Nonaxisymmetric instabilities in self-gravitating disks III. Angular momentum transport

Abstract

We follow the development of nonaxisymmetric instabilities of self-gravitating disks from the linear regime to the nonlinear regime. Particular attention is paid to comparison of nonlinear simulation results with previous linear and quasi-linear modeling results to study the mass and angular momentum transport driven by nonaxisymmetric disk instabilities. Systems with star-to-disk mass ratios of $M_{*}/M_{d} = 0.1$ and 5 and inner-to-outer disk radius ratios of $r_{-}/r_{+} = 0.47$ to 0.66 are investigated. In disks where self-gravity is important, systems with small $M_{*}/M_{d}$ and large $r_{-}/r_{+}$, Jeans-like J modes are dominant and the gravitational stress drives angular momentum transport. In disks where self-gravity is weak, systems with large $M_{*}/M_{d}$ and large $r_{-}/r_{+}$, shear-driven P modes dominate and the Reynolds stress drives angular momentum transport. In disks where self-gravity is intermediate in strength between disks where P modes dominate and disks where J modes dominate, I modes control the evolution of the system and the Reynolds and gravitational stresses both play important roles in the angular momentum transport. In all cases, redistribution of angular momentum takes place on the characteristic disk timescale defined as the orbital period at the location of maximum density in the disk midplane. The disk susceptible to one-armed modes behaves differently than disks dominated by multi-armed spirals. Coupling between the star and the disk driven by one-armed modes leads to angular momentum transfer between the star and disk even when instability is in the linear regime. All modes drive spreading of the disk material and eventually accretion onto the star. The disks dominated by an I mode and one-armed mode do not lead to prompt fission or fragmentation. The J mode dominated disk fragments after instability develops.