2020PASP..132d4301T

We present UBVRI polarization observations of 12 B-type stars in the direction of the young open cluster NGC 6530, nine of them confirmed as probable members. The maximum degree of polarization for our target stars ranges from 0.16% to 2.84%, with an average value of 1.1%. We note variations of the polarization vector orientations across the cluster field. We use the polarization data to infer the wavelength of maximum polarization and obtain the total-to-selective extinction ratio for each star. Based on all 12 observed stars, the weighted average wavelength of the maximum degree of polarization is 0.563 ± 0.001 µm. This yields an average value of R = 3.15 ± 0.06 for the total-to-selective extinction associated with this field. Despite slight variations, R_V is not abnormal. Using UBV photometry combined with recent photometric calibrations yields average E(B - V) = 0.38 ± 0.07 (s.d.) mag and average A_V = 1.20 ± 0.24 (s.d.) mag. Both R_V and E(B - V) show star-to-star variations. Our study yields a photometry-based distance of 1.6 kpc. For more precise distance estimates, especially when seeking agreement with the Gaia parallaxes, better evaluation of peculiarity and duplicity of these stars is needed. We studied comparisons between the photometric UBV absolute magnitudes, Gaia-based absolute magnitudes and magnitudes obtained from direct measurements of the Balmer discontinuity and discuss the presently existing disagreements for the cluster field.