Blue Stragglers in the Galactic globular clusters M3: Evidence for two populations

Abstract

A new VI-CCD survey of the central regions of the Galactic globular cluster M3 has been made in order to search for Blue Straggler Stars (BSS). About 9000 stars within r less than 250 sec from the center have been measured down to the turnoff level, yielding about 70 new BSS candidates. The analysis of a complete sample of bright BSS (B less than 18.6), identified in this study and in previous surveys, clearly shows that the radial distribution of the BSS is nonmonotonic. There is a distant dip in the distribution between 4r<SUB>c</SUB>(approximately 100 sec) and 8r<SUB>c</SUB>(approximately 200 sec). Taken at face value, this result is direct evidence that two distinct populations of BSS exist in M3 or, alternatively, that radially selective effects have been at work in the survival mechanisms or in the BSS dynamics within the cluster. The possible relationship betweeen this result and the BSS population in other clusters is also briefly discussed.