1997A&A...323..139B

We have observed 144 G and K dwarf members of the Pleiades cluster to search for close multiple systems using CFHT's Adaptive Optics adaptor in the near-IR. We detected 22 binary systems and 3 triples, with a separation between 0.08 and 6.9arcsec (11-910AU). After correction for incompleteness, we derive a binary frequency in the orbital period range from 4.2 to 7.1 log days, of 28±4% for G and K Pleiades dwarfs, similar to that of field G-type dwarfs (27%). The distributions of both orbital periods and mass-ratios of the Pleiades systems also appear similar to those of G dwarf binaries of the field. The binary frequency in the 100Myr-old Pleiades cluster is much lower than that observed for Myr-old pre-main sequence (PMS) stars in the Taurus-Auriga cloud. We argue that this difference does not result from the evolution of the binary systems during the pre-main sequence. Instead, we suggest that the low Pleiades binary frequency is typical of stellar populations formed in dense protoclusters, while the higher binary frequency observed among Tau-Aur PMS stars is more typical of loose T associations. The implication is that most field stars are born in dense protostellar clusters. All 144 surveyed stars have known rotational velocities. Based on the current beliefs that i) the rotation rate of Pleiades late-type dwarfs is largely dictated by the lifetime of their pre-main sequence circumstellar disks and that ii) the evolution of the disks is affected by the presence of a close companion, we searched for a relationship between rotational velocity and binarity among Pleiades G and K dwarfs. We find no significant difference between the distribution of rotational velocities of single and binary stars. Unless current models of PMS angular momentum evolution are flawed, this indicates that the presence of a companion within a distance of 10-1000AU does not prevent accretion from occurring onto the primary at a rate similar to that observed for single PMS stars. For the closest systems, this implies that accretion must proceed from the circumbinary disk onto the central stars. For slightly wider systems, it suggests that the truncated circumstellar disks of the primary and of the secondary are fed by an external (circumbinary) reservoir of mass.