  We present near-infrared long slit and multi-slit spectra of low mass brown
dwarf candidates in the Orion Nebula Cluster. The long slit data were observed
in the H- & 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 H-R 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 <1 Myr. We find 38 sources
lie at or below the hydrogen burning limit (0.075 Msun). 10 sources potentially
have masses below the deuterium burning limit (0.012 Msun). We use a Monte
Carlo approach to model the observed luminosity function with a variety of
cluster age and mass distributions. The lowest chi^2 values are produced by an
age distribution centred at 1 Myr, with a mass function that declines at
sub-stellar masses according to an M^alpha power law in the range alpha=0.3 to
0.6. We find that truncating the mass function at 0.012 Msun produces
luminosity functions that are starved of the faintest magnitudes, even when
using bimodal age populations that contain 10 Myr old sources. The results of
these Monte Carlo simulations therefore support the existence of a planetary
mass population in the ONC.
