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A&A 526, A141 (2011) Astronomy DOI: 10.1051/0004-6361/201016034 & c ESO 2011 Astrophysics The HARPS search for southern extra-solar planets, XXVI. Two giant planets around M0 dwarfs T. Forveille1, X. Bonfils1,2, G. Lo Curto3,X.Delfosse1,S.Udry2, F. Bouchy4,5,C.Lovis2, M. Mayor2, C. Moutou6, D. Naef2,3,F.Pepe2, C. Perrier1,D.Queloz2, and N. Santos7,8 1 Laboratoire d’Astrophysique de Grenoble, Observatoire de Grenoble, Université Joseph Fourier, CNRS, UMR 5571, BP 53, 38041 Grenoble Cedex 9, France e-mail: [email protected] 2 Observatoire de Genève, Université de Genève, 51 ch. des Maillettes, 1290 Sauverny, Switzerland 3 European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile 4 Institut d’Astrophysique de Paris, CNRS, Université Pierre et Marie Curie, 98bis Bd Arago, 75014 Paris, France 5 Observatoire de Haute-Provence, CNRS/OAMP, 04870 St Michel l’Observatoire, France 6 Laboratoire d’Astrophysique de Marseille, 38 rue Fréderic Joliot-Curie, 13388 Marseille Cedex 13, France 7 Centro de Astrofísica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal 8 Departamento de Fìsica e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal Received 31 October 2010 / Accepted 1 December 2010 ABSTRACT Fewer giants planets are found around M dwarfs than around more massive stars, and this dependence of planetary characteristics on the mass of the central star is an important observational diagnostic of planetary formation theories. In part to improve on those statistics, we are monitoring the radial velocities of nearby M dwarfs with the HARPS spectrograph on the ESO 3.6 m telescope. We present here the detection of giant planets around two nearby M0 dwarfs: planets, with minimum masses of respectively 5 Jupiter masses and 1 Saturn mass, orbit around Gl 676A and HIP 12961. The latter is, by over a factor of two, the most massive planet found by radial velocity monitoring of an M dwarf, but its being found around an early M-dwarf is in approximate line with the upper envelope of the planetary vs stellar mass diagram. HIP 12961 ([Fe/H] = −0.07) is slightly more metal-rich than the average solar neighborhood ([Fe/H] = −0.17), and Gl 676A ([Fe/H] = 0.18) significantly so. The two stars together therefore reinforce the growing trend for giant planets being more frequent around more metal-rich M dwarfs, and the 5 Jupiter mass Gl 676Ab being found around a metal-rich star is consistent with the expectation that the most massive planets preferentially form in disks with large condensate masses. Key words. stars: individual: Gl 676A – stars: individual: HIP 12961 – planetary systems – stars: late-type – techniques: radial velocities – stars: low-mass 1. Introduction stars, while Neptune-mass planets should inversely be com- mon. Within the same paradigm, but assuming that the prop- Much recent theoretical work has gone into examining how erties of protoplanetary disks, contrary to observations, do not planet formation depends on stellar mass, because stellar mass change with stellar mass, Kornet et al. (2006) predict instead significantly changes the physical conditions which control the that Jupiter-mass planets become more frequent in inverse pro- formation of planets. A comparison, for instance, of the planet portion to the stellar mass. Finally, Boss (2006) examines how populations around Sun-like stars on one hand, and around M planet formation depends on stellar mass for planets formed by dwarfs on the other hand, probes the sensitivity of the plane- disk instability, and concludes that the frequency of Jupiter-mass tary formation process to several physical parameters: around planet is largely independent of stellar mass, as long as disks are lower mass stars gravity (hence disk rotation speed), tempera- massive enough to become unstable. One needs to note, though, ture (which regulates the position of the ice line) are both lower, that proto-planetary disks of a realistic mass are likely to be and, perhaps most importantly, disk mass scales approximately gravitationally stable out to at least 10 AU. Planets can thus linearly with stellar mass (e.g. Scholz et al. 2006). form through gravitational instability only beyond that distance, Within the “core accretion” paradigm, Laughlin et al. (2004), in a separation range only skimmed by radial velocity moni- Ida & Lin (2005), and Kennedy & Kenyon (2008) all predict toring and probed mostly by microlensing searches and direct that giant planet formation is inhibited around very-low-mass imaging. Massive planets formed by gravitational instability and found well within 5 AU must thus then have migrated inward. Based on observations made with the HARPS instrument on the How giant planets migrating in the massive disks needed for ESO 3.6-m telescope at La Silla Observatory under program ID 072.C- gravitational instability can escape accreting enough mass to be- 0488 come a brown dwarf (>13 M ) is unclear (e.g. Stamatellos & Jup Tables 3 and 4 are also available in electronic form at the CDS via Whitworth 2009; Kratter et al. 2010). anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/cgi-bin/qcat?J/A+A/526/A141 Article published by EDP Sciences A141, page 1 of 7 A&A 526, A141 (2011) Observationally, just a dozen of the close to 400 planetary Table 1. Observed and inferred stellar parameters for Gl 676A and systems currently known from radial velocity monitoring, are HIP 12961. 1 centered around M dwarfs (M < 0.6 M) . This no doubt re- flects in part a selection bias, since many more of the intrisically Parameter Gl 676A HIP 12961 Notes brighter solar-type stars than of the fainter M dwarfs have been Spectral Type M0V M0V a searched for planets, but there is increasing statistical evidence V 9.585 ± 0.006 10.31 ± 0.04 b (e.g. Bonfils et al. 2006; Endl et al. 2006; Johnson et al. 2007, J 6.711 ± 0.020 7.558 ± 0.021 c c 2010) that M dwarfs also genuinely have fewer massive plan- Ks 5.825 ± 0.029 6.736 ± 0.018 ∼ d ets ( MJup) than the more massive solar-type stars. They may, BCKs 2.73 2.61 on the other hand, and though no rigorous statistical analysis π [mas] 60.79 ± 1.62 43.45 ± 1.72 e has yet been performed for that planet population, have a larger Distance [pc] 16.45 ± 0.44 23.01 ± 0.91 ± ± prevalence of the harder to detect Neptune-mass and super-Earth MV 8.50 0.06 8.50 0.09 M 4.74 ± 0.06 4.93 ± 0.09 planets: a quarter of the ∼30 planets with M sin(i) < 0.1 M Ks Jup M 7.47 7.54 known to date orbit an M dwarf, when solar-type stars outnum- bol L [L] 0.082 0.076 ber M dwarfs by an order of magnitude in planet-search samples. v sin i [km s−1]1.6± 1.0 1.5 ± 1.0 Conversely, the highest mass planets known around M dwarfs [Fe/H] 0.18 −0.07 f = g are the M sin(i) 2 MJup Gl 876b (Delfosse et al. 1998; Marcy M [ M] 0.71 0.67 et al. 1998) and HIP79431b (Apps et al. 2010), and at a larger (a) orbital separation of ∼3AUstheM =≈ 3.5 MJup OGLE-2005- Notes. Hawley et al. (1996) for Gl 676A, and Luyten (1980)for BLG-071Lb microlensing planet (Dong et al. 2009), when over HIP 12961; (b) Koen et al. (2002) for Gl 676A, and computed from the (c) two dozen planets with masses over 10 MJup are known around TYCHO (BT ,VT ) photometry for HIP 12961; Skrutskie et al. (2006); (d) − − solar type stars. The statistical significance of that difference computed from J Ks with the Leggett et al. (2001)BCKs vs. J Ks (e) ( f ) − relation; van Leeuwen (2007); computed from MKs and V Ks however remains modest, since the M dwarfs searched for plan- (g) ets only number in the few hundreds, when the apparent fraction using the Schlaufman & Laughlin (2010) calibration; computed from MKs using the Delfosse et al. (2000) calibration; both masses are at of these very massive planets is under 1% around solar type stars. the upper edge of the validity range of that calibration, and they might We present here the detection of two giant planets around therefore have somewhat larger errors than its 10% dispersion. M0 dwarfs, a M sin(i) = 0.354 MJup planet around HIP 12961, and a M sin(i) = 4.87 MJup planet around Gl 676A. 2.2. Gl 676A The Gl 676 system (also CCDM J17302-5138) has been rec- 2. Stellar characteristics ognized as a member of the immediate solar neighborhood for much longer, figuring in the original Gliese (1969) catalog of Table 1 summarizes the properties of the two host stars, which the 20 pc volume. It consequently has 15 references listed in we briefly discuss below. SIMBAD, though none of those dedicates more than a few sen- tences to Gl 676. The system comprises Gl 676A (also CD –51 10924, HIP 85647, CPD-51 10396) and Gl 676B, with re- 2.1. HIP 12961 spective spectral types of M0V and M3V (Hawley et al. 1996) and separated by ∼50" on the sky. At the distance of the system HIP 12961 (also CD-23deg1056, LTT 1349, NLTT 8966, ∼ SAO 168043) was not identified as a member of the 25 pc vol- this angular distance translates into an 800 AU projected sepa- ume until the publication of the Perryman & ESA (1997) cat- ration, which is probably far enough that Gl 676B didn’t strongly alog, and has attracted very little attention: it is mentioned in influence the formation of the planetary system of Gl 676A.