Discovery of a Double Blue Straggler Sequence in M15: New Insight into the Core-collapse Process

Abstract

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.

The “dynamical clock”: dating the internal dynamical evolution of star clusters with Blue Straggler Stars

“…M15: another surprise -By applying an advanced photometric de-blending technique to a set of highresolution images, Beccari et al (2019) discovered an even more peculiar double BSS sequence in the innermost regions of the PCC cluster M15 (see Fig. 12).…”

An empirical clock to measure the dynamical age of star clusters

“…In fact two branches are distinguishable: the first branch appears extremely narrow, it extends up to 2.5 mag 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 mag from the MS-TO and it is reproduced by a 5.5-Gyr old collisional isochrone Beccari et al (2019) interpreted these features as the observational signature of two major collisional episodes suffered by M15, likely connected to the collapse of its core: the first one (possibly tracing the beginning of the CC process) occurred approximately 5.5 Gyr ago, while the most recent one (possibly associated with a core oscillation in the post-CC evolution) dates back to ∼2 Gyr ago. This scenario is consistent with results from Monte Carlo simulations.…”

“…Recently Beccari et al (2019) discovered a double BSS sequence in the center of the PCC cluster M15 (see the central panel in Fig. 3).…”

“…(1) mass-transfer (MT) in binary systems McCrea (1964) possibly up to the complete coalescence of the two stars, and (2) direct stellar collisions (COLL; Hills & Day (1976)). The efficiency of both these processes depends on the local environment (Fusi Pecci et al (1992); Ferraro et al (1999Ferraro et al ( , 2006a; Davies et al (2004); Dalessandro et al (2008b); Sollima et al (2008); Knigge et al (2009)), and they can act simultaneously within the same cluster (Ferraro et al (2009); Xin et al (2015); Dalessandro et al (2013a); Beccari et al (2019)).…”

“…The thickline is the 2 Gyr collisional isochrone fromSills et al (2009). Central Panel: The double BSS sequence in M15 (fromBeccari et al (2019). Two branches are apparent along the blue sequence they are reproduced by a 2 Gyr-old and a 5.5 Gyr-old collisional isochrone, respectively.Right Panel: The evolution in time of the 1% Lagrangian radius (top panel) and of the collisional parameter (bottom panel).…”