Astrometric Accelerations as Dynamical Beacons: A Giant Planet Imaged inside the Debris Disk of the Young Star AF Lep

Abstract

We present the direct-imaging discovery of a giant planet orbiting the young star AF Lep, a 1.2 M _⊙ member of the 24 ± 3 Myr β Pic moving group. AF Lep was observed as part of our ongoing high-contrast imaging program targeting stars with astrometric accelerations between Hipparcos and Gaia that indicate the presence of substellar companions. Keck/NIRC2 observations in $L^{\prime} $ with the vector vortex coronagraph reveal a point source, AF Lep b, at ≈340 mas, which exhibits orbital motion at the 6 σ level over the course of 13 months. A joint orbit fit yields precise constraints on the planet’s dynamical mass of ${3.2}_{-0.6}^{+0.7}$ M _Jup , semimajor axis of ${8.4}_{-1.3}^{+1.1}$ au, and eccentricity of ${0.24}_{-0.15}^{+0.27}$ . AF Lep hosts a debris disk located at ∼50 au, but it is unlikely to be sculpted by AF Lep b, implying there may be additional planets in the system at wider separations. The stellar inclination ( i _* = ${54}_{-9}^{{+11}^\circ} $ ) and orbital inclination ( i _o = ${50}_{-12}^{{+9}^\circ} $ ) are in good agreement, which is consistent with the system having spin–orbit alignment. AF Lep b is the lowest-mass imaged planet with a dynamical mass measurement and highlights the promise of using astrometric accelerations as a tool to find and characterize long-period planets.

Keywords

• Extrasolar gaseous giant planets
• Astrometric exoplanet detection
• Direct imaging
• Orbit determination
• Debris disks