Deep N-band nulling interferometric imaging of α Lyrae and β Leo with the LBTI

Since the discovery of an exoplanet around a main-sequence star by Mayor and Queloz (1995) the field of exoplanetology has never stopped to expend. Nowadays, several thousands confirmed exoplanets have been discovered and some other thousands are waiting for confirmation. However, even if the number of "known" exoplanets keeps increasing, we have only very few information about them. Do they have an atmosphere? What is their chemical composition? What is their surface temperature? This is only a few examples of the questions still unanswered for the vast majority of confirmed exoplanets. Characterizing exoplanets is one of the most challenging topic in current exoplanetology, the ultimate goal being to one day be able to determine the habitability of an exoplanet and the possible presence of life. To characterize exoplanets, we nowadays depend on two main methods: the transit method where the atmosphere is characterized using the photons coming from the host star and passing through the atmosphere of the exoplanet before reaching us, and direct imaging where the photons from the planets (reflection of the host star light or thermal emission) are directly recovered and used for the characterization. Direct imaging domain itself decomposed into two main methods: coronography and nulling interferometry. In this master thesis, we will focus on this last method. In both domains the planet faintness is a huge challenge to the characterization. Indeed a terrestrial planet is about 107 fainter than its host star and 109 than the Earth atmospheric background at 10 μm. Transit method is in addition limited to planets orbiting close to their host star, and which actually pass in front of it seen from Earth. On the other hand direct imaging techniques have to deal with the angular separation to the star and face the challenge to detect, let alone characterize, planets close to their host star. The advantage of nulling interferometry is to combine the angular resolution of interferometry and star light suppression. In this master thesis, we will focus on two post-processing methods used in direct imaging: Angular Differential Imaging (ADI, Marois et al 2006) and Reference (star) Differential Imaging (RDI, Lafreniere et al 2007; Ruane et al 2019). We will compare these methods on different datasets obtained by nulling interferometry thanks to NASA’s Large Binocular Telescope Interferometer (LBTI) during the HOSTS survey (Hunt for Observable Signatures of Terrestrial Systems, Ertel et al 2020) which was looking for exozodiacal disks in the habitable zone of the targeted stars to put constraints on their impact on exoplanet imaging. Our goal will be twofold: looking for the signature of resolved circumstellar emission around the target stars and characterize the imaging performance of the LBTI in its nulling mode. Those datasets present different ranges and distributions of parallactic angles, photometry of the star, level of the nulling quality, flux distribution and we intend to retrieve for a given type of dataset (for example with small parallactic angles ranges) an optimal method. In order to achieve this goal we will, in the following, use the VIP python library (Gonzalez et al 2017) to perform analysis with the different methods (ADI, RDI) and different models of PSF subtraction (median subtraction, least-square approximation, full-frame PCA, etc) on all the different datasets. Finally, we will give a conclusion for each method on which type of dataset is the most adapted, and provide an helpful guide on which method is optimal for the processing of a particular dataset.