Blue Stragglers in the Gaia Era: Galactic Open and Globular Clusters
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Date
2023
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Abstract
Blue Straggler Stars (BSSs) are one of the keys to understand in a better way how
stars evolve and interact with each other in different star clusters. They appear
to be hotter and brighter than the Main Sequence Turn Off Point (MSTO) and,
therefore, they should have evolved off the Main-Sequence branch. We properly
select, for Open Clusters (OCs) and Globular Clusters (GCs), star cluster members based on proper motions and parallaxes provided by Gaia Data Release 3
(DR3). Using isochrones models and selection criteria from previous studies, we
select and classify stars as BSS, Yellow Straggler Stars and Red Stragglers Stars
(evolved BSSs). We identify BSSs to be present in all our GCs and in 42/129
(∼ 33 %) of studied OCs. We counted a total of 4399 BSSs; 434 (∼ 10 %) located in OCs and 3965 (90 %) located in GCs. Clusters younger than ∼ 500 Myr
do not show the presence of BSSs in our sample. We obtain astrophysical parameters from 3 different methods (color–temperature relations, isochrone–fitting
models and parameters from Gaia DR3 spectra) such as the effective temperature
Teff, star mass M, and surface gravity log(g). We find values for BSS Teff to be
∼ (6800 ± 585) K in GCs and ∼ (7570 ± 1400) K in OCs; and an average mass
of ⟨MBSS⟩ = (1.75 ± 0.45) M⊙ in OCs and ⟨MBSS⟩ = (1.02 ± 0.1) M⊙ in GCs.
For every BSS, we compute the difference of the BSS mass and the MSTO mass
of its parent cluster, normalized by the MSTO mass, and called it Me based on
previous studies. This parameter is classified as low–Me (Me < 0.5, BSS likely
formed through mass-transfer) and high–Me (0.5 < Me < 1.0, BSS likely formed
through mergers). For OCs we find a percentage 81.34 % high–Me and 18.66 %
low–Me. For GCs, we find 94.25 % low–Me and 5.25 % high–Me. Comparing Me
against the BSS age obtained with isochrone modes, we were able to detect: i)
GC BSSs that are most likely to be formed through collisions show a “boost” in
their percentage/fraction for stars with an age ∼ 1 − 2 Gyr, in agreement with
reported age for core-collapse events in GCs found in previous studies; ii) a double sequence for GC BSSs, where we conclude that these sequences correspond to
a pre merger/closer-binary interaction and post merger/closer-binary interaction
of BSS formation
Description
Tesis (Master in Astrophysics)--Pontificia Universidad Católica de Chile, 2023