Astrometric and photometric initial mass functions from the UKIDSS Galactic Clusters Survey - I. The Pleiades

We present the results of a deep wide-field near-infrared survey of the entire Pleiades cluster recently released as part of the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS) Data Release 9 (DR9). We have identified a sample of ∼1000 Pleiades cluster member candidates combining photometry in five near-infrared passbands and proper motions derived from the multiple epochs provided by the UKIDSS GCS DR9. We also provide revised membership for all previously published Pleiades low-mass stars and brown dwarfs in the past decade recovered in the UKIDSS GCS DR9 Pleiades survey based on the new photometry and astrometry provided by the GCS. We find no evidence of K-band variability in the Pleiades members larger than ∼0.08 mag. In addition, we infer a substellar binary frequency of 22-31 per cent in the 0.075-0.03 M_⊙ range for separations less than ∼100 au. We employed two independent but complementary methods to derive the cluster luminosity and mass functions: a probabilistic analysis and a more standard approach consisting of stricter astrometric and photometric cuts. We found that the resulting luminosity and mass functions obtained from both methods are very similar. We derive the Pleiades mass function in the 0.6-0.03 M_⊙ range and found that it is best reproduced by a lognormal representation with a mean characteristic mass of 0.20 ± 0.05 M_⊙, in agreement with earlier studies and the extrapolation of the field mass function.