GW Orionis: A pre-main-sequence triple with a warped disk and a torn-apart ring as benchmark for disk hydrodynamics

Understanding how bodies interact with each other and with disk material holds the key to understanding the architecture of stellar systems and of planetary systems. While the interactions between point sources can be described by simple gravity, interactions with disk material require further knowledge about the gas viscosity and dust microphysics that needs to be included when simulating disk-body interactions. Pre-main-sequence multiple systems provide us with a unique laboratory to calibrate fundamental parameters such as the viscosity and to test theories of hydrodynamic processes that might shape protoplanetary disk structure and affect the planet populations forming from these disks. In this article I briefly review our knowledge about a particularly intriguing T Tauri triple star system, GW Orionis, that has the potential to serve as a rosetta stone for hydrodynamic studies. The 3-dimensional orbits and masses of the stars in GW Orionis have been constrained by long-term interferometric and radial velocity monitoring. Also, the 3-dimensional geometry of the strongly distorted disk has been tightly contrained based on high-angular resolution thermal dust emission and scattered-light imaging. The disk-tearing effect that we might witness in GW Ori in action constitutes an important new mechanism for moving disk material onto highly oblique or retrograde orbits, even at very wide separations from the star. At the same time, the observed torn ring seems sufficiently massive, and might be sufficiently stable, for planet formation to occur, potentially giving rise to an yet-undiscovered population of circum-multiple planets on highly oblique, long-period orbits.