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Astron. Astrophys. 358, 886-896 (2000)

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2. Compiled catalogue: Cluster structure and stellar content

In Fig. 1 we show the [FORMULA]-plane of NGC 6611 with stars from the CC and the main structure elements of the cluster (the core and corona) as derived in Paper I. In order to get an idea on the patchy distribution of interstellar matter across the cluster surface, we also plotted the isophote outlining the [FORMULA] emission nebulosity as seen on a blue NGPOSS plate (taken from Kamp 1974) and the absorption distribution [FORMULA] according to the absorption map given in Paper I.

[FIGURE] Fig. 1. Distribution of the CC stars in the NGC 6611 region. The coordinates are given with respect to Walker's star 125 ([FORMULA]. The lines divide the cluster area into azimuthal sectors denoted by the corresponding numbers. The thick solid curve marks the isophote of the bright nebula M 16 taken from Kamp (1974). A small circle (thin continuous curve) indicates the cluster core, whereas the cluster edge ([FORMULA]) is shown by a large circle (thin dashed line). The distribution of absorption [FORMULA] as derived in Paper I is illustrated by a grey-scaled color according to the scale plotted in the upper left corner. Areas where no individual data on absorption are available are indicated by white color.

The [FORMULA]-values displayed in Fig. 1 by grey color were determined from individual data on reddening [FORMULA] and total-to-selective absorption ratio [FORMULA] as described in Paper I. For stars in outer parts of the cluster region, no data are available for individual correction of magnitudes due to the absorption. In the following, for these stars we adopted the averaged values [FORMULA] and [FORMULA] estimated by Hillenbrand et al. (1993) for this cluster. This approach could lead to incorrect results in studies of properties of a particular star with an unknown individual extinction law. In the case of statistical investigations of stellar samples (such as the determination of luminosity or mass functions), we can expect, however, that a statistical bias introduced by the averaged reddening values will be of a considerably less significance. We tested various approaches for the treatment of the extinction law (e.g., the normal extinction with [FORMULA], the interpolation of the absorption map, the extinction parameters of the nearest neighbour and the interpolation between the individual values of the nearest neighbours), and we found that the resulting cluster parameters were robust to the extinction laws used, i.e. they either varied within estimated accuracy or did not change at all.

In Fig. 2 the radial density profiles of the distribution of stars from the CC are shown. There is a steady decrease of the total stellar density up to [FORMULA] (Fig. 2a) that is close to the edge of the cluster corona [FORMULA] estimated in Paper I. Assuming the density of field stars to be the minimum density over the area covered the CC, we may conclude that at [FORMULA] no cluster members are virtually present, whereas in the corona there is a mixture of cluster and field stars (from the density relation between the outer region and corona, we can deduce that the contamination of the corona by field stars is more than 80%).

[FIGURE] Fig. 2a - g. Radial density profiles over the cluster area. Panel a shows the density profile for all CC stars (solid line), for the NW segment [FORMULA] (dashed line) and for the SE segment [FORMULA] (dotted line). Panels b through g show the density distribution in each sector. The derived size of the cluster core and corona are marked on the abscissa by small vertical lines. All curves are normalized to the number of the CC stars.

In general, the distribution of CC stars reveals a central symmetry. However, small differences can be seen between the profiles in different azimuthal sections (panels from 2b to 2g). The difference is most prominent between two cluster areas, the "North-West" (NW) and "South-East" (SE) areas (dashed and dotted curves in Fig. 2a), separated by a line


This can be attributed rather to local irregularities of the absorption distribution (cf. Fig. 1) than to a real lack of stars in the NW part of the corona. In the outer areas of the NW corona, the stellar density is lower than in a surrounding field (especially, in the sector [FORMULA]). It appears as if the cluster is still embedded in the interstellar dust in its NW part, whereas in the SE part the cluster is already cleared off by a bubble of the hot gas expanding from the center.

Assuming that 1) the majority of stars observed in the core belongs to the cluster, 2) the sample of stars projected on the corona presents a mixture of cluster members and field stars, and 3) cluster members are practically absent outside the corona, we may expect to see variations in the distribution of stars with apparent magnitude as a function of angular distance from the cluster centre.

The brightness function (BF) based on the CC data i.e., the distribution of the catalogue stars with apparent V magnitude, is shown in Fig. 3a. From the comparison of the BFs for the core and outermost areas covered by the CC, we can conclude that the slope of the core BF differs drastically from the slope of the outer BF which in turn is close to the slope of the BF of all stars included in the CC. In Fig. 3b we show slopes of the BFs derived in concentric rings around the cluster center. According to this plot, we can again discriminate three different groups of stars: the core population dominated by cluster members, the corona population consisting of cluster and field stars, and the "external" population (stars outside the corona border) which is practically free of cluster members.

[FIGURE] Fig. 3a and b. Histograms of brightness distribution (Panel a ) are plotted for all stars from the CC (top), for the outermost area [FORMULA] (middle), and for the cluster core [FORMULA] (bottom). For an easier comparison, the histograms are arbitrary shifted along the ordinate and approximated by lines. The corresponding slopes s of the BFs are given in Panel b as function of radial distance from cluster center. The slope of the brightness distribution of all catalogue stars is [FORMULA]; that is indicated by the horizontal line in Panel b .

The following conclusions could be drawn from the first look on the CC data:

  • the cluster structure consisting of two components (core and corona) as proposed in Paper I is supported by the population analysis;

  • the distribution of the visual absorption in the cluster corona is highly inhomogeneous. In spite of general radial symmetry found for the cluster, there is some lack of stars as counted in the NW part of the cluster area;

  • most of the stars in the CC are field stars, but their fraction is decreasing from the outer areas to the cluster center.

Based on these results, we can conclude that besides the usual methods, additional efforts are required for a proper selection of cluster members in NGC 6611.

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Online publication: June 20, 2000