The EDIBLES survey

III. C₂-DIBs and their profiles

¹ GEPI, Observatoire de Paris, PSL University, CNRS, Place Jules Janssen, 92190 Meudon, France
e-mail: meriem.el-yajouri@obspm.fr
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

Received: 26 March 2018
Accepted: 28 May 2018

Abstract

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.