We present a survey for the tightest visual binaries among 0.3–2 M _ { \odot } members the Orion Nebula Cluster ( ONC ) .
Among 42 targets , we discovered 13 new 0 \farcs 025–0 \farcs 15 companions .
Accounting for the Branch bias , we find a companion star fraction ( CSF ) in the 10–60 au range of 21 ^ { +8 } _ { -5 } % , 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 disk-bearing targets , this indicates that disk 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 neighborhood , possibly due to changes in core fragmentation on Gyr timescales .