Astrometric Accelerations as Dynamical Beacons: A Giant Planet Imaged inside the Debris Disk of the Young Star AF Lep
Jensen-Clem, Rebecca; Crepp, Justin R.; Zhou, Yifan; Dupuy, Trent J.; Bardalez Gagliuffi, Daniella C.; Tran, Quang H.; Bowler, Brendan P.; Brandt, Timothy D.; Faherty, Jacqueline; Franson, Kyle; Morgan, Marvin; Theissen, Christopher A.; Sanghi, Aniket; Pearce, Tim D.; Biddle, Lauren I.; Wolf, Trevor N.
United States, Germany, United Kingdom
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 6σ 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.