Origin of the wide-orbit circumbinary giant planet HD 106906

Abstract

Context. A giant planet has been recently resolved at a projected distance of 730 au from the tight pair of young (∼13 Myr) intermediate-mass stars HD 106906AB in the Lower Centaurus Crux (LCC) group. The stars are surrounded by a debris disk which displays a ring-like morphology and strong asymmetries at multiple scales. Aims. We aim to study the likelihood of a scenario where the planet formed closer to the stars in the disk, underwent inward diskinduced migration, and got scattered away by the binary star before being stabilized by a close encounter (fly-by). Methods. We performed semi-analytical calculations and numerical simulations (Swift_HJS package) to model the interactions between the planet and the two stars. We accounted for the migration as a simple force. We studied the LCC kinematics to set constraints on the local density of stars, and therefore on the fly-by likelihood. We performed N-body simulations to determine the effects of the planet trajectories (ejection and secular effects) onto the disk morphology. Results. The combination of the migration and mean-motion resonances with the binary star (often 1:6) can eject the planet. Nonetheless, we estimate that the fly-by hypothesis decreases the scenario probability to less than 10 −7 for a derived local density of stars of 0.11 stars/pc 3 . We show that the concomitant effect of the planet and stars trajectories induce spiral-features in the disk which may correspond to the observed asymmetries. Moreover, the present disk shape suggests that the planet is on an eccentric orbit. Conclusions. The scenario we explored is a natural hypothesis if the planet formed within a disk. Conversely, its low probability of occurrence and the fact that HD 106906 b shares some characteristics with other systems in Sco-Cen (e.g., HIP 78530, in terms of mass ratio and separation) may indicate an alternative formation pathway for those objects.

High signal-to-noise spectral characterization of the planetary-mass object HD 106906 b

“…Jílková & Portegies Zwart 2015). An ejection scenario with subsequent braking through a stellar fly-by appears unlikely in the light of the simulations by Rodet et al (2017). While not strictly excluded, in situ formation through disk instability appears unlikely (though not excluded) because large disks >700 AU have been rarely observed around forming stars.…”

Discovery of a stellar companion to HD 131399A

“…In the latter scenario, the binary nature of HD 131399A could have played an active role. Rodet et al (2017) recently investigated a model to explain the formation of the distant planet (∼700 au) imaged around HD 106906. In this scenario, the recently discovered binary nature of HD 106906 (Lagrange et al 2016) plays a crucial role.…”

“…A similar scenario could easily be considered for HD 131399Ab. Rodet et al (2017) has shown that a key point for this scenario is the eccentricity of the HD 106906 binary. The resonant eccentricity increase of the planet that furthermore leads to a close encounter is a direct consequence of the binary's eccentricity.…”

“…The main difference is the presence of the outer pair HD 131399BC. The weakest point in the Rodet et al (2017) scenario is the stabilization of the planet on its current wide bound orbit. Most of the time, the close encounter with the central binary actually leads to a definitive ejection of the planet.…”

“…They may alternatively have formed closer in and have then been kicked away by some dynamical mechanism such as planet-planet scattering within the planetary system or via passing-by stars. A recent example of such a scenario was extensively investigated by Rodet et al (2017) for the HD 106906 system, which hosts a planet orbiting at more than 650 au (see Bailey et al 2014) and a debris disk (see Lagrange et al 2016); we recently showed, using HARPS and VLTI/PIONIER data, that the central star is in fact a binary (Lagrange et al 2016). HD 106906 is located in the Lower Centaurus-Crux LCC (Sco-Cen) association, the nearest ( 100-200 pc) OB association and star forming region (de Zeeuw et al 1999).…”