C2 lines. The C2 (2–0) (8750–8849 Å) 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 5769.8 Å. 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.
Key words: line: profiles / ISM: clouds / ISM: molecules
© ESO 2018