Investigating the Enigmatic Orbit of the Suspected 2.5 MJ Planet in the Nu Octantis Binary System
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
ν Octantis is a spectroscopic binary with a semi-major axis and period of 2.55 AU and 2.9 years, respectively. Ramm et al. (2009) discovered a 52 ms^(-1) radial-velocity (RV) perturbation with a period of 417 days in this system. All evidence, both photometric and spectroscopic, suggests the perturbation is the result of a 2.5 MJ planet orbiting the primary star. However, when assuming a “normal” prograde coplanar orbit, celestial mechanics predicts this orbit is unstable, contradicting the observed stability.
Simulations by Eberle and Cuntz (2010) showed a retrograde orbit for the planet to be stable for at least 10^7 years. In this thesis, we performed a 10^8 -yr simulation of the retrograde orbit, and found it remained stable. Simulations over a range of planetary semi-major axes, eccentricities, and primary/secondary masses showed that stable retrograde orbits are not possible past a semi-major axis of 1.315 +/- 0.092 AU . Therefore, planetary retrograde orbits are most likely inherently more stable than prograde orbits owing to the absence of stability at known mean-motion resonances. Eccentricity simulations showed that the period of the planet's dominant eccentricity variation is related to the planet's semi-major axis by a second order exponential. However, retrograde orbits tend to have longer eccentricity periods than prograde orbits at the same semi-major axis. There is also evidence that this eccentricity period is connected to the orbital stability.
By fitting a keplerian to both Ramm et al. (2009) and current radial velocities, the period of the ν Octantis binary was determined to be 1050.04 +/- 0.02 days with an eccentricity of 0.2359 +/- 0.001 . The planetary orbital solution for just the data reduced in this thesis gave a period of 416.9 +/- 2.1 days and an eccentricity of 0.099 +/- 0.015 , with an RMS scatter of 9.6 ms^(-1). Therefore, the orbital elements are within 1σ of the Ramm et al. (2009) elements. Assuming a retrograde coplanar orbit about the primary star then the planet has a mass of M_pl = 2.3 M_J and a semi-major axis of a_pl = 1.21 +/- 0.09 AU.