The spin-orbit alignment of the Fomalhaut planetary system probed by optical long baseline interferometry

Aims. 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. Methods: We use spectrally resolved, near-IR long baseline interferometry to obtain precise spectro-astrometric measurements across the Br-gamma absorption line. The achieved astrometric accuracy of ±3 muas and the spectral resolution R=1500 from the AMBER/VLTI instrument allow us to spatially and spectrally resolve the rotating photosphere. Results: 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)=156.0 ° ± 0.3°). 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. Conclusions: 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. Based on observations collected at the VLTI (ESO Paranal, Chile), with the 082.C-0376 program from the AMBER Guaranteed Time of the Osservatorio Astrofisico di Arcetri (INAF, Italy).