High-contrast spectroscopy with the new VLT/ERIS instrument: Molecular maps and radial velocity of the gas giant AF Lep b

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 H$_2$O and CO but not from CH$_4$ or CO$_2$. This result corroborates the hypothesis of chemical disequilibrium in the atmosphere of AF Lep b. Our measurement of the RV of the planet yields $\Delta$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 $\omega$ = 109$_{-21}^{+13}$ deg. Our detection limits reach a contrast of $\Delta$K = 11.5 mag at 0''.12 for the spectral templates of H$_2$O 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.