Spin-orbit alignment in planetary systems: what can we learn from resolved debris disks?

Since the first detection of the spectroscopic transit of a hot Jupiter by Queloz et al. (2000), the Rossiter-McLaughlin effect, which allows the measurement of the sky-projected angle between the stellar rotation axis and a planet's orbit axis, has been investigated for a large number of planetary systems. These observations have lead to the detection of spin-orbit misalignments for many hot Jupiters, and even of retrograde orbits in some cases (e.g., Triaud et al. 2010). Such misalignments are generally interpreted as the signature of either Kozai migration, or of planet-planet scattering followed by inward migration of the planet (e.g., Morton & Johnson 2011). However, it has recently been proposed that circumstellar disks themselves might not be aligned with the stellar spin axis due to either to early stellar encounters (Bate et al. 2010) or to magnetosphere-disk interactions (Lai et al. 2011). In this talk, I discuss how resolved debris disks may be used to settle this question, in particular by measuring the position angle of oblate stellar photospheres deformed by rapid rotation using spectro-interferometric measurements within the Brackett-gamma line. The method is illustrated by our recent study of Fomalhaut with VLTI/AMBER (Le Bouquin et al. 2009), which confirms the spin-orbit alignment with a high accuracy for the first time in a debris disk system. Possible implications on the nature of the dust grains composing the Fomalhaut debris disk will also be discussed.