2014MNRAS.437..497S

Cool M dwarfs within a few tens of parsecs from the Sun are becoming the focus of dedicated observational programs in the realm of exoplanet astrophysics. Gaia, in its all-sky survey of >109 objects, will deliver precision astrometry for a magnitude-limited (V = 20) sample of M dwarfs. We investigate some aspects of the synergy between the Gaia astrometric data on nearby M dwarfs and other ground-based and space-borne programs for planet detection and characterization. We carry out numerical simulations to gauge the Gaia potential for precision astrometry of exoplanets orbiting a sample of known dM stars within ∼ 30pc from the Sun. We express Gaia detection thresholds as a function of system parameters and in view of the latest mission profile, including the most up-to-date astrometric error model. Our major findings are as follows: (1) it will be possible to accurately determine orbits and masses for Jupiter-mass planets with orbital periods in the range 0.2 ≲ P ≲ 6.0 yr and with an astrometric signal-to-noise ratio σ/σAL ≳ 10. Given present-day estimates of the planet fraction fp around M dwarfs, ~102 giant planets could be found by Gaia around the sample. Comprehensive screening by Gaia of the reservoir of ∼ 4x10⁵ M dwarfs within 100pc could result in ∼ 2600 detections and as many as ∼ 500 accurate orbit determinations. The value of fp could then be determined with an accuracy of 2 percent, an improvement by over an order of magnitude with respect to the most precise values available to-date; (2) in the same period range, inclination angles corresponding to quasi-edge-on configurations will be determined with enough precision (a few percent) so that it will be possible to identify intermediate-separation planets which are potentially transiting within the errors. Gaia could alert us of the existence of 10 such systems. More than 250 candidates could be identified assuming solutions compatible with transit configurations within 10 percent accuracy, although a large fraction of these ( ∼ 85 percent) could be false positives; (3) for well-sampled orbits, the uncertainties on planetary ephemerides, separation ρ and position angle θ will degrade at typical rates of Δρ < 1 mas/yr and Δθ < 2°/yr, respectively. These are over an order of magnitude smaller than the degradation levels attained by present-day ephemerides predictions based on mas-level precision Hubble Space Telescope/Fine Guidance Sensor astrometry; (4) planetary phases will be measured with typical uncertainties Δλ of several degrees, resulting (under the assumption of purely scattering atmospheres) in phase-averaged errors on the phase function ΔΦ(λ) ~ 0.05, and expected uncertainties in the determination of the emergent flux of intermediate-separation (0.3 < a < 2.0 au) giant planets of ∼ 20 percent. Our results help to quantify the actual relevance of the Gaia astrometric observations of the large sample of nearby M dwarfs in a synergetic effort to optimize the planning and interpretation of follow-up/characterization measurements of the discovered systems by means of transit survey programs, and upcoming and planned ground-based as well as space-borne observatories for direct imaging (e.g. Very Large Telescope/Spectro-Polarimetric High-Contrast Exoplanet Research, European Extremely Large Telescope/Planetary Camera and Spectrograph) and simultaneous multiwavelength spectroscopy (e.g. Exoplanet Characterisation Observatory, James Webb Space Telescope).