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 andwide-field ground-based observations to search for blue straggler stars (BSS)over the entire radial extent of the large stellar system omega Centauri. Wehave detected the largest population of BSS ever observed in any stellarsystem. Even though the sample is restricted to the brightest portion of theBSS sequence, more than 300 candidates have been identified. BSS are thought tobe produced by the evolution of binary systems (either formed by stellarcollisions or mass exchange in binary stars). Since systems like Galacticglobular clusters (GGC) and omega Cen evolve dynamically on time-scalessignificantly shorter than their ages, binaries should have settled toward thecenter, showing a more concentrated radial distribution than the ordinary, lessmassive single stars. Indeed, in all GGCs which have been surveyed for BSS, theBSS distribution is peaked at the center. Conversely, in omega Cen we find thatthe BSS share the same radial distribution as the adopted referencepopulations. This is the cleanest evidence ever found that such a stellarsystem is not fully relaxed even in the central region. We further argue thatthe absence of central concentration in the BSS distribution rules out acollisional origin. Thus, the omega Cen BSS are the purest and largestpopulation of non-collisional BSS ever observed. Our results allow the firstempirical quantitative estimate of the production rate of BSS via this channel.BSS in omega Cen may represent the best local template for modeling the BSSpopulations in distant galaxies where they cannot be individually observed.Comment: 21 pages, 6 figures, accepted for publication by Ap