Polarization variability among Wolf-Rayet stars. I. Linear polarization of a complete sample of southern Galactic WC stars

As the first in a series of investigations on polarization variability of Wolf-Rayet (W-R) stars, this paper presents extensive time-dependent linear polarization observations of the seven known southern W-R stars of the carbon sequence brighter than magnitude b = 9. The results can be grouped into three categories: (1) three single-line stars, for which the observed stochastic variability in polarization decreases dramatically for stars with faster winds; the additional fact that the polarization vector of the nonvarying WC 7 star HD 156385 matches the interstellar polarization vector suggests that its wind is highly homogeneous, spherically symmetric, and very stable in time; (2) two short-period WC + O V binaries which show double-wave orbital modulation with very low residuals as expected from (1) for fast winds; and (3) two relatively long-period WC + O I binaries in which the effects of nonradial pulsations of the bright supergiants are important compared to the binary modulation.