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

Discovery of a double Blue Straggler sequence in M15: new insight into the core-collapse process
Discovery of a double Blue Straggler sequence in M15: new insight into the core-collapse process
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.
astro-ph.SR, astro-ph.GA
0004-637X
Beccari, Giacomo
8bd0eff1-764f-417a-8e51-0ae8a3bf2aad
Ferraro, Francesco R.
299ea65b-562d-4a88-9ad6-f51378497e60
Dalessandro, Emanuele
25664aca-7c0e-4048-80ed-e7927192c9d3
Lanzoni, Barbara
0058bcaa-c4aa-47a7-8b1c-eac00986362e
Raso, Silvia
8fb61ad1-285e-4ca6-93b5-c55152a037e7
Origlia, Livia
e623abf8-d5d2-419d-8abe-687ee3694c6f
Vesperini, Enrico
18776122-d88f-4648-9338-ae8b48f0cfb0
Hong, Jongsuk
a4e3f6f8-8c1d-4eb3-b69f-2a06921ae2ac
Sills, Alison
2f7067f6-89a0-4738-b60e-a25e8b7a434d
Dieball, Andrea
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Knigge, Christian
ac320eec-631a-426e-b2db-717c8bf7857e
Beccari, Giacomo
8bd0eff1-764f-417a-8e51-0ae8a3bf2aad
Ferraro, Francesco R.
299ea65b-562d-4a88-9ad6-f51378497e60
Dalessandro, Emanuele
25664aca-7c0e-4048-80ed-e7927192c9d3
Lanzoni, Barbara
0058bcaa-c4aa-47a7-8b1c-eac00986362e
Raso, Silvia
8fb61ad1-285e-4ca6-93b5-c55152a037e7
Origlia, Livia
e623abf8-d5d2-419d-8abe-687ee3694c6f
Vesperini, Enrico
18776122-d88f-4648-9338-ae8b48f0cfb0
Hong, Jongsuk
a4e3f6f8-8c1d-4eb3-b69f-2a06921ae2ac
Sills, Alison
2f7067f6-89a0-4738-b60e-a25e8b7a434d
Dieball, Andrea
5f595453-aa6b-4067-a3e1-026867fb9f7a
Knigge, Christian
ac320eec-631a-426e-b2db-717c8bf7857e

Beccari, Giacomo, Ferraro, Francesco R., Dalessandro, Emanuele, Lanzoni, Barbara, Raso, Silvia, Origlia, Livia, Vesperini, Enrico, Hong, Jongsuk, Sills, Alison, Dieball, Andrea and Knigge, Christian (2019) Discovery of a double Blue Straggler sequence in M15: new insight into the core-collapse process. The Astrophysical Journal, 876 (1), [87]. (doi:10.3847/1538-4357/ab13a4).

Record type: Article

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.

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Discovery of a Double Blue Straggler Sequence in M15: New Insight into the Core-collapse Process - Accepted Manuscript
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Accepted/In Press date: 25 March 2019
e-pub ahead of print date: 7 May 2019
Published date: 7 May 2019
Keywords: astro-ph.SR, astro-ph.GA

Identifiers

Local EPrints ID: 431671
URI: http://eprints.soton.ac.uk/id/eprint/431671
ISSN: 0004-637X
PURE UUID: cd387e93-da6f-4dfb-a211-2f74bea522a2

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Date deposited: 12 Jun 2019 16:30
Last modified: 21 Nov 2021 16:46

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Contributors

Author: Giacomo Beccari
Author: Francesco R. Ferraro
Author: Emanuele Dalessandro
Author: Barbara Lanzoni
Author: Silvia Raso
Author: Livia Origlia
Author: Enrico Vesperini
Author: Jongsuk Hong
Author: Alison Sills
Author: Andrea Dieball

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