Abstract

Stellar magnetic activity induces both distortions and Doppler-shifts in the absorption line profiles of Sun-like stars. Those produce apparent radial velocity (RV) signals which greatly hamper the search for potentially-habitable, Earth-like planets. With the advent of extreme-precision spectrographs, it becomes crucial to develop robust methods to correct for the activity contributions while preserving signatures of planetary origin. In the last few years, innovative methods to exploit the wealth of information present in line profiles rather than just radial velocity time-series have shown promising results.

I propose to review different activity modelling strategies involving the use of stellar cross-correlation functions (CCFs), at the interface between the spectrum and the radial velocity measurement. I will focus on two main types of techniques, both applied to observations of the Sun, intensively monitored with the high-precision spectrograph HARPS -N. In the first strategy, activity-induced line-shape variations are separated from pure Doppler shifts using data-driven processes. The extracted line-shape temporal variations are then included in a multi-dimensional Gaussian process, allowing us to robustly recover RV signatures as low as 0.4 m/s for planets in the Sun’s habitable zone. The second method uses an alternative physically-driven approach based on Doppler Imaging to simultaneously reconstruct the distribution of active regions at the surface of the stars and search for planet signatures shifting the line profiles. Using solar data, I demonstrate that Doppler imaging, initially designed for rapidly-rotating stars, can not only be applied to Sun-like stars, but also give reliable results in term of planet detection. I will discuss the advantages and limitations of both activity mitigation strategy, and potential avenues to overcome them.