Hot Jupiters driven by high-eccentricity migration in globular clusters

Abstract

Hot Jupiters (HJs) are short-period giant planets that are observed around ~ 1% of solar-type field stars. One possible formation scenario for HJs is high-eccentricity (high-e) migration, in which the planet forms at much larger radii, is excited to high eccentricity by some mechanism, and migrates to its current orbit due to tidal dissipation occurring near periapsis. We consider high-e migration in dense stellar systems such as the cores of globular clusters (GCs), in which encounters with passing stars can excite planets to the high eccentricities needed to initiate migration. We study this process via Monte Carlo simulations of encounters with a star+planet system including the effects of tidal dissipation, using an efficient regularized restricted three-body code. HJs are produced in our simulations over a significant range of the stellar number density n_*. Assuming the planet is initially on a low-eccentricity orbit with semimajor axis 1 au, for n_* < 1e3 pc^{-3} the encounter rate is too low to induce orbital migration, whereas for n_* > 1e6 pc^{-3} HJ formation is suppressed because the planet is more likely ejected from its host star, tidally disrupted, or transferred to a perturbing star. The fraction of planets that are converted to HJs peaks at ~ 2% for intermediate number densities of ~ 4e4 pc^{-3}. Warm Jupiters, giant planets with periods between 10 and 100 days, are produced in our simulations with an efficiency of up to ~ 0.5%. Our results suggest that HJs can form through high-e migration induced by stellar encounters in the centers of of dense GCs, but not in their outskirts where the densities are lower.