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

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 with planetary companions. A double-Keplerian orbital fit of the data reveals two Jupiter-like planets with minimum masses m_b sini 1.9 M_J and m_c sini 0.9 M_J, orbiting at semimajor axes of a_b 1.75 au and a_c 2.15 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 this discovery very valuable for planet formation and orbital evolution models.

RV data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/631/A136

Based on observations collected at Lick Observatory, University of California.

Based on observations collected at the European Organization for Astronomical Research in the Southern Hemisphere under ESO programmes 078.C-0751, 079.C-0657, 081.C-0802, 082.C-0427, 289.C-5053, 0100.C-0414 and 0101.C-0232.