High‐Resolution Images of Orbital Motion in the Trapezium Cluster: First Scientific Results from the Multiple Mirror Telescope Deformable Secondary Mirror Adaptive Optics System1

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 θ 1 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 θ 1 Ori B2-B3 (52 AU separation) and θ 1 Ori A1-A2 (94 AU separation) systems. The relative velocity in the θ 1 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 θ 1 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 θ 1 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 V esc ~ 6 km s -1 for the B group). The very lowest mass member of the θ 1 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 θ 1 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 θ 1 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.