Polarization Variability Arising from Clumps in the Winds of Wolf-Rayet Stars

Q. Li$^{1,3}$, J.~P. Cassinelli$^2$, J.~C. Brown$^3$, and R. Ignace$^4$

1 - Dept. of Astronomy, Beijing Normal University, Beijing 100875, China, qkli@bnu.edu.cn; 2 - Dept. of Astronomy, University of Wisconsin-Madison, 53711, USA, cassinelli@astro.wisc.edu; 3 - Dept. of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, Scotland, UK, john@astro.gla.ac.uk; 4 - Dept. of Physics and Astronomy, Eastern Tennessee State University, USA, ignace@etsu.edu

The polarimetric and photometric variability of Wolf-Rayet (WR) stars as caused by clumps in the winds, is revisited. In the model which is improved from Li et al. 2000, the radial expansion of the thickness is accounted for, but we retain the dependence on the $beta$ velocity law, stellar occultation effects. We again search for parameters that can yield results consistent with observations in regards to the mean polarization $bar p$, the ratio $sigma_{rm p}/sigma_{rm phot}$ of polarimetric to photometric variability, and the volume filling factor $f_V$. Clump generation and spatial distribution are randomized by the Monte Carlo method so as to produce clumps which are, in the mean, distributed uniformly in space and have time intervals with a Gaussian distribution. The generated clumps move radially outward with a velocity law determined by a $beta$ index, and the angular size of the clumps is assumed to keep fixed. By fitting the observed $sigma_{rm p}/sigma_{rm {phot}}$ and the volume filling factor $f_V$, the clump velocity law index $beta$ ($sim 2$) and clump ejection rate ($sim 1$) are inferred, and are found to be well constrained. In addition, the subpeak features on broad emission lines seem to support the clump ejection rate. Meanwhile, the fraction of the total mass loss rate that is contained in the clumps is obtained by fitting the observed polarization. We conclude that this picture for the clump properties produces a valuable diagnostic of WR wind structure.

Reference: 2009, Research in Astronomy and Astrophysics (RAA), 9, 558-576
Status: Manuscript has been accepted

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Email: qkli@bnu.edu.cn