3. The cluster
3.1. Preliminary considerations
NGC 6994 is a poorly known object. It is classified as a Trumpler (1930) IV 1 p cluster, say a poorly populated (4 stars?) and compact system. This classification however is doubtful. Many observers detected this stars concentration, which was baptized also as M 73, Cr 426, C 2056-128, and OCL 89 (Ruprecht 1966). Its angular diameter is estimated to range between 1 to 2.8 arcmin, which means that it should be a rather compact cluster (Collinder (1931) suggested it is a globular).
It seems that all these suggestions converge to the conclusion that we have to consider NGC 6994 as a group of four bright stars shown in Fig. 1.
3.2. The structure
NGC 6994 is a cluster located at relatively high galactic latitude (). It appears as a concentration of four bright stars in the DSS image presented in Fig. 1, although the barycenter of these stars is offset with respect to the commonly reported cluster center, which corresponds to the center of the image. These four stars are rather close, justifying the small diameter reported by Lyngå (1987), who presumably referred to these stars when describing NGC 6994 properties. In details, the two brighter stars are HD 358033 and GSC 05778-0082, and their parameters are listed in Table 1, where magnitudes (B and V), distance and proper motions are from Tycho catalogue (Hog et al. 1998). Typical errors affecting these magnitudes are 0.3-0.4 mag. These two stars are well inside the solar neighborhood, and lie 135 and 42 pc from the Sun, respectively. M 73 according to Simbad is a double or multiple star, but most probably represents the whole asterism, and it could be considered as a duplicate name for NGC 6994. Moreover we noticed that the faintest star in the asterism might be a visual binary, or simply the two stars are projected in the same sky direction. It is however rather unlikely to imagine that this binary system actually represents M 73. Apart from these four stars, no many other stars are visible close to the cluster center which could justify the classification of this object as an open cluster. Indeed the surrounding field (see Fig. 1) appears very smoothly populated. Looking at the cluster structure it is reasonable to suggest that these four stars are responsible for the by eye identification of this aggregate as an open cluster.
Table 1. Basic parameters of the two brighter stars in NGC 6994 region. Magnitudes, proper motions and trigonometric parallax are from the Tycho catalogue.
3.3. The CMD
The measured stars in the plane V versus and V versus are shown in Fig. 2 (left and right panel, respectively), and represent a region somewhat smaller ( arcmin2) than that shown in Fig. 1. They define a broad vertical sequence which gets wider at increasing magnitude. The distribution of stars does not exhibit any distinguishing feature, but resembles a typical field stars CMD. The straightforward conclusion is that NGC 6994 is not an open cluster, but the stars in this region define a rather smooth field population, with some voids and less rich in stars than the typical galactic disk fields. We must stress that we are looking at a relatively high galactic latitude, where the thickness of the disk is rather small.
3.4. Is NGC 6994 an open cluster remnant?
de la Fuente Marcos (1998) studied the dynamical evolution of open star clusters, suggesting that the final stage of their evolution consists of a handful of stars which emerges from the general galactic field. The life-time, number of remaining stars and dimension of the OCR depends on the initial cluster size and on the distance to the galactic center. As for NGC 6994, we find that there are 11 stars which significantly emerge from the field (see Fig. 2). This would mean that NGC 6994 was a rich open cluster with an initial population of about 700 stars, and an age of almost a billion yrs, or a younger (half a billion yrs) initially less rich (250 stars) open cluster if the binary population was significant (). Since NGC 6994 does not have bright stars (the two brightest ones are probably dwarfs), the most plausible conclusion would be that it was initially a rich populated cluster, with an age of 1 Gyr or more. There are however not enough arguments leading to this scenario. In fact the lack of any feature in the CMD is a strong argument against the classification of this object as an open cluster. Indeed moving groups and OCR actually maintain in time some structures resembling a cluster CMD (see de la Fuente Marcos 1998, Fig. 3).
The most reasonable conclusion is that we are looking at the general galactic field, with stars at any distance from the Sun.
© European Southern Observatory (ESO) 2000
Online publication: May 3, 2000