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

Context. Most stars form in embedded clusters. Stellar flybys may affect the orbital architecture of the systems by exciting the eccentricity and causing dynamical instability. Aims: Since, incidentally, the timescale on which a cluster loses it gaseous component and begins to disperse is comparable to the circumstellar disk's lifetime, we expect that closer and more perturbing stellar flybys occur when the planets are still embedded in their birth disk. We investigate the effects of the disk on the dynamics of planets after the stellar encounter to test whether it can damp the eccentricity and return the planetary system to a nonexcited state. Methods: We used the hydrodynamical code FARGO to study the disk+planet(s) system during and after the stellar encounter in the context of evolved disk models whose superficial density is 10 times lower than that of the minimum mass solar nebula. Results: The numerical simulations show that the planet's eccentricity, excited during a close stellar flyby, is damped on a short timescale (~10 Kyr) in spite of the disk's low initial density and subsequent tidal truncation. This damping is also effective for a system of 3 giant planets, and the effects of the dynamical instability induced by the passing star are quickly absorbed. Conclusions: If the circumstellar disk is still present around the star during a stellar flyby, a planet (or a planetary system) is returned to a nonexcited state on a short timescale. This does not mean that stellar encounters do not affect the evolution of planets, but they do it in a subtle way with a short period of agitated dynamical evolution. At the end of it, the system resumes a quiet evolution and the planetary orbits are circularized by the interaction with the disk.