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

TL;DR

This study compiles and analyzes a high-quality sample of 53 single-cloud reddening curves in the Milky Way, providing precise measurements of dust extinction properties and confirming the average R_V value around 3.1.

Contribution

The paper presents the largest high-quality sample of single-cloud reddening curves in the Milky Way, reducing systematic uncertainties in dust extinction studies.

Findings

Average R_V = 3.1 ± 0.4, consistent with previous estimates.

Identified 53 high-quality reddening curves suitable for detailed dust analysis.

Refined criteria for selecting reliable reddening curves from diverse data sources.

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 merge a sample of 895 reddening curves of stars for which both FORS2 polarisation spectra and UVES high-resolution spectra are available. The resulting 111 sightlines toward OB-type stars have 175 reddening curves. For these stars, we derive their spectral type from the UVES high-resolution spectroscopy. To obtain high-quality reddening curves we exclude stars with composite spectra in the IUE/FUSE data due to multiple stellar systems. Likewise, we omit stars that have uncertain spectral type designations or stars with photometric variability. We neglect stars that show inconsistent parallaxes when comparing DR2 and DR3 from GAIA. Finally, we identify stars that show differences in the space and ground-based derived reddening curves between $0.28\,\mu$m 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 $R_V = 3.1 \pm 0.4$, confirming previous estimates. A future paper in this series will use the current sample of precise reddening curves and combine them with polarisation data to study the properties of Dark Dust.