The William-Wehlau Spectropolarimeter: Observing hot stars in all four Stokes parameters

We introduce a new polarimeter unit, which, mounted at the Cassegrain focus of any telescope and fiber-connected to a fixed CCD spectrograph, is able to measure all Stokes parameters I, Q, U, and V across spectral Lines of br ight stellar targets and other point sources in a quasi-simultaneous manner . Applying standard reduction techniques for linearly and circularly polari zed light, we an able to obtain photon noise-limited line polarization. We briefly outline the technical design of the polarimeter unit and the linear algebraic Mueller calculus for obtaining polarization parameters of any po int source. In addition, practical limitations of the optical elements are outlined. We present first results obtained with our spectropolarimeter for four bright, hot-star targets. We confirm previous results for H alpha in the bright Be star gamma Cas and find linear depolarization features across the emission-line complex C III/C IV (5696/5808 Angstrom) of the WR + O bi nary gamma(2) Vel. We also find circular line polarization in the strongly magnetic Ap star 53 Cam across its H alpha absorption line. No obvious line polarization features are seen across H alpha in the variable O star theta (1) Ori C above the sigma similar to 0.2% instrumental level.