Interstellar Extinction and Polarization of Stars in the Open Cluster NGC 7160

Abstract

We present polarimetric observations of stars in the field of the open cluster NGC 7160. The multi-wavelength UBVRI polarization measurements of 21 stars that we performed indicate that some of the observed stars follow the typical wavelength dependence indicative of polarization by selective extinction in the interstellar medium, while others display multi-wavelength dependence strikingly different from nominal behavior. Considering only the former stars, we obtained a value of R V = 3.01 ± 0.03 for the total-to-selective extinction ratio toward the cluster, almost identical to the nominal expected value. For the remaining stars we suggest that their wavelength dependence of polarization is most likely altered by the presence of circumstellar material. Patterns in both the wavelength dependence of the degree of polarization and position angle show similarities found in the wavelength dependence of some carbon stars and PMS stars. PMS stars in NGC 7160 have been identified by Gray & Corbally and the multi-wavelength polarization obtained here suggests additional candidates. The rotation of the position vectors for a number of stars seems indicative for mixing of intrinsic and interstellar polarization components. It seems, however, that intra-cluster dust does not play a significant overall role in the polarization of these stars. The distribution of the interstellar extinction with distance as obtained from dust maps supports this finding. We also present V-band polarization measurements for 78 stars in and around the central region of cluster. The polarization ranges from 0.12% to 3.75% with a median value of 1.23%. While the polarization vectors display some scatter, an overall alignment exists, which would indicate polarization by foreground interstellar dust. Using the Planck 353 GHz channel we find agreement between polarization by emission and selective extinction and also that the optical polarization traces a magnetic field that is nearly parallel to the Galactic plane in this region.