Giant planet formation in stellar clusters: the effects of stellar fly-bys

Context. The majority of stars in the Galaxy are thought to have formed within stellar clusters, resulting in occasional close encounters between stars during the epoch of planet formation. Encounters between young stars which possess protoplanetary discs will cause significant modification of the disc structure, and perturb any planets forming within the disc. Aims. The primary aim of this work is to examine the effect of parabolic stellar encounters on the evolution of a Jovian-mass giant planet forming within a protoplanetary disc. We consider the effect on both the mass accretion and the migration history as a function of encounter distance. Methods. We use a grid-based hydrodynamics code to perform 2D simulations of a system consisting of a giant planet embedded within a gaseous disc orbiting around a star, which is perturbed by a passing star on a prograde, parabolic orbit. The disc model extends out to 50 AU, and parabolic encounters are considered with impact parameters ranging from 100-250 AU. Results. In agreement with previous work, we find that the disc is significantly tidally truncated for encounters <150 AU, and the removal of angular momentum from the disc by the passing star causes a substantial inflow of gas through the disc. The gap formed by the embedded planet becomes flooded with gas, causing the gas accretion rate onto the planet to increase abruptly. Gas flow through the gap, and into the inner disc, causes the positive inner disc torques exerted on the planet to increase, resulting in a sustained period of outward migration. For weaker interactions, corresponding to an encounter distance of >= 250 AU, we find that the planet-disc system experiences minimal perturbation. Conclusions. Our results indicate that stellar fly-bys in young clusters may significantly modify the masses and orbital parameters of giant planets forming within protostellar discs. Planets that undergo such encounters are expected to be more massive, and to orbit with larger semimajor axes, than planets in systems which have not experienced parabolic encounters.