This proposal is part of a coordinated project devoted to understand the interplay of globular cluster (GC) dynamics and the formation and evolution of blue straggler stars (BSS). By using a combination of HST and ground-based observations we are constructing complete BSS surveys in a sample of GCs; complete BSS surveys require mid-UV HST observations in the center and wide field CCD ground based observations under excellent seeing conditions of the exterior. Up to now only four clusters have been surveyed in this way and the results are surprising: in three GCs (M3, 47 Tuc, NGC 6752) we have discovered that the BSS radial distribution is bimodal, highly peaked in the cluster center, rapidly decreasing at intermediate radii and rising again at large radii (Ferraro et al. 1997, 2004, Sabbi et al. 2004), conversely BSS population in Omega Centauri does not show any signature of the segregation which would be expected for a class of objects arising from either stellar interactions or binarity (Ferraro et al. 2005). These observational facts are opening a new prospective in the study of the formation processes and evolution of BSS in GCs. By using extensive simulations, we demonstrated that the spatial distribution of BSS observed in 47 Tuc can be only reproduced if a sizable fraction of BSS is generated (via mass transfer in primordial binaries) in the peripheral region of the cluster (Mapelli et al 2004), thus excluding a purely collisional formation scenario. Here we propose mid-UV imaging of a few clusters suspected of harboring a large population of central BSS and a few known to have many BSS the external region. These are good candidates for determining accurate BSS radial distributions. The modest amount of time proposed here will go far to determine the ubiquity of BSS bimodality and to constrain models of dynamical evolution. Since we believe the proposed observations would be useful to the entire stellar community (for multifold purposes) we waive the propretary period.

This is a simple photometric proposal mainly aimed to perform F336W(U) and F255W(UV) observations in the core of 8 Galactic globular clusters in order to search for Blue Stragglers Stars, by using the WFPC2. Accordingly with the approved PCR 1026, 11 orbits have been allocated to this project. Observations are organized in 11 visits (1 orbit per visit) and are planned as follows: NGC 6838 (1 orbit - visit 1); NGC 362 (1 orbit - visit 2);NGC5466 (1 orbit - visit 3); NGC6809 (1 orbit - visit 4); NGC2808 (2 orbits - visits 5 & 6); NGC6656 (1 orbit - visit 7); NGC6864(2 orbits - visits 8 & 9); NGC7078 (2 orbits - visits 10 & 11). Special timing requests have been inserted for the three clusters in which the observations are organized in two orbits, in order to have the two visits executed consecutively: thus visit 6 will be excecuted after visit 5, visit 9 after visit 8 and visit 11 after visit 10. 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