4 Radius of the cluster

We used radial stellar density profile for the determination of cluster radius which is always a difficult task as

(a)

it depends on the limiting magnitude of the members. The more fainter stars one considers, the larger becomes the cluster radius, since such stars occupy larger distances from the cluster center due to two body relaxation;

(b)

the level of surrounding field-stars determines the point where the outer or fainter stars merge with the field, so the radius depends on the density of the background;

(c)

the number of member stars diminishes with increasing distance from the cluster center, and it becomes hard to detect them in the statistical fluctuations of field-stars.

Figure 4: Plot of radial stellar density profiles determined from the compiled catalogue and DSS data having a pixel of size 0 $\hbox{$.\!\!^{\prime\prime}$ }$74 and 1 $\hbox{$.\!\!^{\prime\prime}$ }$7 respectively. Arrows denote the radius where the stellar density seems to merge with the field-star density

The radius at which star density flattens in the radial stellar density profile has been considered as cluster radius. This does not correspond either to the core radius or to the tidal radius but it depends mostly on the cluster richness and the field-star density. For determining star density profile, we used the data from DSS as well as from compiled catalogue. From DSS, we extracted an image of size $30' \times 30'$ in $1059 \times 1059$ pixel² size image frame. Each pixel covers 1 $\hbox{$.\!\!^{\prime\prime}$ }$7 $\times$ 1 $\hbox{$.\!\!^{\prime\prime}$ }7$ of the sky which is more than twice the pixel size of the CCD used by us. We adopted the same cluster center as Sandage (1958) did (star E). Figure 4 shows the plot of radial star density profile for both DSS and compiled catalogue data where arrow indicates the cluster radius. Both the data sets yield almost the same value. The cluster radius obtained in this way is 3 $\hbox{$.\mkern-4mu^\prime$ }7\,\pm\,$0.2, which is larger than the value of 2 $\hbox {$.\mkern -4mu^\prime $ }$5 given in Lyngå (1987) catalogue. The radial stellar density profile based on DSS data shows that the cluster may not have many members in the coronal region i.e. at radial distance of more than 1.5 cluster radius. However, deep proper motion and radial velocity measurements are desired to identify them.