New Spatially Resolved Imaging of the SR 21 Transition Disk and Constraints on the Small-grain Disk Geometry

We present new 0.6-4 m imaging of the SR 21 transition disk from Keck/NIRC2 and Magellan/MagAO. The protoplanetary disk around SR 21 has a large (∼30-40 au) clearing first inferred from its spectral energy distribution and later detected in submillimeter imaging. Both the gas and small dust grains are known to have a different morphology, with an inner truncation in CO at ∼7 au, and micron-sized dust detected within the millimeter clearing. Previous near-infrared imaging could not distinguish between an inner dust disk with a truncation at ∼7 au or one that extended to the sublimation radius. The imaging data presented here require an inner dust disk radius of a few au, and complex structure such as a warp or spiral. We present a parametric warped disk model that can reproduce the observations. Reconciling the images with the spectral energy distribution gathered from the literature suggests grain growth to 2-5 m within the submillimeter clearing. The complex disk structure and possible grain growth can be connected to dynamical shaping by a giant-planet-mass companion, a scenario supported by previous observational and theoretical studies.