Hot Jupiter and ultra-cold Saturn formation in dense star clusters

Yi-Han Wang; Nathan W. C. Leigh; Rosalba Perna; Michael M. Shara

doi:10.3847/1538-4357/abc619

paper

Hot Jupiter and ultra-cold Saturn formation in dense star clusters

arXiv:2011.01236 · doi:10.3847/1538-4357/abc619

Abstract

The discovery of high incidence of hot Jupiters in dense clusters challenges the field-based hot Jupiter formation theory. In dense clusters, interactions between planetary systems and flyby stars are relatively common. This has a significant impact on planetary systems, dominating hot Jupiter formation. In this paper, we perform high precision, few-body simulations of stellar flybys and subsequent planet migration in clusters. A large parameter space exploration demonstrates that close flybys that change the architecture of the planetary system can activate high eccentricity migration mechanisms: Lidov-Kozai and planet-planet scattering, leading to high hot Jupiter formation rate in dense clusters. Our simulations predict that many of the hot Jupiters are accompanied by "ultra-cold Saturns", expelled to apastra of thousands of AU. This increase is particularly remarkable for planetary systems originally hosting two giant planets with semi-major axis ratios $\sim$ 4 and the flyby star approaching nearly perpendicular to the planetary orbital plane. The estimated lower limit to the hot Jupiter formation rate of a virialized cluster is $\sim 1.6 \times 1 0^{- 4} (σ / 1km s^{- 1})^{5} (a_{p} / 20AU) (M_{c} / 1000 M_{⊙})^{- 2}$ Gyr $^{- 1}$ per star, where $σ$ is the cluster velocity dispersion, $a_{p}$ is the size of the planetary system and $M_{c}$ is the mass of the cluster. Our simulations yield a hot Jupiter abundance which is $\sim$ 50 times smaller than that observed in the old open cluster M67. We expect that interactions involving binary stars, as well as a third or more giant planets, will close the discrepancy.

arXiv commentAccepted for publication in ApJS

Hot Jupiter and ultra-cold Saturn formation in dense star clusters

Abstract

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