2012ApJ...749..180C

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 (Brγ: 2.16 µm and [Fe II]: 1.64 µm) images of 47 young (∼1 Myr) Orion Trapezium θ¹ Ori cluster members. The inner ∼41x53'' 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 (0.6 km/s at 450 pc); this is a ∼7ximprovement in orbital velocity accuracy compared to previous efforts. We now detect clear orbital motions in the θ¹ Ori B₂B₃ system of 4.9±0.3 km/s and 7.2±0.8 km/s in the θ¹ Ori A₁A₂ system (with correlations of P.A. versus time at >99% confidence). All five members of the θ¹ Ori B system appear likely a gravitationally bound "mini-cluster." The very lowest mass member of the θ¹ Ori B system (B₄; mass ∼0.2 M_☉) has, for the first time, a clearly detected motion (at 4.3±2.0 km/s; correlation = 99.7%) w.r.t. B₁. However, B₄ 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.