A deep survey of brown dwarfs in Orion with Gemini

Astrophysics, abstract
astro-ph/0504570

From: Philip Lucas [view email]

Date: Tue, 26 Apr 2005 16:31:51 GMT   (590kb)

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 arcmin^2 area, including 138 brown dwarf candidates, defined as M<0.075
Msun for an assumed age of 1 Myr. Only 33 of the brown dwarf candidates are
planetary mass candidates (PMCs) with estimated masses in the range
0.003<M<0.012Msun. In an extinction limited sample (A(V)<5) complete to
approximately 0.005 Msun (5 Mjup) 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 3-13 Mjup number <10% of the cluster population, with an upper limit of 13%.
Analysis of the spatial distribution of stars and brown dwarf candidates
suggests that brown dwarfs and very low mass stars (M<0.1 Msun) are less likely
than more massive stars to have wide (>150 AU) binary companions. This result
has modest statistical significance (96%) in our data but is supported at 93%
confidence by analysis of an completely independent sample taken from the
Subaru data of Kaifu et al.(2000). 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 (Reipurth & Clarke 2001). We also report the discovery of
two new bipolar nebulae, which are interpreted as Class I protostars.

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