On the polarimetric variability of bright O-type stars

Polarimetric data associated with multi-parameter observational campaigns of selected bright O-type stars and their variable winds, are analysed in relation to the outcomes of the UV and optical spectroscopic studies. For the stars xi Per and lambda Cep, individual measurement uncertainties are Delta p ~ +/-0.0002 with nightly mean uncertainties of Delta p ~ +/-0.00007. Although variability is apparent on a night-to-night basis, with differences in delta p ~ 0.0002, no correlations are found between these and the periodic behaviours associated with the stellar Si IV and Hα lines. Similar polarimetric variability is seen in the data for the standard star phi Cas used as a reference in this observing campaign. It is suggested that all of these low level fluctuations are not intrinsic to the stars but are engendered by structured instrumental polarization in the diffraction pattern and depolarization effects in combination with inconsistent target acquisition and with variable seeing conditions in the Earth's atmosphere. Reassessment of older data for lambda Cep from Hayes (\cite{hayes2}) also supports this thesis.