The HARPS search for southern extra-solar planets - XXXI. The M-dwarf sample

Astronomy & Astrophysics, Jan 2013

Context. Searching for planets around stars with different masses helps us to assess the outcome of planetary formation for different initial conditions. The low-mass M dwarfs are also the most frequent stars in our Galaxy and potentially therefore, the most frequent planet hosts.Aims. We present observations of 102 southern nearby M dwarfs, using a fraction of our guaranteed time on the ESO/HARPS spectrograph. We observed for 460 h and gathered 1965 precise (~1–3 m/s) radial velocities (RVs), spanning the period from Feb. 11, 2003 to Apr. 1, 2009.Methods. For each star observed, we derive a time series and its precision as well as its variability. We apply systematic searches for long-term trends, periodic signals, and Keplerian orbits (from one to four planets). We analyze the subset of stars with detected signals and apply several diagnostics to discriminate whether the observed Doppler shifts are caused by either stellar surface inhomogeneities or the radial pull of orbiting planets. To prepare for the statistical view of our survey, we also compute the limits on possible unseen signals, and derive a first estimate of the frequency of planets orbiting M dwarfs.Results. We recover the planetary signals of 9 planets announced by our group (Gl 176 b, Gl 581 b, c, d & e, Gl 674 b, Gl 433 b, Gl 667C b, and Gl 667C c). We present radial velocities confirming that GJ 849 hosts a Jupiter-mass planet, plus a long-term radial-velocity variation. We also present RVs that precise the planetary mass and period of Gl 832b. We detect long-term RV changes for Gl 367, Gl 680, and Gl 880, which are indicative of yet unknown long-period companions. We identify candidate signals in the radial-velocity time series of 11 other M dwarfs. Spectral diagnostics and/or photometric observations demonstrate however that these signals are most probably caused by stellar surface inhomogeneities. Finally, we find that our survey is sensitive to a few Earth-mass planets for periods up to several hundred days. We derive a first estimate of the occurrence of M-dwarf planets as a function of their minimum mass and orbital period. In particular, we find that giant planets (msini = 100 − 1000 M⊕) have a low frequency (e.g. f ≲ 1% for P = 1 − 10 d and f = 0.02+0.03-0.01 for P = 10 − 100 d), whereas super-Earths (msini = 1 − 10 M⊕) are likely very abundant (f = 0.36+0.25-0.10 for P = 1 − 10 d and f = 0.52+0.50-0.16 for P = 10 − 100 d). We also obtained η⊕ = 0.41+0.54-0.13, which is the frequency of habitable planets orbiting M dwarfs (1 ≤ msini ≤ 10 M⊕). For the first time, η⊕ is a direct measure and not a number extrapolated from the statistics of more massive and/or shorter-period planets.

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The HARPS search for southern extra-solar planets - XXXI. The M-dwarf sample

A&A Astronomy & Astrophysics XXXI. The M-dwarf sample X. Bonfils 1 2 X. Delfosse 2 S. Udry 1 T. Forveille 2 M. Mayor 1 C. Perrier 2 F. Bouchy 0 6 M. Gillon 1 5 C. Lovis 1 F. Pepe 1 D. Queloz 1 N. C. Santos 4 D. Ségransan 1 J.-L. Bertaux 3 0 Institut d'Astrophysique de Paris, CNRS, Université Pierre et Marie Curie , 98bis Bd Arago, 75014 Paris , France 1 Observatoire de Genève , 51 Ch. des Maillettes, 1290 Sauverny , Switzerland 2 UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d'Astrophysique de Grenoble (IPAG) UMR 5274 , 38041 Grenoble , France 3 Service d'Aéronomie du CNRS , BP 3, 91371 Verrières-le-Buisson , France 4 Centro de Astrofísica, Universidade do Porto , Rua das Estrelas, 4150-762 Porto , Portugal 5 Institut d'Astrophysique et de Géophysique, Université de Liège , Allée du 6 Août 17, Bat. B5C, 4000 Liège , Belgium 6 Observatoire de Haute-Provence, CNRS/OAMP , 04870 Saint-Michel-l'Observatoire , France Context. Searching for planets around stars with di erent masses helps us to assess the outcome of planetary formation for di erent initial conditions. The low-mass M dwarfs are also the most frequent stars in our Galaxy and potentially therefore, the most frequent planet hosts. Aims. We present observations of 102 southern nearby M dwarfs, using a fraction of our guaranteed time on the ESO/HARPS spectrograph. We observed for 460 h and gathered 1965 precise ( 1-3 m/s) radial velocities (RVs), spanning the period from Feb. 11, 2003 to Apr. 1, 2009. Methods. For each star observed, we derive a time series and its precision as well as its variability. We apply systematic searches for long-term trends, periodic signals, and Keplerian orbits (from one to four planets). We analyze the subset of stars with detected signals and apply several diagnostics to discriminate whether the observed Doppler shifts are caused by either stellar surface inhomogeneities or the radial pull of orbiting planets. To prepare for the statistical view of our survey, we also compute the limits on possible unseen signals, and derive a first estimate of the frequency of planets orbiting M dwarfs. Results. We recover the planetary signals of 9 planets announced by our group (Gl 176 b, Gl 581 b, c, d & e, Gl 674 b, Gl 433 b, Gl 667C b, and Gl 667C c). We present radial velocities confirming that GJ 849 hosts a Jupiter-mass planet, plus a long-term radialvelocity variation. We also present RVs that precise the planetary mass and period of Gl 832b. We detect long-term RV changes for Gl 367, Gl 680, and Gl 880, which are indicative of yet unknown long-period companions. We identify candidate signals in the radial-velocity time series of 11 other M dwarfs. Spectral diagnostics and/or photometric observations demonstrate however that these signals are most probably caused by stellar surface inhomogeneities. Finally, we find that our survey is sensitive to a few Earthmass planets for periods up to several hundred days. We derive a first estimate of the occurrence of M-dwarf planets as a function of their minimum mass and orbital period. In particular, we find that giant planets (m sin i = 100 1000 M ) have a low frequency (e.g. f . 1% for P = 1 10 d and f = 0:02+00::0031 for P = 10 100 d), whereas super-Earths (m sin i = 1 10 M ) are likely very abundant ( f = 0:36+00::2150 for P = 1 10 d and f = 0:52+00::1560 for P = 10 100 d). We also obtained = 0:41+00::5143, which is the frequency of habitable planets orbiting M dwarfs (1 m sin i 10 M ). For the first time, is a direct measure and not a number extrapolated from the statistics of more massive and/or shorter-period planets. planetary systems - techniques; radial velocities - methods; data analysis 1. Introduction M dwarfs are the predominant stellar population of our Galaxy (e.g. Chabrier & Bara e 2000) . Compared to our Sun, they are cooler, smaller, and lower-mass stars. These characteristics ease ? Based on observations made with the HARPS instrument on the ESO 3.6-m telescope at La Silla Observatory under programme ID 072.C-0488(E). ?? Figures 3, 13–19, Tables 3–9, and Appendix A are available in electronic form at http://www.aanda.org ??? Radial velocity data are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/549/A109 the detection of planets for many techniques and M dwarfs were therefore included from an early stage in planet-search samples. While the first claimed detections (e.g. around Barnard’s star – van de Kamp 1963) were later found to be incorrect (Gatewood & Eichhorn 1973; Gatewood 1995) , targeting M dwarfs has since proven to be more successful. At the forefront of planet discoveries, the radial-velocity (RV) technique was first to unveil a candidate giant planet orbiting an M dwarf. Three years after the discovery of 51Peg b (Mayor & Queloz 1995), the detection of a giant planet orbiting the M dwarf GJ 876 (Delfosse et al. 1998; Marcy et al. 1998) p (...truncated)


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X. Bonfils, X. Delfosse, S. Udry, T. Forveille, M. Mayor, C. Perrier, F. Bouchy, M. Gillon, C. Lovis, F. Pepe, D. Queloz, N. C. Santos, D. Ségransan, J.-L. Bertaux. The HARPS search for southern extra-solar planets - XXXI. The M-dwarf sample, Astronomy & Astrophysics, 2013, pp. A109, 549, DOI: 10.1051/0004-6361/201014704