High-resolution images of orbital motion in the Orion Trapezium cluster with the LBT AO system

The new 8.4m 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 47young (∼1 Myr) Orion Trapezium θ¹ 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 15year baseline of high-resolution observations of this cluster. We are now sensitive to relative proper motions of just ∼0.3 mas yr^-1 (0.6kms^-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 θ¹ Ori B₂ B₃ system of 4.9 ± 0.3kms^-1 and 7.2 ± 0.8kms^-1 in the θ¹ Ori A₁ A₂ 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 ± 2.0kms^-1; 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.