Horizontal-branch morphology and dense environments. Hubble Space Telescope observations of globular clusters NGC 2298, 5897, 6535, and 6626

HST observations of the core of four galactic globular clusters allowed us to obtain interesting clues about the influence of the environment on the star distribution along the horizontal branch. The chemical composition and the age of the target clusters have been derived through a detailed analysis of the (V, V-I) color-magnitude diagrams. The photometric metallicities have been found in close agreement with the recent spectroscopic determinations. The four clusters are coeval within the uncertainties. Hence, age and metallicity cannot account for the difference in the horizontal-branch morphologies, as shown by a comparison with theoretical zero-age horizontal-branch sequences and synthetic horizontal branches taken from the literature. Our analysis further supports the hypothesis according to which dense environments cause the occurrence of the so-called "blue tails" in the horizontal branches of galactic globular clusters. The use of HST, as well as of ground-based data for some clusters, enabled us to study the radial distribution of the various evolved populations, starting from the very center. We thus discovered that blue stragglers are, in general, more concentrated while the horizontal and the red giant branch do not display a markedly different trend with the distance from the cluster center, although the red giant branch of NGC 6626 is more concentrated than the horizontal branch.