Searching Habitable Exoplanets with the SPIRou Spectropolarimeter: Tackling Stellar Magnetic Activity

Soutenance de thèse de Stefano BELLOTTI (Salle Coriolis)

Résumé de la thèse :

The search and characterization of habitable Earth-like planets have been substantial drivers for both current (e.g., TESS, CHEOPS, JWST) and mid-term (PLATO, Ariel) space missions. At present, the output of transit photometry campaigns dominates the planet detection statistics, but this method conveys only partial knowledge on the planet (i.e., the radius), and requires complementary follow-up in order to constrain the planetary mass, mainly via radial velocity surveys. M dwarfs represent key targets toward this goal, but they can be highly active, which hampers the success of radial velocity searches. Stellar magnetic activity is in fact responsible for surface inhomogeneities (e.g. dark spots and faculae) that generate spurious radial velocity signatures (so called « activity jitter ») which, in turn, drown the planetary signature completely or prevent a precise mass determination.To tackle this issue and mitigate the stellar contamination, we have to develop physically-motivated, jitter-filtering techniques and understand the jitter sources at play thoroughly, which thus requires an accurate model of the underlying magnetic field.
In the first part of the project, I designed an algorithm that performs a randomised selection of « stable » spectral lines, allowing a reduction of at least 50% in radial velocity dispersion.The algorithm was tested extensively, to corroborate its portability over M dwarfs of different activity levels and spectral types, and its applicability to time series with injected synthetic planets. In the second part, I monitored the large-scale magnetic field at the surface of well-known, active M dwarfs on both sides of the fully-convective boundary. In particular, I sought an evolution of the longitudinal magnetic field, the Full-Width at Half Maximum of unpolarised mean profiles, and the magnetic fluxes obtained via Zeeman broadening modelling, as well as the reconstructed magnetic field topology via Zeeman-Doppler Imaging. I found a variety of long-term trends that have some complementarities with our understanding of the solar magnetic cycle. Ultimately, these findings provide practical feedback to dynamo theories, which are incomplete and still debated even for the Sun.

Composition du jury de thèse :

• Pascal Petit : Supervisor
• Julien Morin: Supervisor
• Gaitee Hussain: Supervisor
• Agnes Lebre: Rapporteur
• Xavier Dumusque: Rapporteur
• Xavier Delfosse: Examinateur
• Jean-Francois Donati: Examinateur
• Theresa Lueftinger: Examinateur