Dynamics of Circumstellar Planets in Binary Systems

Abstract: Radial velocity observations of the $1.9 M_\odot$ K giant star HD 59686 have revealed a binary companion with a minimum mass of $0.53 M_\odot$ in a moderately close ($a_B = 13.6$ AU) but highly eccentric ($e_B = 0.73$) orbit, as well as a planet with a minimum mass of $7 M_{\rm Jup}$ orbiting the primary at $a = 1.09$ AU with $e = 0.05$. The best coplanar prograde fit is highly unstable, but there are nearby narrow regions of stable fits, with the planet locked in secular apsidal alignment with the secondary. Dynamical analysis shows that the best fit is outside the bulk stable region for nearly circular orbits, but that there are narrow regions of eccentric orbits beyond the bulk stable region stabilized by either secular apsidal alignment or mean-motion resonance. On the other hand, the orbit of the planet in HD 59686 is fully stable for a large range of orbital solutions, if it is nearly coplanar and retrograde. The implications for the formation of this planet will be discussed, as well as the evidence for another retrograde circumstellar planet in a binary system.