Spectral characterization of young LT dwarfs

JWST and next-generation facilities are expected to uncover populations of free-floating objects below the deuterium-burning limit in a number of young clusters and star-forming regions. These young planetary-mass brown dwarfs have spectral types L and T; they are shaped by molecular absorption and modified by their low gravity, which makes them distinct from field objects. We aim to provide a detailed characterization of near-infrared spectra for young LT brown dwarfs, including robust spectral typing, calibrating spectral indices, identifying possible binaries, and selecting suitable spectral standards. We processed and analyzed archival spectra from VLT/X-shooter for a sample of 56 dwarfs with ages between 10 and 600\,Myr and spectral types between late-M and mid-T. We redetermined their spectral types by comparing them with a set of literature templates. We assessed a large range of spectral indices, calibrated using a specifically designed literature sample. We identify 15 spectral indices that are useful for spectral typing for specific spectral ranges discussed here and provide the scaling relations with spectral types. We also identify six spectral indices that can be used to separate young L dwarfs from the field ones. The equivalent-widths of the alkali lines show a correlation with age, increasing toward objects with higher surface gravities. From our sample, we confirm three that are likely to be binaries due to their anomalous spectra that appear to be better fitted by a combination of spectral types. We identify 12 objects as preliminary near-infrared spectral standards for young LT dwarfs. This study represents a significant step toward understanding the spectral sequence and properties of young L and T dwarfs. The relations and standards provided here will be useful for future spectroscopic work on young brown dwarfs and giant planets.