Wide binaries in the Orion nebula cluster

Using proper motion data for 894 stars in the Orion nebula cluster (ONC) co mpiled by Jones & Walker in 1988, we search for binaries with apparent sepa rations in the range 1000-5000 au, and find an upper limit of three. Using a Monte Carlo method, we test the consistency of this result with two hypot heses: (i) that the cluster contains a binary population identical to that found in the solar neighbourhood, and (ii) that the cluster contains no bin aries at all in this separation range. We obtain results strongly favouring the latter hypothesis. Star formation in the Galax li is seen to occur in a variety of different e nvironments, but it has been proposed that most stars may be formed in dens e regions similar to the ONC, rather than in less dense groupings like that found in Taurus-Auriga. As roughly 15 per cent of Galactic field stars are known to be in binaries with separations greater than 1000 au, the apparen t absence of such binaries in the ONC places an upper limit on the contribu tion that dense clusters can make to galactic star formation.