Multi-object and long-slit spectroscopy of very low mass brown dwarfs in the Orion Nebular Cluster

Abstract

We present the results of an 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 8.2m Subaru telescope and ISLE on the 1.88m telescope of Okayama Astrophysical Observatory. Eight out of twelve spectra show strong water absorption and we confirm that their effective temperatures are ≤3000K (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