2023A&A...672A..94D

Context. Expanding the sample of directly imaged companions to nearby, young stars that are amenable to detailed astrometric and spectroscopic studies is critical for the continued development and validation of theories of their evolution and atmospheric processes.
Aims. The recent release of the Gaia astrometric catalog allows us to efficiently search for these elusive companions by targeting those stars that exhibit the astrometric reflex motion induced by an orbiting companion. The nearby (27 pc), young (24 Myr) star AF Leporis (AF Lep) was targeted because of its significant astrometric acceleration measured between the HIPPARCOS and Gaia astrometric catalogs, consistent with a wide-orbit planetary-mass companion detectable with high-contrast imaging.
Methods. We used the SPHERE instrument on the VLT to search for faint substellar companions in the immediate vicinity of AF Lep. We used observations of a nearby star interleaved with those of AF Lep to efficiently subtract the residual point spread function. This provided sensitivity to faint planetary-mass companions within 1″ (∼30 au) of the star.
Results. We detected the companion AF Lep b at a separation of 339 mas (9 au) from the host star, at almost the exact location predicted by the astrometric acceleration, and within the inner edge of its unresolved debris disk. The measured K-band contrast and the age of the star yield a model-dependent mass of between 4 and 6 M_Jup, consistent with the mass derived from an orbital fit to the absolute and relative astrometry of 4.3_–1.2^+2.9M_Jup. The near-infrared spectral energy distribution of the planet is consistent with an object at the L-T spectral type transition, but under-luminous with respect to field-gravity objects.
Conclusions. AF Lep b joins a growing number of substellar companions imaged around stars in the young β Pictoris moving group. With a mass of between 3 and 7 M_Jup, it occupies a gap in this isochronal sequence between hotter, more massive companions, such as PZ Tel B and β Pic b, and the cooler 51 Eri b, which is sufficiently cool for methane to form within its photosphere. Lying at the transition between these two classes of objects, AF Lep b will undoubtedly become a benchmark for studies of atmospheric composition and processes, as well as an anchor for models of the formation and evolution of substellar and planetary-mass companions.