Polarimetric versus photometric variability and the density of WR star wind inhomogeneities.

Details Polarimetric versus photometric variability and the density of WR star wind inhomogeneities. Polarimetric versus photometric variability and the density of WR star wind inhomogeneities.

Feb 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...306..519r&link_type=abstract

Astronomy and Astrophysics, v.306, p.519

Astronomy and Astrophysics

Astrophysics

14

Stars: Wr, Polarization: Circumstellar Matter, Stars: Mass Loss

Scientific paper

Intensive broad band observations of Wolf-Rayet (WR) stars by Moffat et al. reveal photometric and polarimetric fluctuations on timescales of hours, similar to the timescale of variation of narrow spectral features on emission lines. These are attributed to localised density enhancements (blobs) in the general wind. The r.m.s. fluctuations in the two observing modes are found to be in the ratio R=σ_pol_/σ_phot_=~0.05 whereas if both were due to electron scattering of starlight in single optically thin blobs far from the star, the ratio should be around unity. It has been suggested that the smallness of R may be attributed to cancellation of the polarisation contributions of significant numbers, N, of blobs emitted in different directions. We prove that this explanation of the small σ ratio is untenable, by calculating the expected values of σ_pol_ and σ_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 discrepancy 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 explanation demands that the blobs be very dense, with n_e_> 5x10^13^ cm^-3^, when the blobs are at distances from the WR centre of around 10^12^ cm.

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