During the past five years, many studies have addressed the issue of multiplicity in low mass star-forming regions. A majority of G-K main sequence (MS) dwarfs belong to multiple systems in the solar vicinity (Duquennoy & Mayor 1991), and several studies (Leinert et al. 1993, Reipurth & Zinnecker 1993, Ghez et al. 1993, Simon et al. 1995) have shown that this is also the case among pre-Main Sequence (PMS) stars. The binary fraction can vary with star formation region (SFR), and in the Taurus cloud, the binary excess over MS stars is of the order of 1.7, indicating that binarity is a fundamental feature of stellar formation, at least in this SFR (see Duchêne, 1999).
Amongst the various mechanisms proposed so far for binary star formation, fragmentation appears as the most likely to meet observational constraints (Boss 1993). Numerical codes have been successful in reproducing the formation of binary or multiple systems (Bonnell et al. 1992, Sigalotti & Klapp 1997a,b, Boss 1997, Burkert et al. 1997). However, current binary formation codes do not offer enough resolution and time span to follow the formation and evolution of circumstellar accretion disks. Only larger structures, which are not necessarily in equilibrium, are predicted, providing only indirect information about these disks, and the fate of the available circumstellar matter remains unclear.
Various authors have studied tidal interaction of circumstellar disks in binary systems for coplanar disks (see a review by Lin & Papaloizou 1993), and demonstrated that Lindblad resonances create a gap in the binary environment, separating two circumstellar disks from a circumbinary one. Accretion from the outer disk onto the inner ones and, eventually, on both stars is prevented by gravitational resonances. However, Artymowicz & Lubow (1996) showed that, under some hypotheses on the disk properties, matter could flow through one or two points of the inner ring of the circumbinary disk toward the central system. If both stars have similar masses, both circumstellar disks are replenished, while, in the case of very unequal masses, the accretion funnel is mainly directed toward the secondary. On the other hand, Bonnell et al. (1992) used a SPH code to study cloud fragmentation processes and concluded that fragmentation of an elongated cloud rotating around an arbitrary axis leads to parallel but non-coplanar accretion disk like structures. They find that, in low mass ratio systems (), accretion of low angular momentum material is directed toward the centre of mass, which is close to the most massive star. Thus, in these systems, the primary appears more obscured and reddenned than its less active companion. The different conclusions about the more actively accreting star are likely due to the different approaches used in these studies: while Artymowicz & Lubow (1996) start with a star+disk system to which they add a second star, Bonnell & Bastien (1992) model the formation of such a binary from the onset of the gravitational collapse. Also, the different initial conditions used in these two studies imply different angular momentum values for the accreting material (see Bate & Bonnell 1997).
The study of accretion activity on both components in PMS binary systems brings insight into the way the residual matter flows onto the central stars. This activity can be traced by spectroscopic measurements. However, up to now, such studies on PMS binaries in Taurus have been limited to wide systems (Hartigan et al. 1994, hereafter H94) due to the limited spatial resolution of the observations. Monin et al. (1998, hereafter Paper I) have started a spectroscopic survey of wide young binaries in Taurus. In this paper, we extend this study to closer systems (down to ), investigating the classification as classical (C) or weak-line (W) TTS of both stars in these binaries, along with a more detailed study of the spectroscopic signature of their accretion activity. We restrict ourself to the Tau-Aur association and we complement our results with those of H94 and Prato & Simon (1997, hereafter PS97) to extend this study to a wider range of systems.
In Sect. 2, we present the observations and the data reduction process. The results and the classification of individual stars as C/W TTS are presented and discussed in Sect. 3, and an evaluation of the random pairing hypothesis is presented in Sect. 4. The accretion activity of each component within binaries is compared in Sect. 5. A discussion and a summary are presented in Sect. 6.
© European Southern Observatory (ESO) 1999
Online publication: November 16, 1999