2013MNRAS.434.3236K

We have determined possible cluster members of the nearby open cluster Praesepe (M44) based on J and K photometry and proper motions from the PPMXL catalogue and z photometry from the Sloan Digital Sky Survey. In total, we identified 893 possible cluster members down to a magnitude of J = 15.5mag, corresponding to a mass of about 0.15M_☉ for an assumed cluster distance modulus of (m - M)0 = 6.30mag (d ~ 182 pc), within a radius of 3 _.^○5 around the cluster centre. We derive a new cluster centre for Praesepe (αcentre = 8h39m37s, δcentre = 19°35'02''). We also derive a total cluster mass of about 630M_☉, and a 2D half-number and half-mass radius of 4.25 and 3.90 pc, respectively. The global mass function of the cluster members shows evidence for a turnover around m = 0.65M_☉. While more massive stars can be fitted by a power law ξ(m) ∼ m-α with slope α = 2.88±0.22, stars less massive than m = 0.65M_☉ are best fitted with α = 0.85±0.10. In agreement with its large dynamical age, we find that Praesepe is strongly mass segregated and that the mass function slope for high-mass stars steepens from a value of α = 2.32±0.24 inside the half-mass radius to α = 4.90±0.51 outside the half-mass radius. We finally identify a significant population of binaries and triples in the colour-magnitude diagram of Praesepe. Assuming non-random pairing of the binary components, a binary fraction of about 35percent for primaries in the mass range 0.6 < m/M_☉ < 2.20 is required to explain the observed number of binaries in the colour-magnitude diagram.