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 (greater than or similar to 75% at 2.16 mu m) near-infrared narrowband (Br gamma: 2.16 mu m and [Fe II]: 1.64 mu m) images of 47 young (similar to 1 Myr) Orion Trapezium theta(1) Ori cluster members. The inner similar to 41 x 53 '' of the cluster was imaged at spatial resolutions of similar to 0 ''.050 (at 1.64 mu m). A combination of high spatial resolution and high S/N yielded relative binary positions to similar 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 similar to 0.3 mas yr(-1) (0.6 km s(-1) at 450 pc); this is a similar to 7x improvement in orbital velocity accuracy compared to previous efforts. We now detect clear orbital motions in the theta(1) Ori B2B3 system of 4.9 +/- 0.3 km s(-1) and 7.2 +/- 0.8 km s(-1) in the theta(1) Ori A(1)A(2) system (with correlations of P. A. versus time at >99% confidence). All five members of the theta(1) Ori B system appear likely a gravitationally bound mini-cluster. The very lowest mass member of the theta(1) Ori B system (B-4; mass similar to 0.2 M-circle dot) 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.