Optical multicolor polarization observations in the region of the open cluster NGC 5749

Abstract

We present (UBVRI) multicolor linear polarimetric data for 31 of the brightest stars in the area of the open cluster NGC 5749 considered to study the properties of the interstellar medium (ISM) towards the cluster. Our data yield a mean polarization percentage of Pv ∼ 1.7%, close to the polarization value produced by the ISM with normal efficiency (P(%) = 3.5E (B-V)0.8) for a mean color excess of EB-V = 0.42. The mean angle of polarization vectors, θ = 74°, agrees quite well with the expected angle produced by dust particles aligned in the direction of the galactic disk (and the magnetic field) in the region. Our analysis indicates that the visual absorption affecting the stars in NGC 5749 is partially produced by a dust layer located up to 300 pc from the Sun and also by a second layer of dust closer to the cluster (located at least at 700 pc). The observed photometry and our polarization data are consistent with the existence of dust within the cluster. We also show in this work how polarimetry could be an excellent technique for identifying nonmember stars.