POLARIMETRIC VERSUS PHOTOMETRIC VARIABILITY AND THE DENSITY OF WR STAR WIND INHOMOGENEITIES

Intensive broad band observations of Wolf-Rayet (WR) stars by Moffat e t al. reveal photometric and polarimetric fluctuations on timescales o f hours, similar to the timescale of variation of narrow spectral feat ures on emission lines. These are attributed to localised density enha ncements (blobs) in the general wind. The r.m.s. fluctuations in the t wo observing modes are found to be in the ratio R = sigma(pol)/sigma(p hot) similar or equal to 0.05 whereas if both were due to electron sca ttering of starlight in single optically thin blobs far from the star, the ratio should be around unity. It has been suggested that the smal lness of R may be attributed to cancellation of the polarisation contr ibutions of sig significant numbers, N, of blobs emitted in different directions. We prove that this explanation of the small sigma ratio is untenable, by calculating the expected values of sigma(pol) and sigma (phot) due to random fluctuations in N and in blob positions. This is done analytically in a simplified case and numerically in the general case. No value of N can yield the observed R. We conclude that the dis crepancy must result from a substantial photometric contribution from broad band light emitted in the blobs and/or substantial reduction in the polarisation by multiple scattering in the blobs. Either explanati on demands that the blobs be very dense, with n(e) greater than or sim ilar to 5 x 10(13) cm(-3), when the blobs are at distances from the WR centre of around 10(12) cm.