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

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

Abstract
Context. Searching for planets around stars with different masses probes 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. This drives observations of a sample of 102 southern nearby M dwarfs, using a fraction of our guaranteed time on the ESO/HARPS spectrograph. We observed 460 hours and gathered 1965 precise (∼ 1 - 3 m/s) radial velocities, spanning the period from Feb. 11th, 2003 to Apr. 1st 2009.
Methods. This paper makes available the sample’s time series, presents their precision and variability. We apply systematic searches for long-term trends, periodic signals and Keplerian orbits (from 1 to 4 planets). We analyze the subset of stars with detected signals and apply several diagnostics to discriminate whether the observed Doppler shifts are caused by stellar surface inhomogeneities or by 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 corresponding to 9 planets already 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 that confirm 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 longterm RV changes for Gl 367, Gl 680 and Gl 880 betraying 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 they are most probably caused by stellar surface inhomogeneities. Finally, we find our survey sensitive to 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 (m sin i = 100 - 1,000 M_⊕) have a low frequency (e.g. ƒ ≲ 1% for P = 1-10 d and ƒ = 0.02^+0.03_-0.01 for P = 10-100 d), whereas super-Earths (m sin i = 1-10 M_⊕) are likely very abundant ( ƒ = 0.36^+0.25_-0.10 for P = 1 - 10 d and ƒ = 0.35^+0.45_-0.11 for P = 10 - 100 d). We also obtained η_⊕ = 0.41^+0.54_-0.13, the frequency of habitable planets orbiting M dwarfs (1 ≤ m sin i ≤ 10 M_⊕). For the first time, η_⊕ is a direct measure and not a number extrapolated from the statistic of more massive and/or shorter-period planets.

Keywords
planetary systems – techniques: radial velocities – methods: data analysis

Notes
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

Astronomy and Astrophysics
Volume 549, Page A109_1
January 2013

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