Brown dwarfs in the Pleiades: spatial distribution and mass function

Abstract

Using new infrared data we have reassessed the membership status of candidate low-mass Pleiads unearthed by the International Telescope Project IZ survey. Those with I–K colours consistent with membership of the Pleiades have been compiled with candidate brown dwarfs identified by three other large, deep far-red CCD surveys of the cluster to yield the biggest magnitude-limited sample of substellar members to date. We fit King profiles to their spatial distribution to determine the Pleiades brown dwarf core radius to be r c = 2.22 +1.36 −0.67 degrees (or 5.0 +3.0 −0.9 pc). This is consistent with a continuation of the r c ∝ m −0.5 relationship found previously for the higher-mass stellar members and suggests that the brown dwarf members are also dynamically relaxed. Using our spatial model we derive the Pleiades mass function in the substellar regime and are able to place stringent limits on its shape. We find that it is well represented by a power law with index α = 0.41 ± 0.08 (0.035 M⊙≲ M ≲ 0.3 M⊙). This result is largely insensitive to our choice of evolutionary model and uncertainties in the cluster age and distance. It is only marginally sensitive to the brown dwarf binary fraction. By assuming that the cluster mass function continues to rise down to the deuterium-burning limit, we estimate that the total brown dwarf mass of the Pleiades is 13 +4 −3 M⊙. This only represents ∼2 per cent of the total cluster mass. Given that the present-day cluster mass function should be a good representation of the initial mass function, we conclude that brown dwarfs do not contribute significantly to disc dark matter.