Visual Binaries in the Orion Nebula Cluster

TL;DR

This study used HST imaging to survey visual binaries in the Orion Nebula Cluster, revealing a lower binary fraction compared to the field and T Tauri associations, with a notable deficit of wide binaries near the cluster center.

Contribution

First comprehensive HST survey of visual binaries in the Orion Nebula Cluster, identifying new binaries and analyzing their distribution and dependence on cluster location.

Findings

Binary fraction in ONC is 8.8%, lower than field and T Tauri associations.

Binary separation distribution shows a steep decline beyond 225 AU.

Wide binary ratio is significantly lower in the cluster's inner region.

Abstract

We have carried out a major survey for visual binaries towards the Orion Nebula Cluster using HST images obtained with an H-alpha filter. Among 781 likely ONC members more than 60" from theta-1 Ori C, we find 78 multiple systems (75 binaries and 3 triples), of which 55 are new discoveries, in the range from 0.1" to 1.5". About 9 binaries are likely line-of-sight associations. We find a binary fraction of 8.8%+-1.1% within the limited separation range from 67.5 to 675 AU. The field binary fraction in the same range is a factor 1.5 higher. Within the range 150 AU to 675 AU we find that T Tauri associations have a factor 2.2 more binaries than the ONC. The binary separation distribution function of the ONC shows unusual structure, with a sudden steep decrease in the number of binaries as the separation increases beyond 0.5", corresponding to 225 AU. We have measured the ratio of binaries wider than 0.5" to binaries closer than 0.5" as a function of distance from the Trapezium, and find that this ratio is significantly depressed in the inner region of the ONC. The deficit of wide binaries in the central part of the cluster is likely due to dissolution or orbital change during their passage through the potential well of the inner cluster region. Many of the companions are likely to be brown dwarfs.