Infrared spectroscopy of substellar objects in Orion

We present broad-band spectra of a sample of 21 low-luminosity sources in the Trapezium cluster, with masses in the range 0.008-0.10 M. (assuming an age of 1 Myr). These were selected for low extinction in most cases, and are located west of the brighter nebulosity. The spectra are in the H bandpass (1.4-1.95 mum) and K bandpass (1.9-2.5 mum) also for most of the brighter sources, with a resolution of 50 nm. They were taken with the United Kingdom Infrared Telescope (UKIRT) using the CGS4 spectrometer. Absorption by water vapour bands is detected in all the substellar candidates except one, which is a highly reddened object with strong H-2 emission and an anomalously blue (I - J) colour, implying that it is a very young cluster member with circumstellar matter. The observation of prominent water vapour bands confirms the low effective temperatures implied by our (I - J) colour measurements in an earlier paper, and would imply late-M or L spectral types if these were older field dwarfs. However, the profiles of the H-bandpass spectra are very different from those of field dwarfs with similar water absorption strength, demonstrating that they are not foreground or background objects. In addition, the CO absorption bands at 2.3 mum and the Na I absorption feature at 2.21 mum. are very weak for such cool sources. All these features are quite well reproduced by the AMES-Dusty-1999 model atmospheres of Allard et al., and arise from the much lower gravities predicted for the Trapezium sources (3.5 < log g < 4.0) compared to evolved objects (log g similar to 5.5). This represents a new proof of the substellar status of our sources, independent of the statistical arguments for low contamination, which are reexamined here. The very late spectral types of the planetary mass objects and very low-mass brown dwarfs demonstrate that they are cluster members, since they are too luminous to be field dwarfs in the background. We also present additional UKIRT photometry of a small region in the south of the Trapezium cluster where the extinction and nebular brightness are low, which permitted the detection of objects with 1-Myr masses slightly lower than our previous least massive source at 8 M-Jup. Following a minor update to our previous J-band photometry, due to a new UKIRT filter calibration, there are similar to 15 planetary mass candidates in the fall data set.