2023MNRAS.523.4158G

This paper describes the specific profile of the diffuse interstellar band (DIB) 6196 Å in objects belonging to the Sco OB2 association. The investigated feature is the narrowest known strong diffuse band. Reddened Sco OB2 association objects show slightly broadened and split (W-shaped) 6196 DIB profiles. The split cannot be explained by the presence of two (or more) clouds of different radial velocities along the line of sight because it is not observed in the much narrower identified atomic/molecular features. The observed DIB profile may (in some cases) be a composition of the band centre and its head of a still unknown molecule. Our estimation of the rotational temperature of C₂ exhibits systematically higher values for objects with the observed split effect (Sco OB2 members). These split profiles are observed in both ζ and σ types of clouds, although in Sco OB2 objects only, suggesting that the phenomenon originates in some specific and currently unclear physical/chemical conditions inside the Sco OB2 association. It could be that a lower amount of small dust particles in Sco OB2 affects the flux of ultraviolet photons, providing a higher rotational temperature of DIB 6196 carriers, which is seen as a profile split.