The Blue Straggler Population of the Globular Cluster M5: Comparison with M3

Abstract

We have surveyed the blue straggler star population of the Galactic globular cluster M5 using high-resolution images of the core along with wide-field ground-based images reaching to more than 19 core radii. To gauge M5's relative efficiency of producing stragglers, we compared our sample to five studies of other globular clusters (mainly Ferraro et al. 1997b; Ferraro et al 2003; and Piotto et al. 2004). Using a "bright" sample selected in the same way as Ferraro et al. 1997b, we found a bimodal radial distribution similar to those found in three other luminous clusters. When the radial distributions for different clusters are scaled using the core radius, there is good cluster-to-cluster agreement in the size of the core straggler sample and the center of the "zone of avoidance". However, M5 has the smallest fraction of stragglers in the zone of avoidance of any of the clusters measured to date, and its zone of avoidance appears to be wider (in r / r_c) than that of M3, which has a very similar surface brightness profile. Both of these facts indicate that M5's straggler population has dynamically evolved to a larger extent than M3. Using an ultraviolet sample from Hubble Space Telescope selected in the same way as Ferraro et al. 2003 and Ferraro et al. 2004, we find that the frequency of blue stragglers in M5 is lower than all but two of the clusters examined. We also identified seven bright blue stragglers that were previously misidentified as HB stars by Sandquist et al. (2004). These bright stragglers are most likely the result of stellar collisions involving binary stars.