Into the blue: AO science in the visible with 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 (60s) r' (0.63μm) 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 C₁C₂ was easily resolved in non-interferometric images for the first time. Relative positions of the bright trapezium binary stars were measured with ~ 0.6 - 5 mas accuracy. We now are sensitive to relative proper motions of just ~ 0.2 mas/yr (~ 0.4 km/s 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 B₂B₃ about θ¹ Ori B₁. All five members of the θ¹ Ori B system appear likely 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 (B₄; mass ~ 0.2M_sun) has a very clearly detected motion (at 4.1 ± 1.3 km/s; correlation=99.9%) w.r.t B₁. Previous work has suggested that B₄ and B₃ 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 B₄ may not occur for many orbits, and B₃ may be stable against ejection 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.