Observatoire de Paris Institut national de recherche scientifique français Univerité Pierre et Marie Curie Université Paris Diderot - Paris 7

Soutenance de thèse de Nour SKAF le jeudi 2 mars 2023

mardi 14 février 2023

La soutenance de thèse de Nour SKAF aura lieu le jeudi 2 mars 2023 à 14h30 dans la salle de conférences du Château à Meudon.

Elle sera diffusée en direct sur la chaîne YouTube du LESIA

La thèse sera soutenue en anglais.



"Self-optimization of adaptative optics and characterization of exoplanetary systems"


With over 5200 exoplanets discovered, our understanding of planet formation and evolution is challenged. Studying their atmosphere and formation processes is crucial to ultimately finding and characterizing Earth-like planets. In this context, my PhD revolves around three aspects aimed at this purpose.

I studied exoplanets’ atmosphere, with three transiting hot Jupiters (WASP-127b, WASP-79b, and WASP-62b), observed in transit with the Hubble Space Telescope in the near-infrared. Their atmospheric analysis revealed significant water features and the possible presence of iron hydrate (FeH).

Besides the transit method, the direct imaging technique is unique in assessing the population of long-period giant planets. However, it is highly challenging and requires overcoming the large contrast at short angular separation, in the presence of optical distortions from the Earth’s atmosphere and the optics within the instrument. They both induce speckles, which mimic exoplanet signals. Adaptive optics technique aims to correct the atmospheric speckles, but is blind to some of the instrument’s ones, also called non-common path aberrations (NCPA). I developed an algorithm to continuously recognize and correct NCPA during night-sky observations. DrWHO (direct reinforcement wavefront heuristic optimization) uses the focal plane camera to self-optimize the AO system. I validated it with simulations, then on-sky, at the SCExAO instrument at the Subaru Telescope in Hawai`i, demonstrating its efficiency, robustness, and flexibility.

Direct imaging is now increasingly powerful in understanding how planets form by observing circumstellar disks. An illustration is the emblematic beta Pictoris system, a star surrounded by a debris disk and at least two giant planets, key to understanding disk-planet interactions and evolution. I analyzed the first high-contrast imaging data in the mid-infrared of the beta Pictoris system, observed with the NEAR-VISIR instrument on the VLT. Although planet b was not detected, I put constraints on a possible circumplanetary disk around it. The data reveals multiple disk structures, including the known southwest dust clump, first observed in 2003. Using archival observations covering a 16-year baseline, I found its orbit to be Keplerian, and possibly indicating a planet.

To conclude, there is room for improvement in the instrumentation development with DrWHO. This opens the road to use focal plane information to reach higher contrast, for the upcoming generation of larger telescopes, and unveiling exoplanets population characteristics.