Exoplanet spectroscopy with VLTI dual-field interferometry

Direct imaging becomes an increasingly important tool in our toolbox for characterising exoplanets, in particular close to the snowline at a few astronomical units where most Jovian planets are expected to reside. GAIA astrometry is destined to discover thousands of planets near the peak of the exoplanet distribution and to populate the mass-luminosity diagram. However, determining the formation pathway of these planets will require spectroscopic follow-up that is very challenging, since most of the planets will be at separations <0.2" from their host star, a regime where even extreme-AO systems struggle to achieve the required contrast due to correlated speckle noise. The GRAVITY instrument recently demonstrated that dual-field interferometry is able to achieve deep contrasts at inner working angles close to the diffraction-limit of the 8.2m VLT telescopes using the phase information provided interferometry. This method effectively combining the star-light suppression from extreme adaptive optics with star-light suppression from interferometry. The aim of this project is to develop this method further, to exploit it for the study of exoplanetary atmospheres, and to apply it for the first time in the shorter-wavelengths Y and J-bands (1.05-1.35 micrometer), a regime that is highly complementary to GRAVITY's K-band (2-2.4 micrometer). For this purpose, we will commission an equivalent dual-field mode with the VLT unit telescopes at the BIFROST instrument that the Exeter group is currently building for the VLTI visitor focus. Building on our experience with GRAVITY, we will implement equivalent starlight suppression algorithm in the BIFROST pipeline and validate them with first BIFROST data as it becomes available in the second year of the project. We will push the science exploitation of GRAVITY and BIFROST for atmospheric retrieval, and combine data from the different wavebands to break current degeneracies between the C/O ratio and surface-gravity/metallicity for giant planets orbiting inside 10 au, such as Beta Pic c.