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Feasibility study of the detection of habitable zone terrestrial exoplanets orbiting K dwarfs and atmospheric characterization with future space-based and ground-based telescopes.

Exoplanets --- K dwarfs --- Habitable --- Physique, chimie, mathématiques & sciences de la terre > Aérospatiale, astronomie & astrophysique

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This thesis deals with the analytical design of a static spectropolarimeter for space use, based on birefringent media. It is about an instrument able to provide access to all four Stokes parameters of the detected light, at different wavelengths. In the first part of the thesis, the most important concepts and methods of polarimetry and of their applications in astrophysics are presented. Further, the proposed instrument is described, and the modulation function is derived, using the Mueller calculus. Because the material of the spectropolarimeter is Magnesium Fluoride (MgF2), an important attention was offered to the variation of the birefringence with wavelength. Two models for the description of the chromaticity of MgF2 were proposed and compared. Then the uncertainty calculus corresponding to this type of instrument was developed. Adjacently, considering the presence of noise, an extraction algorithm for the Stokes parameters was build and the behavior of the instrument in the presence of noise was tested. In the last part of the research, the off-axis case was also studied. After a detailed mathematical determination of the ray tracing with the help of Huygens constructions, several scenarios for the off-axis incidence were analyzed and the main limitations of the system were discovered.

polarization --- spectropolarimeter --- birefringence --- Zeeman effect --- Hanle effect --- polarization exoplanets --- modulation --- Physique, chimie, mathématiques & sciences de la terre > Aérospatiale, astronomie & astrophysique

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This thesis aims at leveraging advances in deep learning, particularly simulation-based inference, to enhance the orbital parameter characterization of exoplanets. The current methods, like MCMC, are computationally expensive and slow to converge. Using Normalizing Flows and the expected forward Kullback-Leibler divergence as a loss function to train the model, we reproduced 
the results of the state-of-the-art method, $alpha$-DPI. 

However, the non-amortized nature of this approach limited its generalizability, necessitating retraining for new datasets or additional observations
of the exoplanet beta-Pic b. To address these limitations, a generic model for exoplanet astrometry was developed using a ResMLP as an embedding network. Using different experiments, we showed that this generic model was able to infer the posterior of the orbital parameters of all four planets of the HR 8799 system, significantly reducing the computational effort compared to MCMC. 

Despite these advancements, challenges remain, particularly in generalizing the model across exoplanets from different systems, as this generic model 
could not infer the posterior of the orbital parameters of beta-Pic b.

Simulation-based inference --- Exoplanets --- astrometry --- normalizing flows --- Ingénierie, informatique & technologie > Sciences informatiques --- Physique, chimie, mathématiques & sciences de la terre > Aérospatiale, astronomie & astrophysique