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Description
Diffuse interstellar bands (DIBs) are broad absorption features measured in sightlines probing the diffuse interstellar medium. Although large carbon-bearing molecules have been proposed as the carriers producing DIBs, their identity remains unknown. DIBs make an important contribution to the extinction curve; the sightline. to the young massive star-forming region M17 shows anomalous extinction in the sense that the total-to-selective extinction parameter (R_V_) differs significantly from the average Galactic value and may reach values R_V_>4. Anomalous DIBs have been reported in the sightline towards Herschel 36 (R_V_=5.5), in the massive star-forming region M8. Higher values of R_V_ have been associated with a relatively higher fraction of large dust grains in the line of sight. Given the high R_V_ values, we investigate whether the DIBs in sightlines towards young OB stars in M17 show a peculiar behaviour. We measure the properties of the most prominent DIBs in M17 and study these as a function of E(B-V) and R_V_. We also analyse the gaseous and dust components contributing to the interstellar extinction. The DIB strengths in M17 concur with the observed relations between DIB equivalent width and reddening E(B-V) in Galactic sightlines. For several DIBs we discover a linear relation between the normalised DIB strength EW/A_V_ and R_V_^-1^. These trends suggest two groups of DIBs: (i) a group of ten moderately strong DIBs that show a sensitivity to changes in R_V_ that is modest and proportional to DIB strength, and (ii) a group of four very strong DIBs that react sensitively and to a similar degree to changes in R_V_, but in a way that does not appear to depend on DIB strength. DIB behaviour as a function of reddening is not peculiar in sightlines to M17. Also, we do not detect anomalous DIB profiles like those seen in Herschel 36. DIBs are stronger, per unit visual extinction, in sightlines characterised by a lower value of R_V_, i.e. those sightlines that contain a relatively large fraction of small dust particles. New relations between extinction normalised DIB strengths, EW/A_V_, and R_V_ support the idea that DIB carriers and interstellar dust are intimately connected. Furthermore, given the distinct behaviour of two groups of DIBs, different types of carriers do not necessarily relate to the dust grains in a similar way.
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