We described a method to use polarimetric measurements to get information on the alignment of the rotation axes of both components of PMS binaries. It gives the projection in the plane of the sky of the symmetry axis of bipolar nebulae. We argued that this method is efficient for classical T Tauri stars seen at a large inclination angle.
This information complements the measure of the inclination angle on the line-of-sight resulting from measurements. We have surveyed the literature for measurements on binaries with separation larger than the seeing, and we used polarimetric imaging, with a simple linear polarizer and a CCD to check for the respective polarization in close binaries. Numerical simulations and careful study of our data show that this method is heavily dependent on the atmospheric conditions, and to a lesser extent on the interstellar polarization. However, qualitative results are found for a few objects.
First results on four wide (AU) PMS low-mass binaries in Taurus suggest that alignement of the symmetry axes occurs at a rate compatible with that measured on the main-sequence. Because it concerns wide binaries, this alignment is thought to result from initial binary formation itself rather than via a posteriori tidal interactions. The method is promising but requires the use of more accurate polarimeters. We demonstrated that in most cases, single-beam polarimeters with polaroid-sheet (in the optical) or wire-grids (in the near-infrared) are inadequate.
To confirm the nature of the binary members, visible spectroscopy was performed. It allowed us to determine accurate spectral types along with Balmer and forbidden lines equivalent widths. Our measures confirm previous results that binaries are not randomly paired. We also find that in some cases the secondary component can have a larger flux than the brighter primary. We will present in a forthcoming paper (Duchêne et al. 1998), an extensive study of the respective accretion activities of both components in tight binaries.
The next step in this disk orientation study would consist of using a high angular resolution device and a polarimeter, to get the same information for closer binaries, where different formation mechanisms may be considered and tidal interaction get more important. From the spectrometric point of view, it will be interesting, in those close binaries, to search for a correlation between primary's and secondary's accretion or mass-loss as a function of the separation, indicating how the presence of a companion tends to influence the disk accretion at small distances.
© European Southern Observatory (ESO) 1998
Online publication: September 30, 1998