Open-cluster density profiles derived using a kernel estimator

Abstract

Surface and spatial radial density profiles in open clusters are derived using a kernel estimatormethod. Formulae are obtained for the contribution of every star into the spatial density profile.The evaluation of spatial density profiles is tested against open-cluster models from N-bodyexperiments with N = 500. Surface density profiles are derived for seven open clusters (NGC1502, 1960, 2287, 2516, 2682, 6819 and 6939) using Two-Micron All-Sky Survey data andfor different limiting magnitudes. The selection of an optimal kernel half-width is discussed.It is shown that open-cluster radius estimates hardly depend on the kernel half-width. Hintsof stellar mass segregation and structural features indicating cluster non-stationarity in theregular force field are found. A comparison with other investigations shows that the data onopen-cluster sizes are often underestimated. The existence of an extended corona around theopen cluster NGC 6939 was confirmed. A combined function composed of the King densityprofile for the cluster core and the uniform sphere for the cluster corona is shown to be a betterapproximation of the surface radial density profile.The King function alone does not reproducesurface density profiles of sample clusters properly. The number of stars, the cluster massesand the tidal radii in the Galactic gravitational field for the sample clusters are estimated. It isshown that NGC 6819 and 6939 are extended beyond their tidal surfaces.