2025ApJ...994...64M

Age-benchmark brown dwarf and planetary-mass-object spectroscopy is key to characterizing substellar evolution. In this paper, we present the JHK medium-resolution (R ∼ 3000) spectra of 25 7-76 M_Jup (spectral types L3.0-M6.0) brown dwarfs and planetary mass objects in the Orion Nebula Cluster, obtained with MOSFIRE installed at the W. M. Keck I telescope. We obtained the spectral types of the targets in our sample using template brown dwarf and planetary-mass-object spectra. We confirmed their extreme youth (<5 Myr) and membership of the cluster using spectral indices as well as the diversity of their spectra, even for targets with similar spectral types. Six of our targets presented Paschen β and Brackett γ emission lines, suggesting the existence of accreting protoplanetary disks for objects with masses as low as 7 M_Jup. After analyzing the emission lines of those objects, and measuring their accretion rates, we compared them to those of stars, brown dwarfs, and planetary mass objects, confirming that planetary mass young objects deplete their disks quickly at young ages. Finally, we illustrate the spectral evolution of a 7-10 M_Jup planetary mass object through its life from 1-3 to 200 Myr old, using one of our latest spectral type targets and other targets from the literature with older ages but similar estimated masses. The spectra are publicly available for the community's use as data behind the figures.