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 bright stellar targets and other point sources in a quasi‐simultaneous manner. Applying standard reduction techniques for linearly and circularly polarized light, we are 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 point 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 Hα in the bright Be star γ Cas and find linear depolarization features across the emission‐line complex C iii /C iv (5696/5808 Å) of the WR + O binary γ 2 Vel. We also find circular line polarization in the strongly magnetic Ap star 53 Cam across its Hα absorption line. No obvious line polarization features are seen across Hα in the variable O star θ 1 Ori C above the σ ∼ 0.2% instrumental level.