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 $M_{\odot }$ member of the 24 $\pm$ 3 Myr ${\hat{I}}^2$ 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 $\approx 340$ mas which exhibits orbital motion at the 6-$\ddot{I}ƒ$ level over the course of 13 months. A joint orbit fit yields precise constraints on the planet's dynamical mass of 3.2${}_{-0.6}^{+0.7}$ $M_{\mathrm{Jup}}$, semi-major 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 $\sim$50 au, but it is unlikely to be sculpted by AF Lep b, implying there may be additional planets in the system at wider separations. The stellar inclination ($i_{\ast }=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.

14 pages, 3 figures, accepted to ApJL