An Adaptive Optics Survey for Close Protostellar Binaries

TL;DR

This study uses adaptive optics observations to investigate the relationship between wide and close binary companions among Class I protostars, providing evidence that many close binaries form through ejections during early stellar evolution.

Contribution

It presents new AO data combined with previous observations to support the hypothesis that close protostellar binaries often form via dynamical ejections in multiple systems.

Findings

Targets with a close companion always have another YSO within 25,000 AU.

Regions with more YSOs within 25,000 AU have higher incidences of close binaries.

No correlation between wide companions (>200 AU) and close companions (<200 AU).

Abstract

In order to test the hypothesis that Class I protostellar binary stars are a product of ejections during the dynamical decay of non-hierarchical multiple systems, we combined the results of new adaptive optics (AO) observations of Class I protostars with our previously published AO data to investigate whether Class I protostars with a widely separated companion (r>200 AU) are more likely to also have a close companion (r<200 AU). In total, we observed 47 embedded young stellar objects (YSOs) with either the Subaru natural guide star AO system or the Keck laser guide star AO system. We found that targets with a widely separated companion within 5,000 AU are not more likely to have a close companion. However, targets with another YSO within a projected separation of 25,000 AU are much more likely to have a close companion. Most importantly, every target with a close companion has another YSO within a projected separation of 25,000 AU. We came to the same conclusions after considering a restricted sample of targets within 500 pc and close companions wider than 50 AU to minimize incompleteness effects. The Orion star forming region was found to have an excess of both close binaries and YSOs within 25,000 AU compared to other star forming regions. We interpret these observations as strong evidence that many close Class I binary stars form via ejections and that many of the ejected stars become unbound during the Class I phase.