Precise radial velocities of giant stars XIII. A second Jupiter orbiting in 4:3 resonance in the 7 CMa system

Abstract

We report the discovery of a second planet orbiting the K giant star 7 CMa based on 166 high-precision radial velocities obtained with Lick, HARPS, UCLES and SONG. The periodogram analysis reveals two periodic signals of approximately 745 and 980 d, associated to planetary companions. A double-Keplerian orbital fit of the data reveals two Jupiter-like planets with minimum masses $m_b\sin i\sim 1.9{\mathrm{M}}_{\mathrm{J}}$ and $m_c\sin i\sim 0.9{\mathrm{M}}_{\mathrm{J}}$, orbiting at semi-major axes of $a_b\sim 1.75\mathrm{au}$ and $a_c\sim 2.15\mathrm{au}$, respectively. Given the small orbital separation and the large minimum masses of the planets close encounters may occur within the time baseline of the observations, thus, a more accurate N-body dynamical modeling of the available data is performed. The dynamical best-fit solution leads to collision of the planets and we explore the long-term stable configuration of the system in a Bayesian framework, confirming that 13% of the posterior samples are stable for at least 10 Myr. The result from the stability analysis indicates that the two-planets are trapped in a low-eccentricity 4:3 mean-motion resonance. This is only the third discovered system to be inside a 4:3 resonance, making it very valuable for planet formation and orbital evolution models.

Accepted in A&A