Astrometric detection of giant planets around nearby M dwarfs: The gaia potential

Abstract

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 ~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 < 1 mas yr-1 and Δθ{symbol} < 2°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 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 per cent. Our results help to quantify the actual relevance of the Gaia astrometric observations of the large sampleof nearby M dwarfs in a synergetic effort to optimize the planning and interpretation of follow-up/characterizationmeasurements 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). © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.