Multi-Object and Long-Slit Spectroscopy of Very Low Mass Brown Dwarfs in the 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/M⊙ < 1) to substellar (0.03< M/M⊙ < 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.