2025A&A...696A...6D

Context. Since the recent discovery of the directly imaged super-Jovian planet AF Lep b, several studies have been conducted to characterize its atmosphere and constrain its orbital parameters. AF Lep b has a measured dynamical mass of 3.68 ± 0.48 M_Jup, radius of 1.3 ± 0.15 R_Jup, nearly circular orbit in spin-orbit alignment with the host star, relatively high metallicity, and near-solar to super-solar C/O ratio. However, key parameters such as the rotational velocity and radial velocity have not been estimated thus far, as they require high-resolution spectroscopic data that are impossible to obtain with classical spectrographs.
Aims. AF Lep b was recently observed with the new HiRISE visitor instrument at the VLT, with the goal of obtaining high-resolution (R ≈ 140 000) spectroscopic observations to better constrain the orbital and atmospheric parameters of the young giant exoplanet.
Methods. We compared the extracted spectrum of AF Lep b to self-consistent atmospheric models using ForMoSA, a forward modeling tool based on Bayesian inference methods. We used our measurements of the planet's radial velocity to offer new constraints on its orbit.
Results. From the forward modeling, we find a C/O ratio that aligns with previous low-resolution analyses and we confirm its supersolar metallicity. We also unambiguously confirm the presence of methane in the atmosphere of the companion. Based on all available relative astrometry and radial velocity measurements of the host star, we show that two distinct orbital populations are possible for the companion. We derived the radial velocity of AF Lep b to be 10.51 ± 1.03 km s^–1 and show that this value is in good agreement with one of the two orbital solutions, allowing us to rule out an entire family of orbits. Additionally, assuming that the rotation and orbit are coplanar, the derived planet's rotation rate is consistent with the observed trend of increasing spin velocity with higher planet mass.