Characterization of AF Lep b at high spectral resolution with VLT/HiRISE^★

¹ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
² Laboratoire J. L. Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, 06304 Nice, France
³ LESIA, Observatoire de Paris, Université PSL, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
⁴ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁵ Department of Physics & Astronomy, John Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
⁸ Millennium Nucleus on Young Exoplanets and their Moons (YEMS), Santiago, Chile
⁹ Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands
¹⁰ Institut de Recherche en Astrophysique et Planétologie (IRAP), 9 avenue Colonel Roche, BP 44346, 31028 Toulouse, France
¹¹ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
¹² European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹³ Universitäts-Sternwarte,Ludwig-Maximilians-Universität München, Scheinerstraße 1, 81679 München, Germany
¹⁴ Exzellenzcluster Origins, Boltzmannstraße 2, 85748 Garching, Germany
¹⁵ Institute for Astrophysics und Geophysik, Georg-August University, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
¹⁶ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
¹⁷ Department of Physics and Astronomy, University of Texas-San Antonio, San Antonio, TX, USA
¹⁸ Center for Advanced Instrumentation, Durham University, Durham, DH1 3LE, UK
¹⁹ Dept. of Astrophysics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
²⁰ Optical and Electronic Systems Department, Kazan National Research Technical University, Russia
²¹ Academia Sinica, Institute of Astronomy and Astrophysics, 11F Astronomy-Mathematics Building, NTU/AS campus, No. 1, Section 4, Roosevelt Rd., Taipei 10617, Taiwan
²² European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
²³ Physics & Astronomy Dpt, University of Exeter, Exeter EX4 4QL, UK
²⁴ Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822, USA

^★★ Corresponding author; allan.denis@lam.fr

Received: 21 November 2024
Accepted: 26 February 2025

Abstract

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.