Predictions of gravity mode pulsations of collisional blue straggler stars in globular clusters

TL;DR

This work investigates whether gravity-mode pulsations can unveil the formation history of blue straggler stars (BSSs) in globular clusters. The authors construct a low-metallicity grid of SSE models ($Z=0.01Z_\odot$) and collisional BSSs via Make Me A Star (MMAS), evolving them with MESA and computing adiabatic $\ell=1$ g-mode frequencies with GYRE to compare period-spacing patterns. They find that collision products, despite resembling SSE tracks in the HR diagram, harbor altered chemical stratification that introduces a secondary peak in the Brunt-Väisälä frequency and a characteristic, periodic modulation in the $g$-mode PSP, signatures that persist during MS evolution. These seismic fingerprints offer a novel route to constrain BSS formation channels in globular clusters and motivate future asteroseismic observations and extended modeling across metallicities and rotation.

Abstract

Blue straggler stars (BSSs) are exotic objects, which, being the results of processes such as mass transfer, mergers, or collisions, are considered key objects in the study of their host clusters' dynamics. While many studies on astrometric, spectroscopic, and photometric properties of BSSs in clusters have been conducted, there are few works in the literature regarding their pulsations and internal structure, which can indeed retain traces of their origin. In this work we computed and analysed a grid of collisional BSSs at low metallicity ($Z = 0.01\; Z_\odot$), finding that collision products present a peculiar chemical stratification that leads to periodicities in the period-spacing pattern of high-order gravity modes. These seismic fingerprints provide a unique opportunity to constrain the formation pathways of BSSs in globular clusters.

Figures (12)

• Figure 1: Representation of BSS grid. The x-axis and y-axis report all the different values of $M_1$ and $M_2$, respectively. Each point is divided into three sectors, indicating the values of $X_\mathrm{c}\xspace$ for the primary (bottom sector) and the secondary star (left sector) at the time of the collision, and the value of $X_\mathrm{c}\xspace$ for the BSS at the zero age MS (ZAMS; top right sector). The colour-coding for $X_\mathrm{c}\xspace$ is reported on the left. The colour-bar ranges from 0.3 to 0.65. Values out of this range are coloured as the closest extreme on the colour scale. The BSSs' masses are reported inside the plot, rounded to the closest multiple of $0.01$.
• Figure 2: HRDs of collision between an $M_1 = 0.61\; \mathrm{M}_\odot\xspace$ star and a $M_2 = 0.51\; \mathrm{M}_\odot\xspace$ star. Left panel: Evolutionary tracks of parent stars (dashed black lines), with the position of the parent stars at the moment of the collision (cyan for the primary, red for the secondary). Right panel: Evolutionary tracks of BSS (orange; dotted lines = relaxation, solid lines = MS) and of the equivalent-mass SSE star (black dashed line). The triangle marks where the relaxation phase starts.
• Figure 3: Comparison between a $1.05\; \mathrm{M}_\odot\xspace$ BSS ($M_1 = 0.61\; \mathrm{M}_\odot\xspace$, $M_2 = 0.51\; \mathrm{M}_\odot\xspace$; orange) and its SSE counterpart (grey) for different values of $X_\mathrm{c}\xspace$ (0.60, 0.35, and 0.10; from top row to bottom row). Left column: Evolution of $X$ profile in g-mode cavity as a function of normalised buoyancy radius, $\Pi_0/\Pi_r$. Centre column: Evolution of $N$ as function of $\Pi_0/\Pi_r$ (solid lines). The dotted orange line represents the FT of the BSS's PSP, with the Nyquist limit ($\Pi_0/\Pi_r = 0.5$) indicated by the dashed black line. Right column: Evolution of PSPs for dipole modes (note that the scales on the period-axis are not the same for all the panels). In the left and centre panels, a light blue shaded region marks the position of the glitch, which corresponds to the steepening of the $X$ profile and to the additional peak in the $N$ profile.
• Figure 4: Representation of our BSSs' grid with points coloured by the percentage of mass lost during the collision.
• Figure 5: Mass (orange) and radius (light blue) of a collision product ($M_1 = 0.61\; \mathrm{M}_\odot\xspace$, $M_2 = 0.51\; \mathrm{M}_\odot\xspace$) as functions of the periastron radius, $r_\mathrm{P}$.