USING POLARIMETRY TO CHECK ROTATION ALIGNMENT IN PMS BINARY STARS - PRINCIPLES OF THE METHOD AND FIRST RESULTS

We present a new method to use polarimetric measurements to estimate t he relative orientations of the star+disk system in each component of Pre Main Sequence (PMS) T Tauri binary stars. The principles of the me thod are presented, together with a review of polarimetric data availa ble in the literature on wide systems (8 - 40'') in Taurus. We show th at this method can be subject to a bias introduced by the interstellar polarization, but we find that in four sources that are not affected by this bias, the rotation axes of both components are preferentially parallel. We also find one system where the axes are clearly not paral lel. Because it concerns wide binaries, this alignment is thought to r esult from initial binary formation itself rather than via a posterior i tidal interactions. The method is promising but requires the use of more accurate polarimeters. In order to test the possibility to study tighter systems, we have also obtained imaging polarimetric measuremen ts and spectra for each components of closer binaries (< 3(.)('')2). O ur first polarimetric measurements lack S/N, but the spectra of indivi dual components of these close binaries provide information on their r espective spectral types and their accretion rate through H alpha equi valent widths and fluxes. We use these spectra to confirm the nature a nd the pairing tendency of the TTS under study. Implications on the bi nary formation mechanism are discussed.