The highly polarized open cluster Trumpler 27

Abstract

We have carried out multicolor linear polarimetry (UBVRI) of the brightest stars in the area of the open cluster Trumpler 27. Our data show a high level of polarization in the stellar light with a considerable dispersion, from $P = 4%$ to $P = 9.5%$. The polarization vectors of the cluster members appear to be aligned. Foreground polarization was estimated from the data of some non-member objects, for which two different components were resolved: the first one associated with a dust cloud close to the Sun producing $P_{λmax}=1.3%$ and $θ=146$ degrees, and a second component, the main source of polarization for the cluster members, originated in another dust cloud, which polarizes the light in the direction of $θ= 29.5$ degrees. From a detailed analysis, we found that the two components have associated values $E_{B-V} < 0.45 $ for the first one, and $E_{B-V} > 0.75$ for the other. Due the difference in the orientation of both polarization vectors, almost 90 degrees (180 degrees at the Stokes representation), the first cloud ($θ\sim 146$ degrees) depolarize the light strongly polarized by the second one ($θ\sim 29.5$ degrees).