The pure noncollisional blue straggler population in the giant stellar system ω Centauri

We have used high spatial resolution data from the Hubble Space Telescope (HST) and wide-field ground-based observations to search for blue straggler stars (BSSs) over the entire radial extent of the large stellar system omega Centauri. We have detected the largest population of BSSs ever observed in any stellar system. Even though the sample is restricted to the brightest portion of the BSS sequence, more than 300 candidates have been identified. BSSs are thought to be produced by the evolution of binary systems (formed either by stellar collisions or mass exchange in binary stars). Since systems like Galactic globular clusters (GGCs) and omega Cen evolve dynamically on timescales significantly shorter than their ages, binaries should have settled toward the center, showing a more concentrated radial distribution than the ordinary, less massive single stars. Indeed, in all GGCs that have been surveyed for BSSs, the BSS distribution is peaked at the center. Conversely, in omega Cen we find that the BSSs share the same radial distribution as the adopted reference populations. This is the cleanest evidence ever found that such a stellar system is not fully relaxed even in the central region. We further argue that the absence of central concentration in the BSS distribution rules out a collisional origin. Thus, the omega Cen BSSs are the purest and largest population of noncollisional BSSs ever observed. Our results allow the first empirical quantitative estimate of the production rate of BSSs via this channel. BSSs in ! Cen may represent the best local template for modeling the BSS populations in distant galaxies where they cannot be individually observed.