Abstract. In order to study the relation between the core and corona in galactic star clusters, the spatial structure of 38 rich open star clusters has been studied using radial density profiles derived from the photometric data of the Digital Sky Survey. The shape of the radial density profile indicates that the corona, most probably, is the outer region around the cluster. It can exist from the very beginning of the cluster formation and dynamical evolution is not the reason for its occurrence. The study does not find any relation between cluster size and age but indicates that the clusters with galacto-centric distances >9.5 kpc have larger sizes. Further, we find that the average value of the core radius is 1.3 ± 0.7 pc and that of annular width of the corona is 5.6 ± 1.9 pc, while average values of densities of cluster members in the core and corona are 15.4 ± 9.9 star/pc 2 and 1.6 ± 0.99 star/pc 2 respectively. Average field star contaminations in the core and corona are ∼35% and 80% respectively. In spite of smaller densities in the coronal region, it contains ∼75% of the cluster members due to its larger area in comparison to the core region. This clearly demonstrates the importance of the coronal region in studies dealing with the entire stellar contents of open star clusters as well as their dynamical evolution. In contrast to the cluster cores, the structure of coronal regions differs significantly from one cluster to other.

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Abstract. CCD UBV IC photometry in a wide field around the open cluster NGC 7654 has been carried out for ∼17 860 stars, down to V ∼ 20. The reddening across the cluster region is found to be variable with E(B −V )min = 0.46 to E(B − V )max = 0.80. The cluster is situated at a distance of 1380 ± 70 pc. The colour magnitude diagrams show a large age spread in the ages. Star formation was biased towards relatively higher masses during the early phase of star formation whereas most of the low mass stars of the cluster were formed during the later phase. The star formation seems to have been a gradual process that proceeded sequentially in mass and terminated with the formation of most massive stars. The present data do not support a uniform mass function (MF) for different regions in the cluster. Although for the whole cluster region, the MF in the observed mass range 0.8 ≤ M ≤ 4.5, can be represented by a single power law with a slope Γ = −1.40 ± 0.07, however it indicates various features when examined carefully. In three subregions of the cluster the slope Γ, for the mass range ∼1.5 < M < 4.0, comes out to be −1.07 ± 0.08 and −1.28 ± 0.20 for the inner and intermediate regions and it becomes steeper in the outermost region (Γ = −2.78 ± 0.21). For stars having masses <∼1.5 M the MF, in the inner and intermediate regions, can be represented by a power law having a steeper slope, whereas in the outer region a turnover can be seen in the MF at M ∼ 1.5 M . The age of NGC 7654 is found to be comparable to its two-body relaxation time-scale; therefore this may also be a reason for the observed mass segregation in the cluster.

Abstract. The CCD observations of the rich open star cluster NGC 2099 and its surrounding field region have been carried out up to a limiting magnitude of V ∼ 22 mag in B, V and I passbands for the first time. A total of ∼12 000 stars have been observed in the area of about 24 × 34 in the cluster region, as well as ∼2180 stars in the ∼12 × 12 area of the field region located ∼45 away from the cluster center. The cluster parameters determined by fitting the convective core overshoot isochrones in the V, (B − V ) and V, (V − I) diagrams are E(B − V ) = 0.30 ± 0.04 mag, distance = 1360 ± 100 pc, age = 400 Myr and metallicity Z = 0.008. A welldefined cluster main sequence spread over about 8 mag in range is observed for the first time. Its intrinsic spread amounting to ∼0.06 mag in colour is almost the same over the entire brightness and can be understood in terms of the presence of physical/optical binaries. The core and cluster radii determined from the radial stellar density profiles are 185 and 1000 respectively. Only about 22% of cluster members are present in the core region. The effects of mass segregation, most probably due to dynamical evolution, have been observed in the cluster. The mass function slope of the entire cluster is ∼−0.67 ± 0.12. It becomes closer to the Salpeter value of −1.35, if flattening in the cluster mass function due to presence of both binaries and a much more extended corona is considered.

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