Assigned ID: 347 ----- Attempting Submission 1 (Fri Feb 25 12:29:58 GMT 2011) ----- ----- Attempting Submission 1 (Thu Jul 07 14:08:44 GMT 2011) ----- COSMIC-LAB: Double BSS sequences as signatures of the Core Collapse phenomenon in star clusters. Globular Clusters (GCs) are old stellar systems tracing key stages of the star formation and chemical enrichment history of the early Universe and the galaxy assembly phase. As part of a project (COSMIC-LAB) aimed at using GCs as natural laboratories to study the complex interplay between dynamics and stellar evolution, here we present a proposal dealing with the role of Blue Straggler Stars (BSS). BSS are core-hydrogen burning stars more massive than the main-sequence turnoff population. The canonical scenarios for BSS formation are either the mass transfer between binary companions, or stellar mergers induced by collisions. We have recently discovered two distinct and parallel sequences of BSS in the core of M30 (Ferraro et al. 2009, Nature 462, 1082). We suggested that each of the two sequences is populated by BSS formed by one of the two processes, both triggered by the cluster core collapse, that, based on the observed BSS properties, must have occurred ~1-2 Gyr ago. Following this scenario, we have identified a powerful "clock" to date the occurrence of this key event in the GC history. Here we propose to secure WFC3 images of 4 post-core collapse GCs, reaching S/N=200 at the BSS magnitude level, in order to determine the ubiquity of the BSS double sequence and calibrate the "dynamical clock". This requires very high spatial resolution and very high precision photometry capabilities that are unique to the HST. The modest amount of requested time will have a deep impact on the current and future generations of dynamical evolutionary models of collisional stellar systems.
Observations are organized in 4 visits (one for each of the selected clusters). Dithering is performed by applying small offsets (via POS TARG keyword) to each exposure. VISIT 1 - target: NGC6681 - 5 orbits - a total of 35 exposures in the selected filters (F390W,F555W,F814W) are planned. A few (5) parallel exposures are planned at the end of each orbit in order to not affect the execution of the primary observations. VISIT 2 - target: NGC6541 - 5 orbits - a total of 35 exposures in the selected filters (F390W,F555W,F814W) are planned. A few (5) parallel exposures are planned at the very end of each orbit in order to not affect the execution of the primary observations. VISIT 3 - target: NGC362 - 5 orbits - a total of 39 exposures in the selected filters (F390W,F555W,F814W) are planned. A few (5) parallel exposures are planned at the very end of each orbit in order to not affect the execution of the primary observations. VISIT 4 - target: NGC6293 - 6 orbits - a total of 24 exposures in the selected filters (F390W,F555W,F814W) are planned. A few (9) parallel exposures are planned in each orbit. HOT STARS 12 false 0.0 0.0 false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER J2000 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 J2000 STELLAR CLUSTER GLOBULAR CLUSTER J2000 J2000 WFC3/UVIS Imaging No No F390W WFC3/UVIS Imaging No No F555W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F390W WFC3/UVIS Imaging No No F555W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F390W WFC3/UVIS Imaging No No F555W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F390W WFC3/UVIS Imaging No No F555W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F814W The scheduling data was generated by CASM 19.3. 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