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R. Koehler (UCSD), Ch. Leinert (Max-Planck-Institute for Astronomy), H. Zinnecker (Astrophysikalisches Institut Potsdam)
It is now a commonly accepted fact that all or nearly all T Tauri stars (TTS) in the Taurus-Auriga star-forming region form in binary systems (Leinert et al. 1993; Ghez et al. 1993; Koehler & Leinert 1998). This means that the binary frequency among TTS in Taurus-Auriga is enhanced by a factor of two compared to solar-type stars on the main sequence (Duqennoy & Mayor 1991).
Surveys in the Orion Trapezium Cluster found a frequency of binary systems with separations between 60 and 500 AU similar to that of main-sequence stars (e.g. Prosser et al. 1994; Petr et al. 1998). Scally et al. (1999) used proper motion data to search for binaries with separations in the range 1000 -- 5000 AU and got the result that the Orion cluster contains no binaries at all in this separation range.
Therefore, it is not possible that all main-sequence field stars formed in regions like Taurus-Auriga or the Trapezium Cluster, but a combination of both can explain the period distribution of main-sequence binaries, at least in the range covered by the surveys in Taurus-Auriga and Orion.
However, we know that a significant fraction of stars form in OB Associations, which have not been included in this combination. Furthermore, it is not yet clear if the high multiplicity in Taurus-Auriga is typical for T Associations. We will review the binary frequency and period distribution in several star forming regions and discuss the implications for the origin of the field star population.