Stellar Populations and Variable Stars in the Core of the Globular Cluster M5

Abstract

We report the discovery of a variable blue straggler in the core of the globular cluster M5, based on a 12 hr-long series of images obtained with the Planetary Camera aboard the Hubble Space Telescope. In addition, we present the light curves of 28 previously unknown or poorly studied large-amplitude variable stars (all but one are RR Lyrae stars) in the cluster core. A (V, U - I) color-magnitude diagram shows 24 blue stragglers within 2 core radii of the cluster center. The blue straggler population is significantly more centrally concentrated than the horizontal-branch and red giant stars. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.