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Charles E. Woodward

High-resolution Images of Orbital Motion in the Orion Trapezium Cluster with the LBT AO System
Close, L. M.; Puglisi, A.; Males, J. R.; Arcidiacono, C.; Skemer, A.; Woodward, C.E.; and 26 coauthors, Astrophysical Journal

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The new 8.4 m LBT adaptive secondary AO system, with its novel pyramid wavefront sensor, was used to produce very high Strehl (gsim 75% at 2.16 μm) near-infrared narrowband (Brγ: 2.16 μm and [Fe II]: 1.64 μm) images of 47 young (~1 Myr) Orion Trapezium θ1 Ori cluster members. The inner ~41 × 53'' of the cluster was imaged at spatial resolutions of ~0farcs050 (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 15 year baseline of high-resolution observations of this cluster. We are now sensitive to relative proper motions of just ~0.3 mas yr-1 (0.6 km s-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 B 2 B 3 system of 4.9 ± 0.3 km s-1 and 7.2 ± 0.8 km s-1 in the θ1 Ori A 1 A 2 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 (B 4; mass ~0.2 M sun) 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.