Multiple Rings and Asymmetric Structures in the Disk of SR 21

Crescent-like asymmetric dust structures discovered in protoplanetary disks indicate dust aggregations. Thus, researching these structures helps us understand the planet formation process. Here we analyze ALMA data of the protoplanetary disk around the T-Tauri star SR 21, which has asymmetric structures that were detected in previous submillimeter observations. Imaged at ALMA Band 6 (1.3 mm) with a spatial resolution of about 0.″04, the disk is found to consist of two rings and three asymmetric structures, with two of the asymmetric structures being in the same ring. Compared to the Band 6 image, the Band 3 (2.7 mm) image also shows the three asymmetric structures, but with some clumps. The elongated asymmetric structures in the outer ring could be due to the interactions of a growing planet. Based on fitting the Band 3 and Band 6 dust continuum data, two branches of solutions for the maximum dust size in the disk are suggested: one is larger than 1 mm, and the other is smaller than 300 μ m. High-resolution continuum observations at longer wavelengths as well as polarization observations can help break the degeneracy. We also suggest that the prominent spiral previously identified in VLT/SPHERE observations south of the star at 0.″25 may be the scattered-light counterpart of the inner arc, and the structure is a dust-trapping vortex in nature. The discovered features in SR 21 make it a good target for studying the evolution of asymmetric structures and planet formation.