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

We present the direct-imaging discovery of a giant planet orbiting the young star AF Lep, a 1.2 M-circle dot member of the 24 +/- 3 Myr beta 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' with the vector vortex coronagraph reveal a point source, AF Lep b, at approximate to 340 mas, which exhibits orbital motion at the 6s 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 similar to 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) degrees) and orbital inclination (i(o) = 50 (+9)(-12) degrees) 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.

Automated summary

We report the direct-imaging discovery of a giant planet orbiting the young star AF Lep, with precise constraints on its dynamical mass, semimajor axis, and eccentricity. The host star also has a debris disk, but it is unlikely to be sculpted by the planet AF Lep b, suggesting the possibility of additional planets in the system. This discovery highlights the potential of using astrometric accelerations to find and characterize long-period planets.