Calibration of radii and masses of open clusters with a simulation

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstrasse 12–14, 69120 Heidelberg, Germany
e-mail: aernst@ari.uni-heidelberg.de
² National Astronomical Observatories of China, Chinese Academy of Sciences, Datun Lu 20A, Chaoyang District, Beijing 100012, PR China
³ Main Astronomical Observatory, National Academy of Sciences of Ukraine, Akademika Zabolotnoho 27, 03680 Kyiv, Ukraine
⁴ Institut für Astronomie der Universität Wien, Türkenschanzstraße 17, 1180 Wien, Austria

Received: 30 April 2010
Accepted: 1 September 2010

Abstract

Context. Piskunov and collaborators developed a method to make a simple mass estimate for tidally limited star clusters based on the identification of the tidal radius in a King profile with the dynamical Jacobi radius. The application of this method to an unbiased open cluster catalog yielded significantly higher cluster masses than the classical methods.

Aims. We quantify the bias in the mass determination as a function of projection direction and cluster age by analyzing a simulated star cluster.

Methods. We use direct N-body simulations of a star cluster in an analytic Milky Way potential that account for stellar evolution and apply a best fit to the projected number density of cluster stars.

Results. We obtain significantly overestimated star cluster masses that depend strongly on the viewing direction. The overestimation is typically in the range of 10–50 percent and reaches a factor of 3.5 for young clusters. Mass segregation reduces the derived limiting radii systematically.