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Astron. Astrophys. 333, L27-L30 (1998)

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1. Introduction

Open clusters are known to disintegrate on a time scale which depends both on their membership and galactocentric distance. The final residue of an open cluster evolution is often called an open cluster remnant (hereafter OCR). The number of disrupted open clusters in our Galaxy is probably very large (Lodén & Rickman 1974).

Ambartsumian (1938) and Spitzer (1940) showed, from a theoretical point of view, that it is impossible for a star cluster to evaporate totally; furthermore, Spitzer pointed out two possible final destinies for a star cluster: evaporation provokes physical collisions between stars, or evaporation proceeds until a stable binary or higher multiplicity system is generated. For rich clusters new theoretical scenarios were found (see e.g. Heggie 1985 for a review) in which the gravitational collapse, predicted by Antonov (1962) and Lynden-Bell and Wood (1968), is arrested by some energetic mechanism: a central black hole, central binary systems or central mass loss. As regards numerical simulations, for systems with some 25 to 250 stars, von Hoerner (1960, 1963), Aarseth (1968) and van Albada (1968) suggested that the final outcome of cluster evolution is one or more tightly bound binaries (or even a hierarchical triple system). Van Albada pointed out several observational candidates ([FORMULA] Ori, ADS 12696, [FORMULA] Oph, 1 Cas, 8 Lac and 67 Oph) as being OCRs and Wielen (1975) indicated another one, the Ursa Major cluster (Collinder 285). Very recently (de la Fuente Marcos 1997b, and references therein), some attention has been paid to the final stages of the evolution of open clusters; this new numerical work suggests that OCRs are regions in the sky where the concentration of binaries (and multiples) is significantly greater than the mean for field stars.

The purpose of this Letter is to shed new light on this topic, by presenting and discussing recent results from simulations for the expected main features of the OCRs. On the basis of detailed analysis of numerical integrations, these computational results are of interest in order to plan future surveys and evaluate the usefulness of the known data on loose clusters.

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998