Binary formation in stellar clusters

We consider how the tidal potential of a stellar cluster or a dense molecul ar cloud affects the fragmentation of gravitationally unstable molecular cl oud cores. We find that molecular cloud cores which would collapse to form a single star in the absence of tidal shear, can be forced to fragment if t hey are subjected to tides. This may enhance the frequency of binaries in s tar-forming regions such as Ophiuchus and the frequency of binaries with se parations less than or similar to 100 au in the Orion Trapezium Cluster. We also find that clouds which collapse to form binary systems in the absence of a tidal potential will form bound binary systems if exposed to weak tid al shear. However, if the tidal shear is sufficiently strong, even though t he cloud still collapses to form two fragments, the fragments are pulled ap art while they are forming by the tidal shear and two single stars are form ed. This sets an upper limit for the separation of binaries that form near dense molecular clouds or in stellar clusters.