High-contrast spectroscopy with the new VLT/ERIS instrument

Molecular maps and radial velocity of the gas giant AF Lep b^★

¹ ETH Zurich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
² Max Planck Institute for Intelligent Systems, Max-Planck-Ring 4, 72076 Tübingen, Germany
³ National Center of Competence in Research PlanetS, Switzerland
⁴ University of Zurich, Rämistrasse 71, 8006 Zurich, Switzerland
⁵ Department of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁶ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁷ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741, Garching, Germany
⁸ Space Sciences, Technologies, and Astrophysics Research Institute, Université de Liège, 4000 Sart Tilman, Belgium
⁹ Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands
¹⁰ ETH Zurich, Department of Earth and Planetary Sciences, Sonneggstrasse 5, 8092 Zurich, Switzerland
¹¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹³ STFC UK ATC, Royal Observatory Edinburgh, Blackford Hill. Edinburgh EH9 3HJ, UK
¹⁴ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937, Köln, Germany
¹⁵ INAF – Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini, 64100 Teramo, Italy

^★★ Corresponding author: jhayoz@phys.ethz.ch

Received: 4 December 2024
Accepted: 20 March 2025

Abstract

Context. The Enhanced Resolution Imager and Spectrograph (ERIS) is the new adaptive optics (AO) assisted infrared instrument at the Very Large Telescope (VLT). Its refurbished integral field spectrograph (IFS) SPIFFIER leverages a new AO module, enabling high-contrast imaging applications and giving access to the orbital and atmospheric characterisation of super-Jovian exoplanets.

Aims. We tested the detection limits of ERIS, and demonstrate its scientific potential by exploring the atmospheric composition of the young super-Jovian AF Lep b. Additionally, we improved its orbital solution by measuring its radial velocity relative to its host star.

Methods. We present new spectroscopic observations of AF Lep b in K-band at R ∼ 11 000 obtained with ERIS/SPIFFIER at the VLT. We reduced the data using the standard pipeline together with a custom wavelength calibration routine, and removed the stellar point spread function using principal component analysis along the spectral axis. We computed molecular maps by cross-correlating the residuals with molecular spectral templates and measured the radial velocity of the planet relative to the star. Furthermore, we computed contrast grids for molecular mapping by injecting fake planets.

Results. We detect a strong signal from H₂O and CO but not from CH₄ or CO₂. This result corroborates the hypothesis of chemical disequilibrium in the atmosphere of AF Lep b. Our measurement of the RV of the planet yields Δ v_{R, P ⋆} = 7.8 ± 1.7 km s⁻¹. This enables us to disentangle the degeneracy of the orbital solution; specifically, the correct longitude of the ascending node is Ω = 248_−0.7^+0.4 deg and the argument of periapsis is ω = 109₋₂₁⁺¹³ deg. Our detection limits reach a contrast of ΔK = 11.5 mag at 0′′.12 for the spectral templates of H₂O and CO, significantly extending the parameter space available to moderately high spectral resolution towards small angular separation.

Conclusions. Our results demonstrate the competitiveness of the new ERIS/SPIFFIER instrument for the orbital and atmospheric characterisation of exoplanets at high contrast and small angular separation.