Dark Dust and single-cloud sightlines in the ISM.

The precise characteristics of clouds and nature of dust in the diffuse ISM can only be extracted by inspecting the rare cases of single-cloud sightlines, which may be identified by the lack of significant Doppler splitting of interstellar lines observed at resolving power$\lambda /\Delta \lambda > 75,000$. We have searched for such sightlines using high-resolution spectroscopy towards reddened OB stars for which far UV extinction curves are known. We have compiled a sample of 186 spectra with 100 obtained specifically for this project with UVES. From our sample we identified 66 single-cloud sightlines, about half of which were previously unknown. We have used the CH/CH$^+$ line ratio of our targets to establish if the sightlines are dominated by warm or cold clouds. We found that CN is detected in all cold (CH/CH$^+ >1$) clouds, but is frequently absent in warm clouds. We inspected the WISE ($3 - 22 \mu$m) observed emission morphology around our sightlines and excluded a circumstellar nature for the observed dust extinction. We found that most sightlines are dominated by cold clouds that are located far away from the heating source. For 132 stars, we derived the spectral type and the associated spectral type-luminosity distance. We also applied the interstellar Ca II distance scale, and compared both these distance estimates with GAIA parallaxes, finding that these distance estimates scatter by ~40%. By comparing spectrophotometric distances with GAIA we detected for nine sightlines a hidden dust component amounting to a few mag of extinction. This {\it Dark Dust} is populated by $>1 \mu$m large grains and appears predominantly in the field of the cold ISM.