Spatial structure of the globular cluster M15

Abstract

The spatial structure of the metal-poor globular cluster M15 is studied as a function of magnitude interval ΔB and the limiting B magnitude of star counts. Astrometric and photometric measurements of two plates obtained with the 2-m reflector of the National Academy of Sciences of Bulgaria were used. Analysis of the differential and integrated apparent (ΔF(r) and F(r)) and spatial (Δf(r) and f(r)) stellar density distributions in different ΔB intervals and to different limiting magnitudes reaching B=21.5m indicates that the cluster structure changes systematically as we consider fainter stars, beginning with the transition region between the subgiant branch and the main-sequence turnoff in the (V, B-V) diagram. This variation is manifest in two ways: a homological growth in the radii of spatial zones of the cluster and of the cluster radius in accordance with a single law, and variations in the stellar density, with the rate of increase in the number of faint stars growing toward the outer zones of the cluster. Empirical relations describing these variations and parameters determining the cluster structure are obtained.