The EDIBLES survey

X. The 6196 Å diffuse interstellar band: Identification of side DIBs as an indication of a small carrier molecule

¹ Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstr. 25/8, 6020 Innsbruck, Austria
² Universität Innsbruck, Institut für Ionenphysik und Angewandte Physik, Technikerstr. 25, 6020 Innsbruck, Austria
³ Univ Paris Est Creteil and Université Paris Cité, CNRS, LISA, 75013 Paris, France
⁴ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
⁵ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁶ GEPI, Observatoire de Paris, PSL Research University, CNRS, Université Paris-Diderot, Sorbonne Paris Cité, Place Jules Janssen, 92195 Meudon, France
⁷ School of Astronomy, Institute for Research in Fundamental Sciences, 19395-5531 Tehran, Iran
⁸ Department of Physics and Astronomy, The University of Western Ontario, London, ON N6A 3K7, Canada
⁹ Institute for Earth and Space Exploration, The University of Western Ontario, London, ON N6A 3K7, Canada
¹⁰ SETI Institute, 189 Bernardo Ave, Suite 100, Mountain View, CA 94043, USA
¹¹ Astrochemistry Laboratory, NASA Goddard Space Flight Center, Code 691, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
¹² Department of Physics, The Catholic University of America, Washington, DC 20064, USA
¹³ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁴ Centre d’Etudes et de Recherche de Grasse, ACRI-ST, Av. Nicolas Copernic, Grasse 06130, France
¹⁵ Lennard-Jones Laboratories, Keele University ST5 5BG, UK
¹⁶ ESTEC, ESA, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
¹⁷ LUNEX EMMESI Euromoonmars Earth Space Innovation, SBIC Noordwijk, The Netherlands

^★ Corresponding author; Alexander.Ebenbichler@uibk.ac.at

Received: 2 December 2024
Accepted: 5 February 2025

Abstract

Context. Numerous studies of diffuse interstellar band (DIB) profiles have detected substructures, which in turn suggests that large molecules are acting as their carriers. However, some of the narrowest DIBs generally do not show such substructures, suggesting the possibility of very small carriers.

Aims. Based on the previously found tight correlation of the three narrow DIBs at 6196, 6440, and 6623 Å and the present detection of weaker side DIBs to each of them in the extensive dataset from the ESO Diffuse Interstellar Bands Large Exploration Survey, we investigated whether they may stem from small linear carrier molecules. This approach can lead to concrete DIB carrier suggestions, which can be tested in laboratory measurements in future studies.

Methods. We suggest that the DIBs we studied here represent individual rotational transitions of a small molecule. We determined the molecular constants from observations and compared them with data from a large set of quantum-chemical calculations to constrain possible carrier candidates. Furthermore, we determined the rotational temperatures by fitting line ratios using the fitted molecular models.

Results. We determined molecular constants for three DIB systems and the corresponding transition types. The fitted rotational temperatures lie within the range of known interstellar diatomic molecules. We identified several DIB carrier candidates, almost all of them molecular ions. Some of them are metastable species, indicating the possibility of collision complexes as DIB carriers.

Conclusions. If our hypothesis holds, this would be a major step towards the identification of a carrier molecule of the 6196 Å DIB, the strongest among the narrow DIBs.