Disc orientations in pre-main-sequence multiple systems - A study in southern star formation regions

Classical T Tauri stars are encircled by accretion discs most of the time unresolved by conventional imaging observation. However, numerical simulations show that unresolved aperture linear polarimetry can be used to extract information about the geometry of the immediate circum-stellar medium that scatter the starlight. Monin et al. (1998, A&A, 339, 113) previously suggested that polarimetry can be used to trace the relative orientation of discs in young binary systems in order to shed light on the stellar and planet formation process. In this paper, we report on new VLT/FORS1 optical linear polarisation measurements of 23 southern binaries spanning a range of separation from 0.8 to 10. In each field, the polarisation of the central binary is extracted, as well as the polarisation of nearby stars in order to estimate the local interstellar polarisation. We find that, in general, the linear polarisation vectors of individual components in binary systems tend to be parallel to each other. The amplitude of their polarisations are also correlated. These findings are in agreement with our previous work and extend the trend to smaller separations. They are also similar to other studies, e. g., Donar et al. (1999, A&AS, 195, 7904), Jensen et al. (2000, IAUS, 200, 85; 2004, ApJ, 600, 789), Wolf et al. (2001, Pre-main sequence binaries with aligned discs?, in The formation of Binary Stars, ed. H. Zinnecker, & R. D. Mathieu, IAU Symp., 200, 295). However, we also find a few systems showing large differences in polarisation level, possibly indicating different inclinations to the line-of-sight for their discs.