Is stellar multiplicity universal? Tight stellar binaries in the Orion nebula Cluster

We present a survey for the tightest visual binaries among 0.3-2M(circle dot) members of the Orion nebula Cluster (ONC). Among 42 targets, we discovered 13 new 0.025-0.15 arcsec companions. Accounting for the Branch bias, we find a companion star fraction (CSF) in the 10-60 au range of 21(-5)(+8) per cent, consistent with that observed in other star-forming regions (SFRs) and twice as high as among field stars; this excess is found with a high level of confidence. Since our sample is dominated by disc-bearing targets, this indicates that disc disruption by close binaries is inefficient, or has not yet taken place, in the ONC. The resulting separation distribution in the ONC drops sharply outside 60 au. These findings are consistent with a scenario in which the initial multiplicity properties, set by the star formation process itself, are identical in the ONC and in other SFRs and subsequently altered by the cluster's dynamical evolution. This implies that the fragmentation process does not depend on the global properties of a molecular cloud, but on the local properties of prestellar cores, and that the latter are self-regulated to be nearly identical in a wide range of environments. These results, however, raise anew the question of the origin of field stars as the tight binaries we have discovered will not be destroyed as the ONC dissolves into the Galactic field. It thus appears that most field stars formed in regions that differ from well-studied SFRs in the solar neighbourhood, possibly due to changes in core fragmentation on Gyr time-scales.