High-precision broad-band linear polarimetry of early-type binaries II. Variable, phase-locked polarization in triple Algol-type system λ Tauri

Aims. To study the binary geometry of the classic Algol-type triple system lambda Tau, we have searched for polarization variations over the orbital cycle of the inner semi-detached binary, arising from light scattering in the circumstellar material formed from ongoing mass transfer. Phase-locked polarization curves provide an independent estimate for the inclination i, orientation Omega, and the direction of the rotation for the inner orbit. Methods. Linear polarization measurements of lambda Tau in the B, V, and R passbands with the high-precision Dipol-2 polarimeter have been carried out. The data have been obtained on the 60 cm KVA (Observatory Roque de los Muchachos, La Palma, Spain) and Tohoku 60 cm (Haleakala, Hawaii, USA) remotely controlled telescopes over 69 observing nights. Analytic and numerical modelling codes are used to interpret the data. Results. Optical polarimetry revealed small intrinsic polarization in lambda Tau with similar to 0.05% peak-to-peak variation over the orbital period of 3.95 d. The variability pattern is typical for binary systems showing strong second harmonic of the orbital period. We apply a standard analytical method and our own light scattering models to derive parameters of the inner binary orbit from the fit to the observed variability of the normalized Stokes parameters. From the analytical method, the average for three passband values of orbit inclination i = 76 degrees + 1 degrees/- 2 degrees and orientation Omega = 15 degrees(195 degrees) +/- 2 degrees are obtained. Scattering models give similar inclination values i = 72-76 degrees and orbit orientation ranging from Omega = 16 degrees (196 degrees) to Omega = 19 degrees (199 degrees), depending on the geometry of the scattering cloud. The rotation of the inner system, as seen on the plane of the sky, is clockwise. We have found that with the scattering model the best fit is obtained for the scattering cloud located between the primary and the secondary, near the inner Lagrangian point or along the Roche lobe surface of the secondary facing the primary. The inclination i, inferred from polarimetry, agrees with the previously made conclusion on the semi-detached nature of the inner binary, whose secondary component is filling its Roche lobe. The non-periodic scatter, which is also present in the polarization data, can be interpreted as being due to sporadic changes in the mass transfer rate.