Optical polarization study in the open cluster NGC 6250

FEINSTEIN, C.; VERGNE, M.M.; MARTINEZ, R. E.; ORSATTI, A.M.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

Año: 2008 vol. 391 p. 447 - 456

0035-8711

We present (UBVRI) multicolour linear polarimetric data for 32 of the brightest stars in the area of the open cluster NGC 6250, with the aim of studying the properties of the interstellar medium (ISM) towards the cluster. Our data yield a mean polarization Pv = 1.83 %,close to the polarization value produced by the ISM with normal efficiency for a mean colour excess of E(B−V) = 0.37. Our analysis indicates that the observed visual absorption in NGC6250  is caused by a nearby dust layer (within 300 pc) producing a polarization with anangle close to the Galactic plane (titaGP = 38◦). In addition, there are at least  two more dustlayers along the line of sight between the Sun and the cluster producing a change in theobserved polarization, making our results compatible with Neckel and Klare¡¦s results. The observations show differences between the orientation of the local magnetic field of the nearby dust layer and the one that is polarizing along the way to the cluster (titav = 20◦). The internal dispersion of the polarization values for the members of NGC 6250 seems to be compatiblewith the presence of intracluster dust. The majority of observed stars do not present evidence of intrinsic polarization in their light. In this work, we also show (as in several previous papers) how polarimetry is an excellent technique for identifying non-member stars.