JWST/NIRCam 4-5 $\hat{I}¼$m Imaging of the Giant Planet AF Lep b

Abstract

With a dynamical mass of $3M_{\mathrm{Jup}}$, the recently discovered giant planet AF Lep b is the lowest-mass imaged planet with a direct mass measurement. Its youth and spectral type near the L/T transition make it a promising target to study the impact of clouds and atmospheric chemistry at low surface gravities. In this work, we present JWST/NIRCam imaging of AF Lep b. Across two epochs, we detect AF Lep b in F444W ($4.4\hat{\mathrm{I}}¼\mathrm{m}$) with S/N ratios of $9.6$ and $8.7$, respectively. At the planet's separation of $320\mathrm{mas}$ during the observations, the coronagraphic throughput is $\approx 7\%$, demonstrating that NIRCam's excellent sensitivity persists down to small separations. The F444W photometry of AF Lep b affirms the presence of disequilibrium carbon chemistry and enhanced atmospheric metallicity. These observations also place deep limits on wider-separation planets in the system, ruling out $1.1M_{\mathrm{Jup}}$ planets beyond $15.6\mathrm{au}$ ($0.58$ arcsec), $1.1M_{\mathrm{Sat}}$ planets beyond $27\mathrm{au}$ ($1$ arcsec), and $2.8M_{\mathrm{Nep}}$ planets beyond $67\mathrm{au}$ ($2.5$ arcsec). We also present new Keck/NIRC2 $L^{\prime }$ imaging of AF Lep b; combining this with the two epochs of F444W photometry and previous Keck $L^{\prime }$ photometry provides limits on the long-term $3-5\hat{\mathrm{I}}¼\mathrm{m}$ variability of AF Lep b on months-to-years timescales. AF Lep b is the closest-separation planet imaged with JWST to date, demonstrating that planets can be recovered well inside the nominal (50\% throughput) NIRCam coronagraph inner working angle.

17 pages, 4 figures, accepted in ApJL