Blue Straggler Stars in the Unusual Globular Cluster NGC 63881

Abstract

We have used multiband high-resolution HST WFPC2 and ACS observationscombined with wide-field ground-based observations to study the bluestraggler star (BSS) population in the Galactic globular cluster NGC6388. As in several other clusters we have studied, the BSS distributionis found to be bimodal: highly peaked in the cluster center, rapidlydecreasing at intermediate radii, and rising again at larger radii. Inother clusters the sparsely populated intermediate-radius region (or''zone of avoidance'') corresponds well to that part of the cluster wheredynamical friction would have caused the more massive BSSs or theirbinary progenitors to settle to the cluster center. Instead, in NGC6388, BSSs still populate a region that should have been cleaned out bydynamical friction effects, thus suggesting that dynamical friction issomehow less efficient than expected. As a by-product of theseobservations, the peculiar morphology of the horizontal branch (HB) isalso confirmed. In particular, within the (very extended) blue portionof the HB we are able to clearly characterize three subpopulations:ordinary blue HB stars, extreme HB stars, and blue hook stars. Each ofthese populations has a radial distribution which is indistinguishablefrom normal cluster stars.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS5-26555. Also based on Wide Field Imagerobservations collected at the European Southern Observatory, La Silla,Chile.