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Astron. Astrophys. 339, 113-122 (1998)

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1. Introduction

Our current understanding of low mass stellar formation has to take into account two very different yet complementary constraints. On one hand, when we consider individual stars, the current model put forward for embedded Young Stellar Objects (YSOs) includes a central stellar core, surrounded by an equatorial accretion disk and a remnant infalling envelope (see e.g., Shu et al. 1987). This stage is frequently associated with energetic bipolar molecular outflows, perpendicular to the disk (e.g., Bachiller, 1996 and references therein) and tracing the symmetry axis of the whole system. Even if the circumstellar disk is warped by the influence of a close companion (see e.g., Terquem and Bertout, 1992), its axis remains very close to the star's rotation one.

On the other hand, we also know that a large fraction of T Tauri stars (TTS) form in binary or multiple ([FORMULA]) systems (e.g., Ghez, et al. 1993; Simon et al. 1995; Ghez et al. 1997; Padgett et al. 1997). This ubiquitous property of the stellar formation process has a potentially enormous influence on the previous one because the circumstellar environment of the individual components of a multiple system can be deeply modified by the presence of a companion. For instance, in the case of a binary, the dust thermal continuum emission at millimeter wavelengths is smaller on average than for singles, indicating that the outer colder part of the circumstellar disks surrounding binary components have been removed, leading to a smaller reservoir of material immediately available for accretion (e.g., Osterloh & Beckwith 1995; Dutrey et al. 1996).

From the theoretical point of view, fragmentation now appears as the best binary formation mechanism to meet the observational constraints (Boss, 1993). Fragmentation mechanisms include fragmentation of a molecular cloud core (e.g., Pringle 1989) and growth of an instability in the outer parts of a massive circumstellar disk (e.g., Bonnell 1994). In the first case, if we neglect long term tidal interactions, fragmentation could yield non co-planar systems, if the initial cloud is elongated and the rotation axis oriented arbitrarily with respect to the cloud axis (see Bonnell et al. 1992). In the second case the disks around both binary components will always be co-planar, thus the stellar spin axes aligned.

The respective orientation of the system components' rotation axes therefore appears as an important geometrical parameter of a forming multiple system to disentangle between the various formation models. Unfortunately, such a determination is currently poorly constrained by observations. Our understanding of the stellar formation will be greatly improved when we establish the respective repartition of the axes orientations. Previous studies of the projected rotational velocities of both components of visual binaries on the main sequence by Weis (1974) and Hale (1994) showed a directional correlation of orbital and rotational axes, indicating a tendency toward spin alignment for systems with separation less than 30-40 AU. Within the framework of star formation theory, this distance is smaller than the size of an accretion disk, leading to think that star-disk interactions are indeed important in determining the final system structure. However, these studies concern stars on the main sequence where all the star-disk and disk-disk interaction processes between the components are likely to have ceased and the state of the system do not reflect the initial binary formation conditions anymore.

In PMS binary systems, the projection of the rotation axes of both components on the line-of-sight can be obtained through the combination of the projected rotational velocity, [FORMULA], the rotational period, and an estimate of the stellar radius. This determination is quite indirect and can induce large uncertainties. In this paper, we propose to use linear polarization measurements to determine the other angle that defines the three-dimensional orientation: the projection of the rotation axis in the plane of the sky. The basic idea relies on the fact that in T Tauri stars, linear polarization in the visible range is caused mostly by scattering. As a consequence, it retains information about the geometry of the circumstellar environment of each component, allowing evaluation of the symmetry axis of these environments. We have started a study of the linear polarization of individual components in PMS binaries in nearby star formation regions, including Taurus-Auriga.

Sect. 2 presents a description of our method, together with the expected limitations resulting from the finite signal-to-noise ratio. A review of polarimetric measurements on wide binaries ([FORMULA]) in the literature is presented in Sect. 3, together with a test of the method on closer binaries ([FORMULA], all but one being [FORMULA]), for which we have also observed individual spectra in order to assess their nature, classical or weak-line TTS (resp. CTTS or WTTS). The results are presented in Sect. 4 and discussed in Sect. 5. Expected improvements in the near future and a conclusion are given in Sect. 5 and Sect. 6.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998