0 Assigned ID: 235 ----- Attempting Submission 1 (Thu Mar 05 15:59:21 GMT 2020) ----- 0 ----- Attempting Submission 1 (Tue Jun 30 14:04:27 GMT 2020) ----- A new reading of the core size-age distribution of LMC clusters: changing the paradigm of star cluster formation The distribution of core radius (rc) versus chronological age for the globular clusters (GCs) in the Large Magellanic Cloud (LMC) is very peculiar: old GCs show a large spread in rc, while young clusters are all compact. The origin of such a distribution is largely debated and mostly interpreted in terms of an evolutionary sequence: all GCs formed with compact cores (rc<3 pc), then most of them maintained small cores, while several others experienced core expansion due to the heating action of binary black holes. This scenario is becoming the new paradigm of GC formation and evolution. However a new and more natural interpretation could exist: the spread in rc observed at fixed chronological age could be due to different levels of dynamical evolution (dynamical ages), with large-core GCs being dynamically-younger than those with small rc. This alternative scenario can now be tested since the level of dynamical evolution reached by a star cluster can be empirically measured from the level of central segregation of its Blue Straggler Star population. Here we propose deep WFC3 observations of 3 of the most compact (rc<0.8 pc), old, and still unexplored GCs in the LMC, to determine their dynamical age and definitely confirm this scenario. The scientific return of the project is huge: in fact, the correct interpretation of the rc-age distribution would not only shed light on the correct paradigm for GC formation, but would also raise new fundamental questions concerning the processes responsible for the observed differences between the structural properties of old and young LMC GCs. Because of the legacy value of the proposed observations, we waive the proprietary period. The aim of the program is to measure the level of radial segregation of Blue Straggler Stars (BSSs) in 3 compact and old globular clusters (NGC1835, NGC1916, NGC1754) in the Large Magellanic Cloud, with the purpose of determining their dynamical age. A total of 12 primary orbits with the WFC3-UVIS are allocated to this program. To reach the goal, for each cluster we plan to secure 16 properly-dithered images in three filters: 6 images in filter F606W, 4 exposures in the F814W, and 6 in filter F300X. This guarantees to reach an adequate S/N ratio (>40) at the base of the BSS sequence. The exposures are organized in 2 visits per each cluster. Each visit is 2-orbits long. ACS/WFC Parallel observations through filters F606W and F814W (9 orbits in total, 3 per each clusters) are also planned to sample the LMC field population for a proper statistical decontamination of the cluster CMD from field interlopers. The PI of the project (Prof. Ferraro) is an expert of stellar populations in star clusters and has led several HST observing campaigns with more than 300 orbits awarded over the last ten Cycles. He is currently member of the HST User Committee. The team has a large experience in performing high-precision stellar photometry in crowded environments with the current and the past instrumentation onboard the HST. With respect to the specific subject of this proposal (i.e. the study of Blue Straggler Stars and their role in tracing the parent cluster dynamical evolution), this group has already published a number of outstanding discoveries based on HST data: Ferraro et al. 2019, Nature Astronomy, 3, 1149 Ferraro et al, 2012, Nature, 492, 393 Ferraro et al., 2009, Nature, 462, 1028 Ferraro et al., 2018, ApJ, 860, 36 Ferraro et al., 2016, ApJ, 816, 70 Ferraro et al., 2015, ApJ, 807, L1 Sollima & Ferraro, 2019, MNRAS,483,1523 Dalessandro et al, 2019, A&A,621, 45 Dalessandro et al., 2013, ApJ,778,135 Lanzoni et al., 2016, ApJ, 833, L29 Lanzoni et al., 2019, ApJ, 887, 176 Beccari et al, 2019, ApJ, 876, 87 Raso et al., 2017, ApJ, 839, 64 Alessandrini et al., 2016, ApJ, ApJ,833, 252 Pallanca et al., 2017, ApJ, 845, 4 Pallanca et al, 2019, ApJ, 882, 159 Cadelano et al, 2019, ApJ, 875, 25 Cadelano et al., 2017, ApJ, 844, 53 Specific skills of the team: High quality photometry in crowded fields (Pallanca, Cadelano, Beccari), proper motion analysis (Dalessandro), N-body simulations and dynamical models (Lanzoni, Vesperini,Bhat), spectroscopic follow-up (Mucciarelli). Publications:More than 80 papers based on HST data and dealing with the study of stellar populations and dynamics in Globular Clusters have been published in the last 10 years by this team (see the ADS). 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