Polarimetric Variations of Binary Stars. VI. Orbit-Induced Variations in the Pre-Main-Sequence Binary AK Scorpii

We present simultaneous UBV polarimetric and photometric observations of thepre-main-sequence binary AK Sco, obtained over 12 nights, slightly less thanthe orbital period of 13.6 days. The polarization is a sum of interstellar andintrinsic polarization, with a significant intrinsic polarization of 1% at5250A, indicating the presence of circumstellar matter distributed in anasymmetric geometry. The polarization and its position angle are clearlyvariable on time scales of hours and nights, in all 3 wavelengths, with abehavior related to the orbital motion. The variations have the highestamplitudes seen so far for pre-main-sequence binaries (~1%, ~30deg) and aresinusoidal with periods similar to the orbital period and half of it. Thepolarization variations are generally correlated with the photometric ones:when the star gets fainter, it also gets redder and its polarization increases.The color-magnitude diagram B-V, V exhibits a ratio of total to selectiveabsorption R=4.3 higher than in normal interstellar clouds (R=3.1). Theinterpretation of the simultaneous photometric and polarimetric observations isthat a cloud of circumstellar matter passes in front of the star, decreasingthe amount of direct, unpolarized light, and hence increasing the contributionof scattered (blue) light. We show that the large amplitude of the polarizationvariations can not be reproduced with a single scattering model and axiallysymmetric circumbinary or circumstellar disks.Comment: 24 pages, 10 figures, accepted for publication in the Astronomical Journa