Dark dust

III. The high-quality single-cloud reddening curve sample: Scrutinizing extinction curves in the Milky Way^★

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
e-mail: Ralf.Siebenmorgen@eso.org
² European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
³ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁴ Materials Spectroscopy Laboratory, University of Rzeszów, Pigonia 1 Street, 35-310, Rzeszów, Poland
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780 Bochum, Germany
⁷ Universidad Católica del Norte, Instituto de Astronomía, Avenida Angamos 0610, Antofagasta, Chile
⁸ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland

Received: 25 July 2022
Accepted: 1 July 2023

Abstract

The nature of dust in the diffuse interstellar medium can be best investigated by means of reddening curves where only a single interstellar cloud lies between the observer and the background source. Published reddening curves often suffer from various systematic uncertainties. We merged a sample of 820 reddening curves of stars for which both FORS2 polarization spectra and UVES highresolution spectra are available. The resulting 111 sightlines towards OB-type stars have 175 reddening curves. For these stars, we derived their spectral-type from the UVES high-resolution spectroscopy. To obtain high-quality reddening curves, we excluded stars with composite spectra in the IUE/FUSE data due to multiple stellar systems. Likewise, we omitted stars that have uncertain spectral-type designations or stars with photometric variability. We neglected stars that show inconsistent parallaxes when comparing data releases two and three from Gaia. Finally, we identified stars that show differences in the space- and ground-based-derived reddening curves between 0.28 µm and the U band or in R_V. In total, we find 53 stars with one or more reddening curves passing the rejection criteria. This provides the highest-quality Milky Way reddening curve sample available today. Averaging the curves from our high-quality sample, we find R_V = 3.1 ± 0.4, confirming previous estimates. A future paper in this series will use the current sample of precise reddening curves and combine them with polarization data to study the properties of dark dust.