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Population of very young brown dwarfs in Orion
Phil Lucas and Pat Roche (Oxford)
We describe the results of a very deep imaging survey of the
Trapezium Cluster in the IJH bands, using the UKIRT high resolution camera
UFTI. Extinction is calculated from the J-H colours, permitting accurate
luminosity estimates and temperatures are derived from the I-J colours,
using the UFTI long I band filter. Approximately half of the point sources
detected are brown dwarf candidates, including some free floating objects
with masses below the deuterium burning (planetary) threshold near 0.013
solar masses; these are detectable because of their extreme youth.
However, there appears to be a cut-off in the mass function below the
level of several Jupiter masses, which may represent the bottom end of the
IMF. Since star formation is complete in the Trapezium this limit could
have general significance. Foreground contamination is negligible in the
33 arcmin2 area
surveyed and the dense backdrop of OMC-1 obscures all
background stars, so we have confidence that the sources detected are
cluster members.
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