Spatial structure of some globular and open star clusters: I. Results of the investigations

The article presents the general conclusions of our program for investigation of the deep spatial structure of seven globular (M56, M12, NGC6535, NGC6171, NGC5466, M92 and M15) and four open (Pleiades, Praesepe, Coma Berenices and NGC2420) clusters on our Galaxy. The investigation of the globular clusters is carried out on deep plates of the 2/16 m RCC telescope of the Rozhen National Astronomical Observatory within the field of view 60' x 60'. Open clusters are investigated by data from the literature. The spatial structure of different subsystems in the clusters which contain stars over, around and under the main sequence turnoff in the (V, B-V) diagram is investigated based on the analysis of the differential and integral functions of distribution of the surface (Delta F(r) and F(r) and spatial (Delta f (r) and f (r)) star density in the clusters obtained by the method of star counts carried out in different intervals Delta B and Delta V up to different limit B and V magnitudes. It is shown that: (1) Different spatial zones distinguished by their gradient of star density have been observed in the structure of all clusters (globular and open). (2) The spatial structure has in general a layered character where the different layers coincide with the observed spatial zones. (3) The spatial structure of all clusters varies systematically as we proceed to fainter stars. This variation starts from the brightest stars of the main sequence and consist of the following: a homologous increase of the radii of each spatial zone of the cluster and of the cluster as a whole in accordance with a uniform law, and changes in the stellar density distribution - the rate of increase in the number of faint stars with increase in star magnitude grows as we move outwards in the cluster. General parameters defining the structure of the cluster are introduced and empirical relationships describing the variation of the structure are derived.