Letter to the Editor

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

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile e-mail: jlebouqu@eso.org
² Institut d'Astrophysique et de Géophysique, Université de Liège, 17 allée du Six Août, 4000 Sart Tilman, Belgium
³ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 16 February 2009
Accepted: 9 April 2009

Abstract

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- absorption line. The achieved astrometric accuracy of as and the spectral resolution from the AMBER/VLTI instrument allow us to spatially and spectrally resolve the rotating photosphere.

Results. We find a position angle  ± for the stellar rotation axis, perpendicular to the literature measurement for the disk position angle ( ± ). 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.