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 _ { \odot } ( 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 \mu m ) and K bandpass ( 1.9-2.5 \mu m ) 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 \mu m and the NaI absorption feature at 2.21 \mu m 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 .
( 2000,2001 ) , and arise from the much lower gravities predicted for the Trapezium sources ( 3.5 < log ( g ) < 4.0 ) compared to evolved objects ( log g \sim 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 \sim 15 planetary mass candidates in the full dataset .