The EDIBLES survey. X. The 6196 {\AA} diffuse interstellar band: Identification of side DIBs as an indication of a small carrier molecule

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 A and the present detection of weaker side DIBs to each of them in the extensive data set 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 A DIB, the strongest among the narrow DIBs.