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In this paper we report on the discovery of a double blue straggler star(BSS) sequence in the core of the core-collapsed cluster M15 (NGC 7078). Weperformed a detailed photometric analysis of the extremely dense core of thecluster using a set of images secured with the Advanced Camera for Survey inthe High Resolution Channel mode on-board the Hubble Space Telescope. Theproper combination of the large number of single frames in the near-UV (F220W),and blue (F435W) filters allowed us to perform a superb modeling of the PointSpread Function and an accurate deblending procedure. The Color-Magnitudediagram revealed the presence of two distinct parallel sequences of bluestragglers. In particular, the blue BSS sequence is characterized by theintriguing presence of two different branches. The first branch appearsextremely narrow, it extends up to 2.5 magnitudes brighter than the clustermain-sequence turnoff (MS-TO) point, and it is nicely reproduced by a 2 Gyr-oldcollisional isochrone. The second branch extends up to 1.5 magnitudes from theMS-TO and it is reproduced by a 5.5 Gyr-old collisional isochrone. Ourobservations suggest that each of these branches is mainly constituted by apopulation of nearly coeval collisional BSS of different masses generatedduring two episodes of high collisional activity. We discuss the possibilitythat the oldest episode corresponds to the core-collapse event (occurred about5.5 Gyr ago), while the most recent one (occurred about 2 Gyr ago) isassociated with a core oscillation in the post-core collapse evolution. Thediscovery of these features provides further strong evidence in support of theconnection between the BSS properties and GC dynamical evolution, and it opensnew perspectives on the study of core-collapse and post core-collapseevolution.

Discovery of a double Blue Straggler sequence in M15: new insight into the core-collapse process