A search for intrinsic polarization in O stars with variable winds

New observations of nine of the brightest northern O stars have been made w ith the Breger polarimeter on the 0.9 m telescope at McDonald Observatory a nd the AnyPol polarimeter on the 0.4 m telescope at Limber Observatory, usi ng the Johnson-Cousins UBVRI broadband filter system. Comparison with earli er measurements shows no clearly defined long-term polarization variability . For all nine stars the wavelength dependence of the degree of polarizatio n in the optical range can be fitted by a normal interstellar polarization law. The polarization position angles are practically constant with wavelen gth and are consistent with those of neighboring stars. Thus the simplest c onclusion is that the polarization of all the program stars is primarily in terstellar. The O stars chosen for this study are generally known from ultr aviolet and optical spectroscopy to have substantial mass loss rates and va riable winds, as well as occasional circumstellar emission. Their lack of i ntrinsic polarization in comparison with the similar Be stars may be explai ned by the dominance of radiation as a wind driving force due to higher lum inosity, which results in lower density and less rotational Battening in th e electron scattering inner envelopes where the polarization is produced. H owever, time series of polarization measurements taken simultaneously with H alpha and UV spectroscopy during several coordinated multiwavelength camp aigns suggest two cases of possible small-amplitude, periodic short-term po larization variability, and therefore intrinsic polarization, which may be correlated with the more widely recognized spectroscopic variations.