Transiting exoplanets and magnetic spots characterized with optical interferometry

Context. Stellar activity causes difficulties in the characterization of transiting exoplanets. In particular, the magnetic spots present on most exoplanet host stars can lead to false detections with radial velocity, photometry, or astrometry techniques. Studies have been performed to quantify their impact on infrared interferometry, but no such studies have been performed in the visible domain. This wavelength domain, however, allows reaching better angular resolution than in the infrared and is also the wavelength most often used for spectroscopic and photometric measurements.
Aims: We use a standard case to completely analyse the impact of an exoplanet and a spot on interferometric observables and relate it to current instrument capabilities, taking into account realistic achievable precisions.
Methods: We built a numerical code called COMETS using analytical formulae to perform a simple comparison of exoplanet and spot signals. We explored instrumental specificities needed to detect them, such as the required baseline length, the accuracy, and signal-to-noise ratio. We also discuss the impact of exoplanet and spot parameters on squared visibility and phase: exoplanet diameter and size, exoplanet position, spot temperature, star diameter.
Results: According to our study, the main improvement to achieve is the instrument sensitivity. The accuracy on squared visibilities has to be improved by a factor 10 to detect an exoplanet of 0.10 mas, leading to <0.5% precision, along with phase measurements of ~5° accuracy beyond the first null of visibility. For an exoplanet of 0.05 mas, accuracies of ~0.1% and ~1° from the first null are required on squared visibilities and phases. Magnetic spots can mimic these signals, leading to false exoplanet characterization. Phase measurements from the third lobe are needed to distinguish between the spot and the exoplanet if they have the same radius.
Conclusions: By increasing interferometer sensitivity, more objects will become common between interferometric and photometric targets. Furthermore, new missions such as PLATO, CHEOPS, or TESS will provide bright exoplanet host stars. Measurements will thus overlap and provide a better characterization of stellar activity and exoplanet.