SCYON Abstract

Received on December 9 2008

Shape parameters of Galactic open clusters

AuthorsN.V. Kharchenko (1,2,3), P. Berczik (1,2), M.I. Petrov (2,4), A.E. Piskunov (1,3,5), S. Röser (1), E. Schilbach (1), and R.-D. Scholz (3)
Affiliation(1) Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg (ZAH), Mönchhofstrasse 12-14, D-69120 Heidelberg, Germany
(2) Main Astronomical Observatory, 27 Academica Zabolotnogo Str., 03680 Kiev, Ukraine
(3) Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany
(4) Institut für Astronomie der Universität Wien, Türkenschanzstrasse 17, A-1180 Wien, Austria
(5) Institute of Astronomy of the Russian Acad. Sci., 48 Pyatnitskaya Str., 109017 Moscow, Russia
Accepted byAstronomy & Astrophysics
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Abstract

There are only a few tens of open clusters for which ellipticities have been determined in the past. In this paper we derive observed and modelled shape parameters (apparent ellipticity and orientation of the ellipse) of 650 Galactic open clusters identified in the ASCC catalogue. We provide the observed shape parameters of Galactic open clusters, computed with the help of 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 the specially developed φGRAPE code providing models of clusters of different initial masses, Galactocentric distances and rotation velocities. The comparison of models and observations of about 150 clusters reveals ellipticities of observed clusters which are too low (0.2 vs. 0.3), and offers the basis to find the main reason for this discrepancy. The models predict that after ≈50 Myr clusters reach an oblate shape with an axes ratio of 1.65:1.35:1, and with the major axis tilted by an angle of qXY ≈ 30o with respect to the Galactocentric radius due to differential rotation of the Galaxy. Unbiased estimates of cluster shape parameters requires reliable membership determination in large cluster areas up to 2-3 tidal radii where the density of cluster stars is considerably lower than the background. Although dynamically bound stars outside the tidal radius contribute insignificantly to the cluster mass, 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, though deep surveys allow to identify more cluster members and, therefore, to increase the accuracy of the observed shape parameters.