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

TL;DR

This study uses optical long baseline interferometry to measure the spin-orbit alignment of the Fomalhaut system, confirming the star's rotation axis is perpendicular to its debris disk, supporting standard star and planet formation theories.

Contribution

First spectro-astrometric measurement of a debris disk's orientation using interferometry, providing new insights into star-disk alignment in non-eclipsing systems.

Findings

Stellar rotation axis PA=65° pm 3°

Disk position angle PAdisk=156° pm 0.3°

Supports star and planet formation models

Abstract

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 brG absorption line. The achieved astrometric accuracy of 3 nu-as 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 PAstar=65deg pm 3deg for the stellar rotation axis, perfectly perpendicular with the literature measurement for the disk position angle (PAdisk=156deg pm 0.3deg). This is the first time such test can 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 collinear with the stellar spins.