High-Resolution Images of Orbital Motion in the Orion Trapezium Cluster With the LBT Ao System

The new 8.4 m LBT adaptive secondary AO system, with its novel pyramid wavefront sensor, was used to produce very high Strehl (≳ 75% at 2.16 μm) near-infrared narrowband ( Br γ: 2.16 μm and [Fe ii ]: 1.64 μm) images of 47 young (∼1 Myr) Orion Trapezium θ 1 Ori cluster members. The inner ∼41 × 53'' of the cluster was imaged at spatial resolutions of ∼0 050 (at 1.64 μm). A combination of high spatial resolution and high S/N yielded relative binary positions to ∼0.5 mas accuracies. Including previous speckle data, we analyze a 15 year baseline of high-resolution observations of this cluster. We are now sensitive to relative proper motions of just ∼0.3 mas yr −1 (0.6 km s −1 at 450 pc); this is a ∼7 × improvement in orbital velocity accuracy compared to previous efforts. We now detect clear orbital motions in the θ 1 Ori B 2 B 3 system of 4.9 ± 0.3 km s −1 and 7.2 ± 0.8 km s −1 in the θ 1 Ori A 1 A 2 system (with correlations of P.A. versus time at >99% confidence). All five members of the θ 1 Ori B system appear likely a gravitationally bound "mini-cluster." The very lowest mass member of the θ 1 Ori B system ( B 4 ; mass ∼0.2 M ☉ ) has, for the first time, a clearly detected motion (at 4.3 ± 2.0 km s −1 ; correlation = 99.7%) w.r.t. B 1 . However, B 4 is most likely in a long-term unstable (non-hierarchical) orbit and may "soon" be ejected from this "mini-cluster." This "ejection" process could play a major role in the formation of low-mass stars and brown dwarfs.