Forming short period sub-stellar companions in 47 Tuc: I. Dynamical
  model and brown dwarf tidal capture rates
  Stars in globular clusters formed and evolved in the most extreme
environment: high density and low metallicity. If the formation of stars and
planets are at all sensitive to environmental conditions, this should therefore
be evident in globular clusters. Observations have indicated that hot Jupiters
are at least an order of magnitude less prevalent in the central region of the
globular cluster 47 Tucanae than in the field. In this work, we explore the
claims in the literature for additional consequences for the low mass stellar
initial mass function. Tidal capture, the mechanism that produces X-ray
binaries in globular clusters, applies also to brown dwarfs (BDs). This process
produces tight stellar-BD binaries that would be detectable by transit surveys.
Applying a Monte Carlo dynamical evolution model, we compute the overall BD
capture rates. We find that the number of captures is lower than previous
estimates. Capture efficiency increases steeply with stellar mass, which means
that mass segregation reduces capture efficiency as BDs and low mass stars
occupy the same regions. The result of this effect is that the current
constraints on the short period companion fraction remains marginally
consistent with initially equal numbers of BDs and stars. However, our findings
suggest that expanding the sample in 47 Tuc or surveying other globular
clusters for close sub-stellar companions can yield constraints on the
sub-stellar initial mass function in these environments. We estimate the
capture rates in other globular clusters and suggest that 47 Tuc remains a
promising target for future transit surveys.
