Title: The stellar mass function, binary content and radial structure of the open cluster Praesepe derived from PPMXL and SDSS data
Abstract: 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 = 15.5 mag, corresponding to a mass of about 0.15 M⊙ for an assumed cluster distance modulus of (m − M)0 = 6.30 mag (d ≈ 182 pc), within a radius of 3| $_{.}^{\circ}$|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 630 M⊙, 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 = 0.65 M⊙. 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 = 0.65 M⊙ 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 35 per cent 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.