Spiral arms in accretion disk encounters

In high-density star-forming regions, encounters of protostars surrounded by accretion disks are thought to be quite likely events. There is increasing numerical and observational evidence that disks around early stars may display characteristic spiral patterns, become elliptical, or display a ring structure. In this paper simulations of prograde encounters between a disk-surrounded protostar and a secondary star are used to investigate the mass transport within the disk induced by such encounters. It is demonstrated that in this context the appearance of super-Keplerian velocities in the tidal tail is the key to predicting properties such as spiral arms, rings, and elliptic disks. A detailed parameter study reveals the dependence of mass transport on the actual encounter parameters and the influence of hydrodynamic effects and self-gravity on the disk dynamics. Further simulation results show that there are two different mechanisms at work to create the first two spiral arms. The first spiral arm resembles a shocklike structure, whereas the second arm ( and possibly subsequent arms) is caused by a relative movement between the central star and the center of mass of the disk, caused by the mass imbalance in the disk.