During the past five years, many studies have addressed the question of the multiplicity in star-forming regions (SFR). While about 60% of G-K main sequence dwarfs belong to multiple systems in the solar vicinity (Duquennoy & Mayor 1991), several papers (e.g. Leinert et al. 1993) have pointed out that 80%, and maybe up to 100%, of Taurus young stars are not formed singly. More recently, this binarity excess has been found in other SFR (e.g. Ghez et al. 1997). It is a major theoretical challenge to explain several points including why do stars form in multiple system, and why is the degree of multiplicity different in different SFR and in the solar environment.
Various binary formation mechanisms have been proposed. Although fragmentation is the more widely accepted idea nowadays, many details remain to be investigated observationally and numerically to fully understand the details (and initial conditions) of the process. Binary formation has been studied using SPH codes by various authors.
When the formation results in a central binary surrounded by a circumbinary disk, Artymowicz & Lubow (1996) showed that matter could flow through one or two points of the inner ring of the circumbinary disk, and when the stars have very unequal masses, the accretion funnel is preferentially directed towards the star orbiting closer to the circumbinary disk. A different prediction applying to younger objects has been made by Bonnell & Bastien (1992): in high mass ratio systems, accretion of low angular momentum material is directed toward the center of mass which is close to the more massive star.
The study of the accretion activity on one or both components in PMS binary systems will tell us a lot about the way the residual matter flows onto the central star. Such a study can be performed by spectroscopic measurements. However, up to now, spectroscopic studies of PMS binaries have been limited to somewhat wide systems due to the spatial resolution limit of the observations.
To compare the accretion activity of both components in all systems studied here, we evaluated the H luminosity ratios (see Figure 5), assuming that the H luminosity is propotional to the energy dissipated in the accretion phenomenon, and thus to the accretion rate, and that extinction is of the same order on both stars.
The absence of close mixed systems is a constraint on binary formation mechanisms. Both component must share a common environment, as the stars cannot have been formed independently from different cloud cores and then paired (via capture). Concerning the C/C pairs, we see a trend for closer pairs to have higher activity in the primaries. Spectral types determinations show that the star accreting more matter appears to be the more massive one. This effect cannot be accounted for by a failure of the equal extinction hypothesis. Despite small samples, this provides another hint for close pairs (with separation smaller than 350 AU) to behave in a different way than wider systems. It is unlikely that the difference is due to undiscovered wide, faint companions.
Our results show that accretion is preferentially directed toward the center of mass of the binary system, ie. the primary component. We confirm the previous results from Prato & Simon (1997) that the circumstellar properties of both components are linked, with a higher statistical significance. These finding are consistent with the idea that accretion on both components is linked to the presence of a circumbinary reservoir: circumbinary accretion replenishes the circumstellar disks and, when the outer disk dissapears, both stars becomes WTTS quickly since dissipation time for truncated disks are very small, and we do not see mixed pairs in close systems. For wider pairs, dissipation time are only linked to stellar masses and disk structure, so that they are independent. To discriminate between close and wide binaries, we propose a 350 - 600 AU limit, which agrees with current models of circumbinary gap formation.
Allen L., Strom K., 1995, AJ, 109, 1379