Astrometric detection of giant planets around nearby M dwarfs: the Gaia potential

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 > 10(9) 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 similar to 30 pc 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 less than or similar to P less than or similar to 6.0 yr and with an astrometric signal-to-noise ratio zeta/sigma(AL) greater than or similar to 10. Given present-day estimates of the planet fraction f(p) around M dwarfs, approximate to 10(2) giant planets could be found by Gaia around the sample. Comprehensive screening by Gaia of the reservoir of similar to 4 x 10(5) M dwarfs within 100 pc could result in similar to 2600 detections and as many as similar to 500 accurate orbit determinations. The value of f(p) could then be determined with an accuracy of 2 per cent, 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 per cent) so that it will be possible to identify intermediateseparation 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 per cent accuracy, although a large fraction of these (similar to 85 per cent) could be false positives; (3) for well-sampled orbits, the uncertainties on planetary ephemerides, separation rho and position angle upsilon will degrade at typical rates of Delta rho < 1 mas yr(-1) and Delta upsilon < 2 degrees yr(-1), 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 Delta lambda of several degrees, resulting (under the assumption of purely scattering atmospheres) in phase-averaged errors on the phase function Delta Phi(lambda) approximate to 0.05, and expected uncertainties in the determination of the emergent flux of intermediate-separation (0.3 < a < 2.0 au) giant planets of similar to 20 per cent. 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).