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Astron. Astrophys. 357, 145-148 (2000)

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1. Introduction

The disk of the Milky Way harbors about 1200 open clusters. The youngest ones trace the spiral pattern in the disk, whereas the oldest ones are ideal templates to study the chemical and dynamical evolution of the disk, together with the Star Formation History (Friel 1995).

Open star clusters are usually studied by analyzing the Color Magnitude Diagram (CMD, Chiosi et al. 1992).

Despite the importance of these objects for our understanding of the disk properties, CMDs are at present available only for a small fraction of the cluster population, say 40[FORMULA]. However the situation is rapidly changing thanks to dedicated surveys like for instance the PLCON (Palomar Las Campanas Observatory NOAO) Open Clusters Survey (Phelps 1999).

One of the crucial problem with open clusters is that they are highly contaminated by field stars in the disk, so that it is rather difficult to obtain precise estimates of their fundamental parameters.

This way many clusters remain completely unstudied but for the identification, which in most cases is done on a by eye basis, by inspecting at several different sky charts.

In some cases the identification is made by recognizing a peak of star concentration in the field. This is naturally a good criterion, which however deserves further studies to confirm the real nature of the star enhancement.

This is not only a semantic question. A star cluster is doubtless a star enhancement in the field.

Nonetheless the real nature can be unraveled with higher degree of confidence by looking at the CMD, and recognizing typical features, like a Main Sequence of H-burning stars, a Red Giant Branch, and/or a clump of He-burning stars (Chiosi et al. 1992).

For instance Carraro & Patat (1995) analyzed photometry for the presumed old cluster Ruprecht 46, and concluded that it is not a cluster, but only a random enhancement of bright stars.

Recently, de la Fuente Marcos (1998) has performed numerical simulations of open clusters evolution, suggesting that many star concentrations in the sky may be OCRs. The basic criterion is the evidence of a star concentration higher than the field stars. So in principle an enhancement of stars could be just the final stage of the evolution of an open cluster.

In this paper we present B, V and I photometry for NGC 6994, an object which appears in the Lyngå (1987) open clusters catalogue and that it is classified as open cluster also by SIMBAD.

The aim is to provide some photometric data for this cluster, complementing the very poor information we have, which basically consists of the cluster identification and diameter, which according to Lyngå (1987) is about 1.0 arcmin. Its coordinates for the 2000.0 equinox are: [FORMULA], [FORMULA], [FORMULA], [FORMULA].

NGC 6994 is expected to lie close to HD 358033, GSC 05778-0082, and M 73. This latter might be a binary or multiple system according to SIMBAD, but more probably represents all the asterism.

From the acquired data, it turns out that NGC 6994 is not a cluster, but just a random enhancement of the four mentioned bright stars.

Sect. 2 is dedicated to a brief description of the data acquisition and reduction; Sect. 3 deals with the cluster structure and CMD. Finally Sect. 4 summarizes the results.

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000