A deep survey of brown dwarfs in Orion with Gemini

We report the results of a deep near-infrared (JHK) survey of the outer parts of the Trapezium Cluster with Gemini South/Flamingos. 396 sources were detected in a 26 arcmin² area, including 138 brown dwarf candidates, defined as M < 0.075 M_solar for an assumed age of 1 Myr. Only 33 of the brown dwarf candidates are planetary mass candidates with estimated masses in the range 0.003 < M < 0.012 M_solar. In an extinction-limited sample (A_V < 5) complete to approximately 0.005 M_solar (5M_Jup) the mass function appears to drop by a factor of 2 at the deuterium burning threshold, i.e. at planetary masses. After allowing for background contamination it is likely that planetary mass objects at 3-13M_Jup number <10 per cent of the cluster population, with an upper limit of 13 per cent. Analysis of the spatial distribution of stars and brown dwarf candidates suggests that brown dwarfs and very low-mass stars (M < 0.1 M_solar) are less likely than more massive stars to have wide (>150 au) binary companions. This result has modest statistical significance (96 per cent) in our data but is supported at 93 per cent confidence by analysis of a completely independent sample taken from the Subaru data of Kaifu et al. There is a statistically very significant excess of both stars and brown dwarfs with small separations from each other (<6 arcsec or 2600 au). This appears to be due to the presence of small-N subgroups, which are likely to be dynamically unstable in the long term. Hence these results are consistent with the `ejected stellar embryo' hypothesis for brown dwarf formation. We also report the discovery of two new bipolar nebulae, which are interpreted as Class I protostars.