First ultraviolet spectropolarimetry of hot supergiants
Abstract
In this Letter we present the first UV spectropolarimetric observations of the early-type supergiant stars P Cygni and κ Cassiopeiae obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE). Near-simultaneous spectropolarimetric observations of each of these stars were also obtained in the optical region of the spectrum providing complete wavelength coverage from 1400 to 7600 Å. Contrary to model predictions, the intrinsic polarization of P Cyg remains constant in the Balmer continuum except for a broad dip between 2600 and 3000 Å. We discuss this decrease in polarization in terms of possible Fe line blanketing effects. The intrinsic position angle is essentially constant in the UV except for a rotation across the feature between 2600 and 3000 Å. The intrinsic polarization of κ Cas is small; most of the observed polarization is interstellar in nature. We note that the Serkowski curve extrapolated into the UV falls consistently below the observed polarization. Also, a slight rotation in position angle from optical to UV wavelengths is noted. We find very good agreement between the mean continuum polarization obtained by WUPPE and the broad-band UV polarization measurement obtained by Gehrels in 1968.