Shape parameters of Galactic open clusters

Context. Ellipticities have been determined for only a few tens of open clusters. Aims. We derive the observed and modelled shape parameters (apparent ellipticity and orientation of the ellipse) of 650 Galactic open clusters identified in the ASCC-2.5 catalogue. Methods. We compute the observed shape parameters of Galactic open clusters with a multi-component analysis. For the vast majority of clusters, these parameters are determined for the first time. High resolution (star by star) N-body simulations are carried out with a specially developed phi GRAPE code providing models of clusters of different initial masses, Galactocentric distances, and rotation velocities. Results. By comparing models and observations for about 150 clusters, we find that the ellipticities of observed clusters are too low (0.2 vs. 0.3), and take a first step in identifying the main reason for this discrepancy. After approximate to 50 Myr, the models predict that clusters exhibit an oblate shape with an axis ratio of 1.65:1.35:1, and a major axis tilt by an angle of q(XY) approximate to 30 degrees. with respect to the Galactocentric radius due to the differential rotation of the Galaxy. Conclusions. Unbiased estimates of cluster shape parameters require reliable membership determination in large cluster areas out to 2-3 tidal radii, where the density of cluster stars is considerably lower than the background. Although dynamically bound stars beyond the tidal radius contribute insignificantly to the cluster mass, knowledge of their distribution is essential for a correct determination of cluster shape parameters. In contrast, a restricted mass range of cluster stars does not play such a dramatic role, although deep surveys allow us to identify more cluster members and, therefore, to increase the accuracy of the observed shape parameters.