" Or maybe RU Lup is interacting with another star. Another possibility is that the disk is interacting with its environment, accreting interstellar material along the spiral arms."
"None of these scenarios completely explain what we have observed, said team-member Sean Andrews of CfA. "There might be
unknown processes happening during planet formation that we have not yet accounted for in our models. bold added. cited from:
ScienceDaily, 3 August 2020. <
www.sciencedaily.com/releases/2020/08/200803140010.htm>.
Jane Huang, Sean M. Andrews, Karin I. Öberg, Megan Ansdell, Myriam Benisty, John M. Carpenter, Andrea Isella, Laura M. Pérez, Luca Ricci, Jonathan P. Williams, David J. Wilner, Zhaohuan Zhu.
Large-scale CO Spiral Arms and Complex Kinematics Associated with the T Tauri Star RU Lup.
The Astrophysical Journal, 2020; 898 (2): 140 DOI:
10.3847/1538-4357/aba1e1
Spiral arms are quasi-standing waves. standing waves are often created by resonances. so: not just ring shaped, concentric but traverse spoke shaped waves in proto - planetary disks?
"interacting with the environment" , possibly the pulsations, frequencies of the newly fired up star?