Wolf-Rayet wind structure and optical variability

Results are presented on the expected variability of Wolf-Rayet (WR) stars in broad-band optical polarimetry and photometry, and in emission line profiles, due to an inhomogeneous random distribution of blobs in spherical geometry. Time dependent 3−D simulations are carried out with blob ejection random in time and direction, and the radiation properties are evaluated in the optically thin limit. In contrast with previous purely statistical analyses, inclusion in the present treatment of a β velocity law and stellar occultation effects can yield results consistent with observations of the mean polarisation p̄ and the ratio R = σp/σphot of polarimetric to photometric variability. Such consistency puts constraints on model parameters. Indeed by considering combinations of the three observables σp, σphot, and p̄ it is possible to infer reasonably good estimates of three wind-blob parameters – theβ of their velocity law, the total mass loss rate Ṁ in blobs and the total number N of blobs emitted per wind flow time (R∗/v∞), provided the blobs are reasonably localised in angular and radial extent. It is found that typical data requires β ≥ 1.5, N ≈ 20–50 and Ṁ ≈ 10−4M /year. The N estimate is consistent with the number of observed narrow features on emission lines. Smaller values of β are excluded. This improved model shows that data do not after all demand very dense blobs as previously suggested, and offers a valuable diagnostic of WR wind structure parameters.