2009A&A...498L..41L

We discuss the spin-orbit orientation of the Fomalhaut planetary system composed of a central A4V star, a debris disk, and a recently discovered planetary companion. We use spectrally resolved, near-IR long baseline interferometry to obtain precise spectro-astrometric measurements across the Br-γ absorption line. The achieved astrometric accuracy of ±3µas and the spectral resolution R=1500 from the AMBER/VLTI instrument allow us to spatially and spectrally resolve the rotating photosphere. We find a position angle PA_star=65°±3° for the stellar rotation axis, perpendicular to the literature measurement for the disk position angle (PA_disk=156.0deg±0.3deg). This is the first time such a test could be performed for a debris disk, and in a non-eclipsing system. Additionally, our measurements suggest unexpected backward-scattering properties for the circumstellar dust grains. Our observations validate the standard scenario for star and planet formation in which the angular momentum of the planetary systems are expected to be colinear with the stellar spins.