Studying the Fomalhaut debris disk with infrared interferometry

In this talk, I will describe two recent studies carried out with infrared interferometry to characterise the planetary system around Fomalhaut, and its debris disk in particular. In the first study, we aimed to determine whether the debris disk is located within the equatorial plane of the stellar photosphere, in an attempt to improve our understanding of spin-orbit misalignements in planetary systems in general. We measured the orientation of the rotationnally-distorted stellar photosphere using micro-arcsecond precision VLTI/AMBER spectro-astrometry within the Br-gamma line. The derived poition angle is in perfect agreement with the position angle of the cold debris disk measured in visible and sub-millimeter images. We discuss the implications of this result on our understanding of the dust grain properties in the Fomalhaut disk. In the second study, we aimed at characterising the dust content of the innermost part of the debris disk. We used archival high-precision K-band visibility measurements with VLTI/VINCI and obtained N-band nulling observations with the Keck Interferometer Nuller. We report a significant excess emission at K band, and a marginal excess emission at N band, that we attempt to reproduce with a 2D debris disk model. A comprehensive Bayesian analysis of the main disk parameters is performed to derive most-probable values. Our analysis points towards a very compact ring of hot dust close to the sublimation radius as the origin of the reported excess emission