On the evolution of the resonant planetary system HD 128311

Context. A significant number of the known multiple exoplanetary systems contain a pair of giant planets engaged in low-order mean-motion resonance. Such a resonant condition protects the dynamics of these planets resulting in very stable orbits. According to recent studies, this capture into a resonance is the result of a planetary migration process induced by interaction of the planets with a protoplanetary disk. If the migration is slow enough ( adiabatic) near a mean motion resonance, the two planets will also be in apsidal corotation. Aims. The recently refined orbital parameters of the system HD 128311 suggest that the two giant planets are in a 2: 1 mean motion resonance, however, without exhibiting apsidal corotation. Thus the evolution of this system cannot be described by an adiabatic migration process alone. We present possible evolution scenarios of this system by combining migration processes and sudden perturbations. Methods. We model migration scenarios through numerical integration of the gravitational N-body problem with additional non-conservative forces. Planet-planet scattering has been investigated by N-body simulations. Results. We show that the present dynamical state of the system HD 128311 may be explained by such evolutionary processes.