Astronomy & Astrophysics manuscript no. 0774 c ESO 2018 November 11, 2018 The HARPS search for southern extra-solar planets⋆ XVI. HD 45364, a pair of planets in a 3:2 mean motion resonance A.C.M. Correia1, S. Udry2, M. Mayor2, W. Benz3, J.-L. Bertaux4, F. Bouchy5, J. Laskar6, C. Lovis2, C. Mordasini3, F. Pepe2, and D. Queloz2 1 Departamento de F´ısica, Universidade de Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal e-mail: [email protected] 2 Observatoire de Gen`eve, Universit´ede Gen`eve, 51 ch. des Maillettes, 1290 Sauverny, Switzerland e-mail: [email protected] 3 Physikalisches Institut, Universit¨at Bern, Silderstrasse 5, CH-3012 Bern, Switzerland 4 Service d’A´eronomie du CNRS/IPSL, Universit´ede Versailles Saint-Quentin, BP3, 91371 Verri`eres-le-Buisson, France 5 Institut d’Astrophysique de Paris, CNRS, Universit´ePierre et Marie Curie, 98bis Bd Arago, 75014 Paris, France 6 IMCCE, CNRS-UMR8028, Observatoire de Paris, UPMC, 77 avenue Denfert-Rochereau, 75014 Paris, France Received ; accepted To be inserted later ABSTRACT Precise radial-velocity measurements with the HARPS spectrograph reveal the presence of two planets orbiting the solar-type star HD 45364. The companion masses are m sin i = 0.187 MJup and 0.658 MJup, with semi-major axes of a = 0.681 AU and 0.897 AU, and eccentricities of e = 0.168 and 0.097, respectively. A dynamical analysis of the system further shows a 3:2 mean motion resonance between the two planets, which prevents close encounters and ensures the stability of the system over 5 Gyr. This is the first time that such a resonant configuration has been observed for extra-solar planets, although there is an analogue in our Solar System formed by Neptune and Pluto. This singular planetary system may provide important constraints on planetary formation and migration scenarios. Key words. stars: individual: HD 45364 – stars: planetary systems – techniques: radial velocities – methods: observational 1. Introduction Ganymede system in a 4:2:1 resonance, or the Neptune-Pluto system in a 3:2 resonance. While the satellites are believed to At present, about 25% of the known exoplanets are in multi- achieve resonant configurations after tidal evolution of their or- planetary systems. The older the radial-velocity planet-search bits, the resonances between planets and asteroids were probably surveys, the higher the fraction of multi-planet families detected formed after the inward or outward migration of the planets dur- in these programs. Taking into account the still strong bias ing the early stages of the evolution of the Solar System. against long-period and/or low-mass planet detection, and the enhanced difficulty of fully characterizing systems with more The presence of two or more interacting planets in a system than one planet, the fraction of known multi-planet systems is dramatically increases our potential ability to constrain and un- certainly still a lower limit. It appears likely that a high number, derstand the processes of planetary formation and evolution. The if not the majority, of planet-host stars form systems of planets dynamical analysis of such systems is then very useful, first for arXiv:0902.0597v1 [astro-ph.EP] 3 Feb 2009 rather than isolated, single planetary companions. constraining the system evolution history and second for deter- Among the known multi-planet systems, a significant frac- mining the system “structure” in terms of orbital content. tion are in mean motion resonances, the majority of which are in low-order resonances. The 2:1 resonance is the most com- Multi-planet systems are naturally found in planet-search mon (HD73526,HD82943,HD128311,GJ876), but other con- programs. However, improving the precision of the radial- figurations are observed as well, such as the 3:1 resonance in velocity measurements greatly helps their detection and com- HD75732 or the 5:1 resonance in HD202206. These resonances plete characterization. The HARPS search for southern extra- most probably arise from evolutionary processes as migrating solar planet is an extensive radial-velocity survey of some 2000 planets forming in the protoplanetary disc become trapped. In stars of the solar neighborhoodconductedwith the HARPS spec- our Solar System, we also find mean motion resonances in the trograph on the ESO 3.6-m telescope at LaSilla (Chile) in the satellites of the giant planets, or between these planets and sev- framework of the Guaranteed Time Observations granted to the eral asteroids. The most well-known example is the Io-Europa- HARPS building consortium (Mayor et al. 2003). About half of the HARPS GTO time is dedicated to very high-precision Send offprint requests to: A.C.M. Correia measurements of non-active stars selected from the CORALIE ⋆ Based on observations made with the HARPS instrument on the ESO 3.6 m telescope at La Silla Observatory un- planet-search program (Udry et al. 2000) and stars with already der the GTO programme ID 072.C-0488. The table with the known giant planets, while searching for lower mass compan- radial velocities is available in electronic form at the CDS ions. This program reveals itself as very efficient in finding via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via multi-Neptune (Lovis et al. 2006) and multi-Super Earth sys- http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/???? tems (Mayor et al. 2008). 2 A.C.M. Correia et al.: HD 45364, a pair of planets in a 3:2 mean motion resonance Table 1. Observed and inferred stellar parameters of HD45364. Table 2. Orbital parameters for the two bodies orbiting HD45364, obtained with a 3-body Newtonian fit to observa- tional data (Fig. 1). Parameter HD 40307 Sp K0V V [mag] 8.08 Param. [unit] HD 45364 b HD 45364 c B − V [mag] 0.72 Date [JD-2400000] 53500.00 (fixed) π [mas] 30.69 ± 0.81 V [km/s] 16.4665 ± 0.0002 MV [mag] 5.51 P [day] 226.93 ± 0.37 342.85 ± 0.28 Teff [K] 5434 ± 20 λ [deg] 105.76 ± 1.41 269.52 ± 0.58 log g [cgs] 4.38 ± 0.03 e 0.1684 ± 0.0190 0.0974 ± 0.012 [Fe/H] [dex] −0.17 ± 0.01 ω [deg] 162.58 ± 6.34 7.41 ± 4.30 L [L⊙] 0.57 K [m/s] 7.22 ± 0.14 21.92 ± 0.43 M∗ [M⊙] 0.82 ± 0.05 i [deg] 90 (fixed) 90 (fixed) −1 v sin i [km s ] 1 a sin i [10−3 AU] 0.1485 0.6874 ′ 1 log R −4.94 −9 HK f (M) [10 M⊙] 0.0085 0.3687 P (log R′ ) [day] 32 rot HK M sin i [MJup] 0.1872 0.6579 a [AU] 0.6813 0.8972 Photometric and astrometric data are from the Hipparcos catalogue (ESA 1997) and the stellar physical quantities from Sousa et al. (2008). Nmeas 58 Span [day] 1583 rms [m/s] 1.417 From the HARPS high-precision survey, we present an in- pχ2 2.789 teresting system of two planets in a 3:2 mean motion reso- nance around HD45364, a configuration not observed previ- Errors are given by the standard deviation σ and λ is the mean longitude ously among extra-solar planet, but similar to the Neptune-Pluto of the date (λ = ω + M). The orbits are assumed co-planar. pair in the Solar System. Section 2 gathers useful stellar infor- mation about HD45364, its derived orbital solution is described HD 45364 HARPS 16.50 in Sect.3, and the dynamical analysis of the system is discussed in Sect. 4. Finally, conclusions are drawn in Sect. 5. 16.49 16.48 2. Stellar characteristics of HD 45364 The basic photometric (K0V, V = 8.08, B − V = 0.72) and as- 16.47 trometric (π = 30.69mas) properties of HD45364 were taken 16.46 from the Hipparcos catalogue (ESA 1997). They are recalled in Table1 with inferred quantities such as the absolute magnitude Radial velocity [km/s] 16.45 (MV = 5.51) and the stellar physical characteristics derived from the HARPS spectra by Sousa et al. (2008). For the complete 16.44 high-precision HARPS sample (including HD45364), these au- thors provided homogeneous estimates of effective temper- 10 5 ature (Teff = 5434 ± 20 K), metallicity ([Fe/H] = −0.17 ± 0.01), and surface gravity (log g = 4.38 ± 0.03) of the stars. A projected 0 − 1 O-C [m/s] low rotational velocity of the star, v sin i = 1kms was derived -5 from a calibration of the width of the cross-correlation function -10 used in the radial-velocity estimate (Santos et al. 2002). 53000 53200 53400 53600 53800 54000 54200 54400 54600 HD45364 is a non-active star in our sample with an activ- JD-2400000 [days] ′ ity indicator log RHK of −4.94. No significant radial-velocity Fig. 1. HARPS radial velocities for HD45364, superimposed on jitter is thus expected for the star. From the activity indicator, a 3-body Newtonian orbital solution (Table2). we also derive a stellar rotation period Prot = 32day (following Noyes et al. 1984). HD45364 has a subsolar metallicity with [Fe/H] = −0.17 un- the first stages of the observations, peculiar variations in the like most of the gaseous giant-planet host stars (Santos et al. radial velocities (Fig.1) have shown the presence of one or 2004). According to simulations of planet formation based more companions in the system. After 58 measurements, we on the core-accretion paradigm, moderate metal-deficiency are now able to determine the nature of these bodies. Using does not, however, prevent planet formation (Ida & Lin 2004; a genetic algorithm combined with the iterative Levenberg- Mordasini et al.