Wednesday, July 24, 2013

1307.5849 (Benjamin T. Montet et al.)

The TRENDS High-Contrast Imaging Survey. IV. The Occurrence Rate of Giant Planets around M-Dwarfs    [PDF]

Benjamin T. Montet, Justin R. Crepp, John Asher Johnson, Andrew W. Howard, Geoffrey W. Marcy
Doppler-based planet surveys have discovered numerous giant planets with short orbital periods but are currently incomplete beyond several AU. At larger star-planet separations direct planet detection through high-contrast imaging has proven successful, but this technique is sensitive to only the youngest planets and characterization relies upon theoretical evolution models which are poorly constrained. Here we demonstrate that a combination of precise radial velocity measurements and high-contrast imaging can be used to overcome these issues. The presence of widely separated companions can be deduced by identifying an acceleration (long-term trend) in the radial velocity of a star as only a small fraction of its orbit is observed. By obtaining high spatial resolution follow-up imaging observations, we rule out scenarios in which such accelerations are caused by stellar binary companions with high statistical confidence. We report results from an analysis of Doppler measurements of a uniform sample of 111 M-dwarf stars with a median of 29 radial velocity observations over a median time baseline of 11.8 years. By targeting stars that exhibit a radial velocity acceleration ("trend") with adaptive optics imaging, we determine that 6.5% +/- 3.0% of all M dwarf stars have a gas giant companion with 1 M_J < m < 13 M_J and a < 20 AU, and that there are 0.083 +/- 0.019 planets in this parameter space per M-dwarf star. These results are lower than previous analyses of the planet occurrence rate around higher mass stars. Additionally we find a strong correlation between giant planet occurrence and stellar metallicity inside of the M-dwarf spectral class. Our results are consistent with gravitational microlensing measurements of the planet occurrence rate; this study represents the first model-independent comparison with microlensing observations.
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