Steady Warps: Linear, Nonlinear, and Breaking
by
Tsung-Dao Lee Institute/N6F-N601 - Meeting Room
Tsung-Dao Lee Institute
Host: Dong Lai
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Abstract:
An increasing number of protoplanetary disks shows observational signatures of warps and misalignments, raising questions of how disks sustain coherent warps and how they may break into misaligned pieces. We study the steady-state structures and breaking conditions of warped disks. To focus on the hydrodynamics, while remaining agnostic about what forces the warp, we adopt a simple but physically motivated setup: rather than including an explicit perturber or external torque, we fix the disk's inner and outer boundaries at different inclinations. The disk is hence constrained to accommodate a warp between the boundaries. By varying the boundary misalignment, we can explore the linear regime, the nonlinear regime, and the onset of breaking, while having good control over the warp amplitude. Combining this model with analytical theories and three-dimensional hydrodynamic simulations, we carry out a clean and systematic investigation of the hydrodynamic behaviors of warped disks. We find that, with small warps, disks settle into warp steady states that are well described by the linear theory. Moderately warped disks enter the nonlinear regime, showing several distinct features such as torque saturation, vertical “bouncing” motion of gas, and enhanced mass accretion rates. Measurements of these effects in our simulations show good quantitative agreement with nonlinear theories. Strongly warped disks are unstable: these disks are susceptible to a runaway growth of warp amplitude that ultimately leads to disk breaking. This instability may be caused by the nonlinear saturation of the disk's internal torque, which occurs roughly when the warp amplitude exceeds a critical value ~2\sqrt{\alpha} for Keplerian disks.
Biography:
Jiaru Li is a CIERA Postdoctoral Fellow at Northwestern University. His research interests include exoplanetary dynamics, protoplanetary disks, and compact objects embedded in AGN disks, while he primarily studies these topics using N-body and hydrodynamics simulations.