Disk eccentricity and embedded planets

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 v = 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.