The planetary-mass-limit VLT/SINFONI library: Spectral extraction and atmospheric characterization via forward modeling

We aim to deepen our understanding of the M-L transition on planetary-mass companions and isolated brown dwarfs, and search for evidence of possible differences between these two populations of objects. To this end, we present a set of 21 VLT/SINFONI K-band observations from five archival programs at a spectral resolution of 4000. We aim to measure atmospheric properties, such as temperature, surface gravity, and metallicity, to understand the similarities and differences between objects ranging from M5 to L5 in spectral type. We extracted the spectra of these targets with the TExTRIS code. Subsequently, we model them using ForMoSA, a Bayesian forward modeling tool for spectral analysis, exploring four families of self-consistent atmospheric models: ATMO, BT-Settl, Exo-REM, and Sonora. Here we present the spectra of our targets and the derived parameters from the atmospheric modeling. We observed a drop in effective temperature of more than 500 K as a function of spectral type at the M/L transition, likely related to limitations in the current atmospheric models. In addition, we report carbon-to-oxygen ratio measurements for three companions (2M 0103 AB b, AB Pic b, and CD-35 2722 b), which contribute to the growing list of exoplanets for which this value has been measured. In conclusion, the VLT/SINFONI Library highlights two key points. First, there is a critical need to further investigate the discrepancies among grids of spectra generated by self-consistent models, as these models yield varying results and do not uniformly explore the parameter space. Second, we do not observe apparent discrepancies in the K-band spectra between companions and isolated brown dwarfs, which potentially suggests that these super-Jupiter objects formed through a similar process; however, this warrants further investigation.