Observational constraints on the chemical tracers of planet formation history

Context. Constraining the link between the atmospheric composition of giant exoplanets and their formation history is a key goal of exoplanet studies. In particular, the atmospheric carbon-to-oxygen (C/O) ratio and metallicity – which are readily measurable with direct spectroscopic observations – are believed to be chemical tracers of the birth location of substellar and planetary companions. Aims. We aim to collect observational constraints for planet formation theories by performing a large and systematic survey of the atmospheric C/O ratio and metallicity. Methods. We collected new K-band moderate-resolution (R ~ 11 000) spectroscopic observations of 13 directly imaged planetary-mass companions spanning the molecular snowlines of CO2, CO, and CH4 and with masses 4–30 MJ with VLT/ERIS/SPIFFIER. Additionally, we gathered a large portion of the available archival observations for these targets, amounting to 40 spectra and 140 photometric fluxes. We performed spectral fits using self-consistent grid models and freely parametrisable models. We obtained robust estimates of the key atmospheric parameters by aggregating the results from the different models, thereby reducing modelling systematics. Results. Using molecular mapping, we detected H2O and CO in 12 of our targets as well as 13CO in HR 2562 B. With our multimodel spectral fitting strategy, we obtained stellar to superstellar C/O ratios – ranging between 0.3 and 0.8 – and predominantly superstellar metallicities – between −1 and 1 dex – across all targets. We measured a substantial enrichment of 13CO for HR 2562 B with 12CO/13CO= 12.0−3.3+4.5. If corroborated by independent observations, it could indicate that the companion might have formed beyond the CO snowline and later migrated inwards to its current location. We find an anti-correlation (R = −0.64) between the C/O ratio and the companion mass, consolidating a previous result. Conclusions. Our work demonstrates the scientific potential of the ERIS/SPIFFIER instrument for the orbital and atmospheric characterisation of close-in substellar and exoplanet companions.