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The stellar mass function, binary content and radial structure of the open cluster Praesepe derived from PPMXL and SDSS data

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 (SDSS). 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 Msun for an assumed cluster distance modulus of (m-M)_0 = 6.30 mag (d ~ 182 pc), within a radius of 3.5° around the cluster centre. We derive a new cluster centre for Praesepe (α_centre = 8h 39m 37s, δ_centre = 19° 35&#39; 02&#34;). We also derive a total cluster mass of about 630 Msun and a 2D half-number and half-mass radius of 4.25 pc and 3.90 pc respectively. The global mass function (MF) of the cluster members shows evidence for a turnover around m = 0.65 Msun. While more massive stars can be fit by a power-law ξ(m) ~ m^(-α) with slope α = 2.88 +/- 0.22, stars less massive than m = 0.65 Msun 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% for primaries in the mass range 0.6 < m/Msun < 2.20 is required to explain the observed number of binaries in the colour-magnitude diagram (CMD).