The formation and evolution of binary systems - III. Low-mass binaries in the Praesepe cluster

With the aim of investigating the binary population of the 700 Myr old Prae sepe cluster, we have observed 149 G and K-type cluster members using adapt ive optics. We detected 26 binary systems with an angular separation rangin g from less than 0.08 to 3.3 arcsec (15-600 AU). After correcting for detec tion biases, we derive a binary frequency (BF) in the log P (days) range fr om 4.4 to 6.9 of 25.3 +/-5.4%, which is similar to that of field G-type dwa rfs (23.8%, Duquennoy & Mayor 1991). This result, complemented by similar o nes obtained for the 2 Myr old star forming cluster IC 348 (Paper II) and t he 120 Myr old Pleiades open cluster (Paper I), indicates that the fraction of long-period binaries does not significantly evolve over the lifetime of galactic open clusters. We compare the distribution of cluster binaries to the binary populations of star forming regions, most notably Orion and Tau rus, to critically review current ideas regarding the binary formation proc ess. We conclude that it is still unclear whether the lower binary fraction observed in young clusters compared to T associations is purely the result of the early dynamical disruption of primordial binaries in dense clusters or whether it reflects intrinsically different modes of star formation in clusters and associations. We also note that if Taurus binaries result from the dynamical decay of small-N protostellar aggregates, one would predict the existence of a yet to be found dispersed population of mostly single su bstellar objects in the Taurus cloud.