AF Lep b: The lowest-mass planet detected by coupling astrometric and direct imaging data

Abstract

Aims. Using the direct-imaging technique, we searched for low-mass companions around the star AF Lep, which presents a significant proper-motion anomaly (PMa) signal obtained from the comparison of HIPPARCOS and Gaia eDR3 catalogs. Methods. We observed AF Lep in two epochs with VLT/SPHERE using its subsystems IFS and IRDIS in the near-infrared, covering wavelengths ranging from the Y to the K spectral bands (between 0.95 and 2.3 μm). We then reduced the data using the high-contrast imaging techniques angular differential imaging (ADI) and spectral differential imaging in order to be able to retrieve the signal from low-mass companions of the star. Results. A faint companion was retrieved at a separation of ~0.335″ from the star and with a position angle of ~70.5° in the first epoch and with a similar position in the second epoch. This corresponds to a projected separation of ~9 au. The extracted photometry allowed us to estimate a mass for the companion of between 2 and 5 MJup. This mass is in good agreement with astrometric measurements of the dynamic mass of the companion, which give 5.2–5.5 MJup. This is the first companion with a mass well below the deuterium burning limit that was discovered by coupling direct imaging with PMa measurements. Orbital fitting done using the orvara tool allowed us to further confirm the companion mass and to define its main orbital parameters.