Infrared spectroscopy and analysis of brown dwarf and planetary mass objects in the Orion nebula cluster

We present near-infrared long-slit and multislit spectra of low-mass brown dwarf candidates in the Orion nebula cluster. The long-slit data were observed in the H and K bands using NIRI on the Gemini-North Telescope. The multi-object spectroscopic observations were made using IRIS2 on the Anglo-Australian Telescope at H band. We develop a spectral typing scheme based on optically calibrated, near-infrared spectra of young sources in the Taurus and IC 348 star-forming regions with spectral types M3.0 to M9.5. We apply our spectral typing scheme to 52 sources, including previously published UKIRT and GNIRS spectra. 40 objects show strong water absorption with spectral types of M3 to >M9.5. The latest type objects are provisionally classified as early L types. We plot our sources on Hertzsprung–Russell diagrams overlaid with theoretical pre-main-sequence isochrones. The majority of our objects lie close to or above the 1-Myr isochrone, leading to an average cluster age that is