A Large-Sample Study of Interstellar Reddening in Galactic Star Clusters
The interstellar reddening (extinction or absorption) is caused mainly by the dust grains of different sizes and compounds. All astrophysical observations are somehow affected by this effect, depending on the wavelength and location of the observed object. Different methods and techniques have been developed over the last nearly one hundred years to determine the reddening and to map the solar neighbourhood. The members of open clusters are a consistent population of stars formed together with a homogeneous metallicity. Therefore, the reddening can be estimated in a statistical way, avoiding all problems of single stars and spectral-energy-distribution fitting, for example. Other than for most available works, we can also use more massive upper main-sequence stars for the analysis. We used classical colour-colour diagrams (also known as the Q-method) for (B-V), (BP-RP), (U-B), (U-BP), (U-RP), and (U-G) to determine consistent mean reddening values and their errors. The corresponding standard lines for the fitting are independent of the age and metallicity. We determined colour excesses for 5377 Galactic open clusters covering Galactocentric distances from two to twenty kpc, respectively. The derived values were checked with existing dust and extinction maps. In general, we found good agreement. The solar neighbourhood of +-1500 pc shows a very patchy structure of the dust composition. Our results lay the foundations for constructing a new map of the reddening (dust) distribution and estimating clusters' metallicities by isochrone fitting, keeping the distances and reddening values fixed.