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 (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{\deg} around the cluster centre. We derive a new cluster centre for Praesepe ({\alpha}_centre = 8h 39m 37s, {\delta}_centre = 19{\deg} 35' 02"). 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 {\xi}(m) ~ m^-{\lpha}) with slope {\alpha} = 2.88 +/- 0.22, stars less massive than m = 0.65 Msun are best fitted with {\alpha} = 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 {\alpha} = 2.32 +/- 0.24 inside the half-mass radius to {\alpha} = 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).