Astron. Astrophys. 356, 619-626 (2000)

1. Introduction

Intrinsic polarisation from electron scattering polarisation in hot star winds requires an aspherical distribution of electron density. The quasi-steady, slowly varying, component of observed polarisation may be attributed to a mean rotationally symmetric wind structure, the magnitude of polarisation being determined by the product of envelope optical depth, a shape (asphericity) factor, and a viewing angle factor (Brown & McLean 1977). This description applies strictly in the single scattering limit, but is a reasonable approximation even for moderate optical depths (e.g., Daniel 1980). The more rapid variations (of order the wind flow time ) are usually attributed to localised density enhancements, or "blobs", moving outward in the wind, these features also producing the photometric variability and transient narrow emission line features in Wolf-Rayet stars (e.g., Moffat et al. 1994; Robert 1994; Brown et al. 1995). Such blobs could originate in a number of ways, including local mass loss enhancements due to non-radial pulsations of a star near its Eddington limit and/or rotational limits (Langer 1998), or possibly localised aspherical features arising within the wind itself by processes such as radiative instability or radiatively driven shocks (Lucy 1982; Owocki & Rybicki 1984; Gayley & Owocki 1995). The amplitude of polarimetric variability can provide valuable information on the nature and origin of such blobs.

Brown et al. (1995), Richardson et al. (1996) and Li et al. (2000) have addressed the problem of the effect on polarimetric variability by the presence of substantial numbers of blobs, the random polarisations of which result in partial cancellation. For a large number N of blobs with a fixed total number of electrons in all N blobs, Brown et al. and Richardson et al. found that the polarisation declines as owing to cancellation efffects. Here we address a different issue first raised by Brown (1994) concerning the polarimetric variability, namely the distinction between the effect of blobs created from local mass loss rate variations at the star and those arising from redistribution of the scattering electrons in the wind. The point is that density enhancements arising solely from the redistribution of electrons within an optically thin wind may produce little change in polarisation (tending to zero in some cases) unless the redistribution occurs over radial or angular scales that are substantial compared to (a) the radius of the star or (b) the angular extent over which the scattering angle influences the polarisation. The reason is that, in the optically thin limit, the contribution to the polarisation by the blob is proportional to the total number of scatterers, which does not vary upon redistribution, so that if the redistribution occurs over a small range of radius and angle, the polarisation is little changed. In this paper we quantify Brown's discussion for a number of simple cases relevant to stellar winds and discuss implications for shocks and other models of wind variability.

In Sect. 2, we specify our representation for wind clump density and derive the polarisation for several particular forms. In Sect. 3, the derived forms are used in considerations of redistribution of wind material from (a) conical slices that collapse to small dense bullets and (b) cones that collapse to conical caps. The former is a schematic representation for the case of a radiative instability and the latter for the case of a driven shock. A discussion of the results and applications is given in Sect. 4.

© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000
helpdesk.link@springer.de