Extremely Inclined Orbit of S-type Planet $\gamma$ Cep Ab Induced by Eccentric Kozai-Lidov Mechanism

Abstract

$\gamma$ Cep Ab is a typical S-type planet, which occupies a nearly perpendicular planetary orbit relative to the binary. Here we use the Markov Chain Monte Carlo (MCMC) sampler to conduct full N-body fitting and derive self-consistent orbital solutions for this hierarchical system. Then we employ the Eccentric Kozai-Lidov (EKL) mechanism to explain the extremely inclined orbit of S-type planet $\gamma$ Cep Ab. The EKL mechanism plays an essential role in exploring significant oscillations of the mutual inclination $i_{\mathrm{mut}}$ between the planet and the secondary star. We perform qualitative analysis and extensive numerical integrations to investigate the flip conditions and timescales of $\gamma$ Cep Ab's orbit. When the planetary mass is 15 $M_{\mathrm{Jup}}$, the planet can reach $i_{\mathrm{mut}} \sim$ 113$^{\circ}$ with the critical initial conditions of $i_{\mathrm{mut}} < 60^{\circ}$ and $e_1<0.7$. The timescale for the first orbital flip decreases with the increase of the perturbation Hamiltonian. Flipping orbits of $\gamma$ Cep Ab are confirmed to have a large possibility to retain stable based on surfaces of section and the secular stability criterion. Furthermore, we extend the application of EKL to general S-type planetary systems with $a_1/a_2\leq0.1$, where the most intense excitation of $i_{\mathrm{mut}}$ occurs when $a_1/a_2=0.1$ and $e_2 \sim 0.8$, and the variation of planetary mass mainly affect the flip possibility where $e_1\leq 0.3$.