Diffraction-limited visible light images of orion trapezium cluster with the Magellan Adaptive Secondary Adaptive Optics System (MagAO)

We utilized the new high-order (250-378 mode) Magellan Adaptive Optics system (MagAO) to obtain very high spatial resolution observations in "visible light" with MagAO's VisAO CCD camera. In the good-median seeing conditions of Magellan (0.″5-0.″7), we find MagAO delivers individual short exposure images as good as 19 mas optical resolution. Due to telescope vibrations, long exposure (60 s) r′ (0.63 μm) images are slightly coarser at FWHM = 23-29 mas (Strehl ∼28%) with bright (R < 9 mag) guide stars. These are the highest resolution filled-aperture images published to date. Images of the young (∼1 Myr) Orion Trapezium θ¹ Ori A, B, and C cluster members were obtained with VisAO. In particular, the 32 mas binary θ¹ Ori C 1 C 2 was easily resolved in non-interferometric images for the first time. The relative positions of the bright trapezium binary stars were measured with ∼0.6-5 mas accuracy. We are now sensitive to relative proper motions of just ∼0.2 mas yr^-1 (∼0.4 km s^-1 at 414 pc) - this is a ∼2-10 × improvement in orbital velocity accuracy compared to previous efforts. For the first time, we see clear motion of the barycenter of θ¹ Ori B 2 B 3 about θ¹ Ori B 1. All five members of the θ¹ Ori B system appear likely to be a gravitationally bound "mini cluster," but we find that not all the orbits can be both circular and co-planar. The lowest mass member of the θ¹ Ori B system (B 4; mass ∼0.2 M ) has a very clearly detected motion (at 4.1 ± 1.3 km s^-1; correlation = 99.9%) w.r.t. B 1. Previous work has suggested that B 4 and B 3 are on long-term unstable orbits and will be ejected from this "mini cluster." However, our new "baseline" model of the θ¹ Ori B system suggests a more hierarchical system than previously thought, and so the ejection of B 4 may not occur for many orbits, and B 3 may be stable against ejection in the long-term. This "ejection" process of the lowest mass member of a "mini cluster" could play a major role in the formation of low-mass stars and brown dwarfs.