Multi-Object and Long-Slit Spectroscopy of Very Low Mass Brown Dwarfs in
  Orion Nebular Cluster
  We present the results of a H- and K-band multi-object and long-slit
spectroscopic survey of substellar mass candidates in the outer regions of the
Orion Nebula Cluster. The spectra were obtained using MOIRCS on the 8.2-m
Subaru telescope and ISLE on the 1.88-m telescope of Okayama Astronomical
Observatory. Eight out of twelve spectra show strong water absorptions and we
confirm that their effective temperatures are < 3000 K (spectral type > M6)
from a chi-square fit to synthetic spectra. We plot our sources on an HR
diagram overlaid with theoretical isochrones of low-mass objects and identify
three new young brown dwarf candidates. One of the three new candidates is a
cool object near the brown dwarf and planetary mass boundary. Based on our
observations and those of previous studies, we determine the stellar (0.08 <
M/Msun < 1) to substellar (0.03 < M/Msun < 0.08) mass number ratio in the outer
regions of the Orion nebular cluster to be 3.5 +/- 0.8. In combination with the
number ratio reported for the central region (3.3+0.8/-0.7), this result
suggests the number ratio does not simply change with the distance from the
center of the Orion nebular cluster.
