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

Abstract

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.

The effects of fly-bys on planetary systems

“…Essentially, in clusters with a high binary fraction, exchange encounters are likely to dominate, while in clusters with a low binary fraction, fly-bys are likely play the most important role. Fly-bys can, if they occur during the protoplanetary disc stage, change the frequency of planet formation (Forgan & Rice 2009) or the orbital and physical properties of planets formed (Fragner & Nelson 2009).…”

Circumstellar disks do erase the effects of stellar flybys on planetary systems

“…To make the model more precise, we should have run the code without the passing star and with the planet on a fixed orbit to give it the time to carve a gap in the disk at its location prior to the stellar flyby. However, as also shown by Fragner & Nelson (2009), a close stellar encounter strongly perturbs the disk, destroying any previous structure present in it. A pre-existing planetary gap would be fully erased by the tidal perturbations of the passing star.…”

“…By inspecting the dynamical properties and architecture of planetary systems around stars that were members of clusters, it would be difficult "a posteriori" to distinguish between systems whose evolution was influenced by stellar flybys and those that were not. The influence of stellar flybys will be detectable only on a statistical basis as shown by the modeling of Fragner & Nelson (2009). Many parameters are indeed affecting the behavior of the disk+planet system during the stellar flyby, and they include the initial disk density profile and the architecture of the planetary system.…”

“…We concentrate in this paper on the effects of close stellar encounters on planets still embedded in their birth disk. Fragner & Nelson (2009) have shown that stellar flybys significantly altering the orbital parameters of planets are also expected to affect the disk structure. Via hydrodynamical simulations they find that stellar encounters with distances less than three times the disk radius r d can cause a significant shrinkage of the disk.…”

Tails and bridges in the parabolic restricted three-body problem

“…They concluded that, in the coplanar case, there were more change of particles between stars when the encounter was prograde. Fragner & Nelson (2009) studied the effect of parabolic encounters in the formation of Jovian-mass planets. They concluded that planets that have been formed after encounters are more massive and also have greater semimajor axes.…”