Disk eccentricity and embedded planets

Context: .
Aims: .We investigate the response of an accretion disk to the presence of a perturbing protoplanet embedded in the disk through time dependent hydrodynamical simulations.
Methods: .The disk is treated as a two-dimensional viscous fluid and the planet is kept on a fixed orbit. We run a set of simulations varying the planet mass, and the viscosity and temperature of the disk. All runs are followed until they reach a quasi-equilibrium state.
Results: .We find that for planetary masses above a certain minimum mass, already 3~ M_Jup for a viscosity of ν = 10^-5, the disk makes a transition from a nearly circular state into an eccentric state. Increasing the planetary mass leads to a saturation of disk eccentricity with a maximum value of around 0.25. The transition to the eccentric state is driven by the excitation of an m=2 spiral wave at the outer 1:3 Lindblad resonance. The effect occurs only if the planetary mass is large enough to clear a sufficiently wide and deep gap to reduce the damping effect of the outer 1:2 Lindblad resonance. An increase in viscosity or temperature in the disk, which both tend to close the gap, have an adverse influence on the disk eccentricity.
Conclusions: .In the eccentric state the mass accretion rate onto the planet is greatly enhanced, an effect that may ease the formation of massive planets beyond about 5 M_Jup that are otherwise difficult to reach.