Polarization and Interstellar Extinction Towards the Open Star Cluster NGC 6633

Abstract

We present optical polarization data of 110 stars in the direction of NGC 6633. From multi-wavelength BVRI measurements of 64 of these stars we obtained a wavelength of maximum polarization that yielded an estimate of total-to-selective extinction ratio R_V = 3.18 ± 0.04 over the observed region. The distribution of the polarization position angles overall reveals a fairly uniform Galactic magnetic field. The fractional V-band polarization of the cluster's members is between 0.8% and 1.4%, clearly above the 0.5% average value measured for the nearby field stars in the sample. We analyze the V-band polarization and maximum polarization as a function of color excess and extinction based on dust maps, Gaia, and uvbyβ photometry to discuss the polarization efficiency. The interpretation depends on the set of color excess or extinction data used, but for the majority of stars the polarization is confined below the empirical p_max ∼ 9%E(B ‑ V) upper limit for the maximum polarization. On average, the normalized Stokes parameters show a slight offset of the foreground and background stars in comparison to the cluster's members, but a significant overlap for many stars along the line of sight is present. We discuss a comparison of our optical polarization measurements to the Planck data in the context of the interpretation that the dust causing the infrared emission is also responsible for the optical polarization. We suggest that the polarization we measure for stars at the cluster's distance around 400 pc is due to foreground dust between 100 and 350 pc and that multiple clouds along the line of sight could be the reason for some of the variations we observe.