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

TL;DR

This study uses new high-resolution imaging to analyze the SR 21 transition disk, revealing a complex inner disk structure, possible warps or spirals, and grain growth, which suggest dynamical shaping by a giant planet.

Contribution

It provides the first detailed imaging-based constraints on the small-grain disk geometry and introduces a parametric warped disk model to explain the observations.

Findings

Inner dust disk radius of a few AU is required by the data.

Evidence of complex structures such as warps or spirals in the disk.

Grain growth to 2-5 μm within the sub-millimeter clearing.

Abstract

We present new 0.6 - 4 $\mu$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 sub-millimeter 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 $\mu$m within the sub-millimeter 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.