Spectropolarimetry of O supergiants

We present medium‐resolution spectropolarimetry at high signal‐to‐noise ratio of the Hα emission line of 20 O‐type supergiants. Five stars (25 per cent) of the sample show a statistically significant change in polarization through the line. We combine our Hα data with new K ‐band spectropolarimetry and archival low‐resolution optical spectropolarimetry to determine the polarigenic mechanism in the stars that show a line effect. We show that the line polarization change in the binary systems is caused by the classical ‘dilution’ mechanism, in which the Hα emission is essentially unpolarized and the continuum polarization is caused by intrabinary scattering. We find that the line effect in HD 108 is also well modelled by pure dilution, but suggest that the continuum polarization is the result of stochastic wind clumping. A similar description applies to the continuum polarization of HD 188001, although the line effect cannot be reproduced by pure dilution. We use low‐resolution spectropolarimetry to determine the interstellar polarization vector to λ Cephei, and confirm that the intrinsic polarization of the object is very low (<0.1 per cent, corresponding to an equator:pole density ratio of <1.25). The line polarization of this star is modelled using the torus three‐dimensional radiative‐transfer code. We show that the line effect is a consequence of symmetry breaking caused by the rapid rotation of the system (>200 km s −1 ), and that the system is similar polarimetrically to the O4 supergiant ζ Puppis. Finally, we note that the precision of current photo‐ and spectro‐polarimetric observations is insufficient to test structured wind models, which predict a continuum polarization of ∼0.1 per cent.