  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). We
performed a detailed photometric analysis of the extremely dense core of the
cluster using a set of images secured with the Advanced Camera for Survey in
the High Resolution Channel mode on-board the Hubble Space Telescope. The
proper 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 Point
Spread Function and an accurate deblending procedure. The Color-Magnitude
diagram revealed the presence of two distinct parallel sequences of blue
stragglers. In particular, the blue BSS sequence is characterized by the
intriguing presence of two different branches. The first branch appears
extremely narrow, it extends up to 2.5 magnitudes brighter than the cluster
main-sequence turnoff (MS-TO) point, and it is nicely reproduced by a 2 Gyr-old
collisional isochrone. The second branch extends up to 1.5 magnitudes from the
MS-TO and it is reproduced by a 5.5 Gyr-old collisional isochrone. Our
observations suggest that each of these branches is mainly constituted by a
population of nearly coeval collisional BSS of different masses generated
during two episodes of high collisional activity. We discuss the possibility
that the oldest episode corresponds to the core-collapse event (occurred about
5.5 Gyr ago), while the most recent one (occurred about 2 Gyr ago) is
associated with a core oscillation in the post-core collapse evolution. The
discovery of these features provides further strong evidence in support of the
connection between the BSS properties and GC dynamical evolution, and it opens
new perspectives on the study of core-collapse and post core-collapse
evolution.
