Precise radial velocities of giant stars VI. A possible 2:1 resonant planet pair around the K giant star $\eta$ Cet

Abstract

We report the discovery of a new planetary system around the K giant $\eta$ Cet (HIP 5364, HD 6805) based on 118 high-precision optical radial velocities taken at Lick Observatory since July 2000. Since October 2011 an additional nine near-infrared Doppler measurements have been taken using the ESO CRIRES spectrograph (VLT, UT1). The visible data set shows two clear periodicities. Although we cannot completely rule out that the shorter period is due to rotational modulation of stellar features, the infrared data show the same variations as in the optical, which strongly supports that the variations are caused by two planets. Assuming the mass of $\eta$ Cet to be 1.7 $M_{\odot }$, the best edge-on coplanar dynamical fit to the data is consistent with two massive planets ($m_b\sin i$ = 2.6 $\pm$ 0.2 $M_{\mathrm{Jup}}$, $m_c\sin i$ = 3.3 $\pm$ 0.2 $M_{\mathrm{Jup}}$), with periods of $P_b$ = 407 $\pm$ 3 days and $P_c$ = 740 $\pm$ 5 days and eccentricities of $e_b$ = 0.12 $\pm$ 0.05 and $e_c$ = 0.08 $\pm$ 0.03. We tested a wide variety of edge-on coplanar and inclined planetary configurations for stability, which agree with the derived radial velocities. We find that in certain coplanar orbital configurations with moderate $e_b$ eccentricity, the planets can be effectively trapped in an anti-aligned 2:1 mean motion resonance. A much larger non-resonant stable region exists in low-eccentricity parameter space, although it appears to be much farther from the best fit than the 2:1 resonant region. In all other cases, the system is categorized as unstable or chaotic. Another conclusion from the coplanar inclined dynamical test is that the planets can be at most a factor of $\sim$ 1.4 more massive than their suggested minimum masses. This stability constraint on the inclination excludes the possibility of two brown dwarfs, and strongly favors a planetary system.

arXiv comment15 pages, 11 figures, accepted for publication in A&A on June 20, 2014

References in corpus (24)

• Validation of the new Hipparcos reduction
• Evolutionary tracks and isochrones for low- and intermediate-mass stars: from 0.15 to 7 M_sun, and from Z=0.0004 to 0.03
• Images of a fourth planet orbiting HR 8799
• Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars
• Quantifying the Uncertainty in the Orbits of Extrasolar Planets
• Discovery of a Jupiter/Saturn Analog with Gravitational Microlensing
• Five Planets Orbiting 55 Cancri
• TW Hydrae: evidence of stellar spots instead of a Hot Jupiter
• The sdB+M Eclipsing System HW Virginis and its Circumbinary Planets
• Extrasolar Planets in Mean-Motion Resonance: Apses Alignment and Asymmetric Stationary Solutions
• Detection of Solar-like Oscillations in the G7 Giant Star xi Hya
• Amplitudes of solar-like oscillations: a new scaling relation
• Confirmation of the Planet Hypothesis for the Long-period Radial Velocity Variations of Beta Geminorum
• The HARPS search for southern extra-solar planets. XXVII. Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems
• A Planetary Mass Companion to the K0 Giant HD 17092
• HD45364, a pair of planets in a 3:2 mean motion resonance
• Evidence for 9 planets in the HD 10180 system
• A Planet in a 0.6-AU Orbit Around the K0 Giant HD 102272
• Extrasolar planets and brown dwarfs around A-F type stars V. A planetary system found with HARPS around the F6IV-V star HD 60532
• Characterizing the Orbital and Dynamical State of the HD 82943 Planetary System With Keck Radial Velocity Data
• Planetary companion candidates around the K giant stars 42 Dra and HD 139357
• Precise radial velocities of giant stars V. A brown dwarf and a planet orbiting the K giant stars tau Gem and 91 Aqr
• The Discovery of Stellar Oscillations in the K Giant $\iota$ Dra
• A Dynamical Investigation of the Proposed BD +20 2457 System

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