Polarimetric Variations of Binary Stars. IV. Pre–Main-Sequence Spectroscopic Binaries Located in Taurus, Auriga, and Orion

We present polarimetric observations of 14 pre-main-sequence (PMS) binarieslocated in the Taurus, Auriga, and Orion star forming regions. The majority ofthe average observed polarizations are below 0.5%, and none are above 0.9%.After removal of estimates of the interstellar polarization, about half thebinaries have an intrinsic polarization above 0.5%, even though most of them donot present other evidences for the presence of circumstellar dust. Varioustests reveal that 77% of the PMS binaries have or possibly have a variablepolarization. LkCa3, Par1540, and Par2494 present detectable periodic andphase-locked variations. The periodic polarimetric variations are noisier andof a lesser amplitude (~0.1%) than for other types of binaries, such as hotstars. This could be due to stochastic events that produce deviations in theaverage polarization, a non-favorable geometry (circumbinary envelope), or thenature of the scatterers (dust grains are less efficient polarizers thanelectrons). Par1540 is a Weak-line TTauri Star, but nonetheless has enough dustin its environment to produce detectable levels of polarization and variations.A fourth interesting case is W134, which displays rapid changes in polarizationthat could be due to eclipses. We compare the observations with some of ournumerical simulations, and also show that an analysis of the periodicpolarimetric variations with the Brown, McLean, & Emslie (BME) formalism tofind the orbital inclination is for the moment premature: non-periodic eventsintroduce stochastic noise that partially masks the periodic low-amplitudevariations and prevents the BME formalism from finding a reasonable estimate ofthe orbital inclination.Comment: 70 pages, 20 figures, to be published in the Astronomical Journa