The rotationally modulated polarization of ξ Boo A

Overview & Synopsis

We have observed the active starξBoo A (HD 131156A) with high precision broadbandlinear polarimetry contemporaneously with circular spectropolarimetry. We find both signalsare modulated by the 6.43 d rotation period ofξBoo A. The signals from the two techniques are0.25 out of phase, consistent with the broadband linear polarization resulting from differentialsaturation of spectral lines in the global transverse magnetic field. The mean magnitude ofthe linear polarization signal is∼4 ppm G–1but its structure is complex and the amplitude ofthe variations suppressed relative to the longitudinal magnetic field. The result has importantimplications for current attempts to detect polarized light from hot Jupiters orbiting activestars in the combined light of the star and planet. In such work stellar activity will manifest asnoise, both on the time-scale of stellar rotation, and on longer time-scales – where changes inactivity level will manifest as a baseline shift between observing runs