Polarization in the Young Cluster NGC 6611: Circumstellar, Interstellar, or ... Both?

Abstract

We obtained linear polarization observations of 82 A/B-type stars in the young cluster NGC 6611, in order to probe the circumstellar material and to search for any evidence of intracluster or interstellar material that could also contribute to the polarization. We found linear polarization values that reach up to 14%. We consider the distribution of the polarization, its position angle, correlations with extinction and membership probability, polarization variability and wavelength distribution to identify the origin of the polarization toward NGC 6611. The polarization is found to be dominated by interstellar polarization, although some stars also have some circumstellar polarization. There is no evidence for intracluster dust. Rather, the dust must be located in a low density cloud toward the general line of sight to NGC 6611 and in front of it. The depth of that cloud along the line of sight increases slowly from the south—east to the north—west. The cloud is threaded by a very uniform magnetic field.

Based on observations obtained on the 1.6 m telescope at the mont Mégantic Observatory (Canada) and on the 2m Bernard-Lyot telescope (TBL) operated by INSU/CNRS and Pic du Midi observatory (CNRS USR 5026). Financial support for the observations at Pic du Midi was provided by the Programme National de Physique Stellaire (PNPS) of CNRS/INSU, France.