High-Resolution Images of Orbital Motion in the Trapezium Cluster: First Scientific Results from the Multiple Mirror Telescope Deformable Secondary Mirror Adaptive Optics System

We present the first scientific images obtained with a deformable secondary mirror adaptive optics (AO) system. We utilized the 6.5 m Multiple Mirror Telescope adaptive optics system to produce high-resolution (FWHM = 0.″07) near-infrared (1.6 μm) images of the young (~1 Myr) Orion Trapezium θ1 Ori cluster members. A combination of high spatial resolution and high signal-to-noise ratio allowed the positions of these stars to be measured to within ~0.″003 accuracies. We also present slightly lower resolution (FWHM ~ 0.″085) images from Gemini with the Hokupa`a AO system as well. Including previous speckle data from Weigelt et al., we analyze a 6 yr baseline of high-resolution observations of this cluster. Over this baseline we are sensitive to relative proper motions of only ~0.″002 yr-1 (4.2 km s-1 at 450 pc). At such sensitivities we detect orbital motion in the very tight θ1 Ori B2-B3 (52 AU separation) and θ1 Ori A1-A2 (94 AU separation) systems. The relative velocity in the θ1 Ori B2-B3 system is 4.2 ± 2.1 km s-1. We observe 16.5 ± 5.7 km s-1 of relative motion in the θ1 Ori A1-A2 system. These velocities are consistent with those independently observed by Schertl et al. with speckle interferometry, giving us confidence that these very small (~0.″002 yr-1) orbital motions are real. All five members of the θ1 Ori B system appear likely gravitationally bound (B2-B3 is moving at ~1.4 km s-1 in the plane of the sky with respect to B1, where Vesc ~ 6 km s-1 for the B group). The very lowest mass member of the θ1 Ori B system (B4) has K′ ~ 11.66 and an estimated mass of ~0.2 M☉. Very little motion (4 ± 15 km s-1) of B4 was detected with respect to B1 or B2; hence, B4 is possibly part of the θ1 Ori B group. We suspect that if this very low mass member is physically associated, it most likely is in an unstable (nonhierarchical) orbital position and will soon be ejected from the group. The θ1 Ori B system appears to be a good example of a star formation "minicluster," which may eject the lowest mass members of the cluster in the near future. This "ejection" process could play a major role in the formation of low-mass stars and brown dwarfs.