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 $\beta$ 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 ${ \approx}340$ mas which exhibits orbital motion at the 6-$\sigma$ level over the course of 13 months. A joint orbit fit yields precise constraints on the planet's dynamical mass of 3.2$^{ +0.7}_{ -0.6}$ $M_\mathrm{ Jup}$, semi-major axis of $8.4^{ +1.1}_{ -1.3}$ au, and eccentricity of $0.24^{ +0.27}_{ -0.15}$. 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_* = 54^{ +11}_{ -9} { }^\circ$) and orbital inclination ($i_o = 50^{ +9}_{ -12} { }^\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.
Categories: astro-ph.EP astro-ph.SR