Polarimetric versus photometric variability of Wolf-Rayet star winds

Abstract

Intensive broad band observations of WR stars by Moffat et al. reveals photometric and polarimetric variability on time-scales of hours, similar to that seen on narrow spectral features. The r.m.s. fractional photometric variation is found to be about 20 times the r.m.s. polarimetric variation whilst simple scattering theory predicts ratios of order unity. The possibility that this discrepancy is due to polarimetric cancellation of the Stokes parameters of light scattered from many dense blobs at random positions (known from spectrometric data to be present) is investigated analytically and by simulations. It is found that factors of 20 reduction in the polarimetric variability cannot be achieved for reasonable numbers of blobs and it is concluded that further polarimetric suppression by substantial blob optical depth and/or intrinsic blob emission must play a role. These results put further constraints on physical conditions in WR wind inhomogeneities.