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 V_esc ∼ 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.