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

Abstract

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.

The GAPS programme with HARPS-N at TNG

“…In this case, the dynamical excitation of a stellar encounter is fully damped by the circumstellar disc and any trace of the flyby is erased (Marzari & Picogna 2013;Picogna & Marzari 2014). The probability of stellar encounters at later time, however, could be increased if the cluster is the outcome of two smaller clusters merging, as it has been suggested for M 44 (Holland et al 2000).…”

Significant interstellar object production by close stellar flybys

“…Only in high-mass clusters do hyperbolic encounters become essential. Generally, hyperbolic flybys lead to weaker effects on discs (Olczak et al 2012;Marzari & Picogna 2013;Picogna & Marzari 2014). Therefore, the results obtained here, can also be applied in high-mass clusters but should be interpreted as upper limits of the ISO release (see discussion in Sect.…”

Flyby-induced misalignments in planet-hosting discs

“…Three-dimensional simulations suggest that while moderate misalignments can be obtained between the planet and disc (Picogna & Marzari 2014), large misalignments or eccentricities are not likely to be driven. These studies suggest that planet-disc interactions rapidly removes any evidence of the flyby (even when the disc mass is low, as in Marzari & Picogna 2013).…”

“…Breslau & Pfalzner 2019) or increasing both the eccentricity and inclination of the planet orbit (Hao et al 2013). In the latter case, with a hyperbolic flyby and multiple discs, Marzari & Picogna (2013) showed that these features are damped within about 10 kyr due to interactions between discs. Three-dimensional simulations suggest that while moderate misalignments can be obtained between the planet and disc (Picogna & Marzari 2014), large misalignments or eccentricities are not likely to be driven.…”