THE CHALLENGE OF ROBUST ORBITAL MEASUREMENTS OF DIRECTLY-IMAGED EXOPLANETS AND BROWN DWARFS

Anne-Lise Maire – University of Liège

Orbital monitoring of circumstellar systems is complementary to the spectrometry of the atmospheres as it allows for analyzing their architecture, dynamical state and evolution, and formation. It is required to derive the orbital parameters and robust model-independent mass measurements. Measuring these parameters for imaged exoplanets and brown dwarfs can lead to degeneracies and biases because a small fraction of the orbit is typically sampled (<20%). I will discuss these issues and methods that I developed to mitigate them in my studies. Precise and robust measurements of the position of the companions over time are critical. I showed the good and stable astrometric calibration of VLT/SPHERE since it has been made available to the community in 2015. This has allowed for precise position measurements down to ~1–3 mas, whereas the precision reached in pre-SPHERE studies was ~10 mas. My work has served as a reference for various studies carried out in the guaranteed-time survey and in open-time programs: the discovery of several exoplanets and brown dwarfs, the rejection of companion candidates as background objects, the measurement of the position and orbit of a few tens of (sub)stellar companions, the analysis of companion-disk dynamical interactions and of the motion of disk features. I will also discuss the lessons learned from the astrometric study of SPHERE for the development of the high-contrast imaging instruments on the ELT.