Diffuse interstellar bands in the HII region M17: Insights into their
  relation with the total-to-selective visual extinction $R_V$
  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. The sight line to the young massive
star-forming region M17 shows anomalous extinction in the sense that the
total-to-selective extinction parameter differs significantly from the average
Galactic value and may reach values $R_{V} > 4$. Given the high $R_V$ values,
we investigate whether the DIBs in sight lines 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 $R_{V}$. The DIB strengths
in M17 concur with the observed relations between DIB equivalent width and
reddening $E(B-V)$ in Galactic sight lines. 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: (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. The DIB behaviour as a function
of reddening is not peculiar in sight lines to M17. Also, we do not detect
anomalous DIB profiles as seen in Her 36. DIBs are stronger, per unit visual
extinction, in sight lines characterised by a smaller value of $R_{V}$ (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 connected. 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.
