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Dynamical evolution of lower mass binary stars in the Milky Way Nuclear Stellar Cluster

Published online by Cambridge University Press:  15 October 2026

Arn Marklund Open the ORCID record for Arn Marklund [Opens in a new window] ,
Ross P. Church  and
Alessandro A. Trani Open the ORCID record for Alessandro A. Trani [Opens in a new window]
Arn Marklund*
Affiliation:
Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France Division of Astrophysics, Department of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden
Ross P. Church
Affiliation:
Division of Astrophysics, Department of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden
Alessandro A. Trani
Affiliation:
Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen, Denmark National Institute for Nuclear Physics – INFN, Sezione di Trieste, I-34127, Trieste, Italy Departamento de Astronomía, Facultad Ciencias Físicas y Matemáticas, Universidad de Concepción, Avenida Esteban Iturra, Casilla 160-C, Concepción, 4030000, Chile
*
email: arn.marklund@astro.unistra.fr
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Abstract

Most intermediate-mass galaxies, including the Milky Way, host both a supermassive black hole (SMBH) and a nuclear stellar cluster (NSC). We simulate the 10 Gyr evolution of lower-mass binaries near the hard-soft boundary, including three-body encounters and von Zeipel-Lidov-Kozai oscillations. Encounter-driven kicks cause both inward and outward migration for hard binaries on timescales shorter than classical binary mass segregation, determining whether the systems survive, merge, or evaporate. The process is, however, too slow to bring binaries into the inner arcsecond within the ages of the S-stars or G-objects, but suggests that i) the S-cluster is contaminated by merger remnants from earlier star formation events and more generally that ii) a primordial population of blue stragglers ($\lesssim 2\mathrm{M}_\odot$) should be scattered towards the outer parts of the inner parsec – providing new constraints on NSC stellar populations and dynamical histories.

Keywords

Nuclear Stellar ClustersNumerical modellingSimulationsBinariesBlue Stragglers

Information

Type
Contributed Paper
Information
Proceedings of the International Astronomical Union , Volume 21 , Symposium S398: Compact Objects and Binaries in Dense Star Clusters , September 2025 , pp. 165 - 168
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Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2026. Published by Cambridge University Press on behalf of The International Astronomical Union

1. Introduction

The Milky Way (MW) Nuclear Stellar Cluster (NSC) is home to many special objects; a supermassive black hole (SMBH, see Neumayer et al. Reference Neumayer, Seth and Böker2020, and references therein), the S-stars (Schödel et al. Reference Schödel, Nogueras-Lara and Gallego-Cano2020, and references therein), G-objects (Ghez et al. Reference Ghez, Salim and Hornstein2005; Ciurlo et al. Reference Ciurlo, Campbell and Morris2020), Blue Straggler stars (BSSs, Thorsbro et al. Reference Thorsbro, Forsberg and Kordopatis2023) and more. The origin of these objects remains somewhat debatable, but they are typically believed to originate from binaries (see also e.g. Trani et al. Reference Trani, Fujii and Spera2019). Binaries make up most of the systems in the Galactic field (Raghavan et al. Reference Raghavan, McAlister and Henry2010; Moe and Di Stefano Reference Moe and Di Stefano2017), yet in the NSC, only a handful of stellar binaries have been observed (Martins et al. Reference Martins, Trippe and Paumard2006; Pfuhl et al. Reference Pfuhl, Alexander and Gillessen2014; Peiß ker et al. Reference Peißker, Zajaček and Labadie2024). This lack of binaries can be partially explained by the extreme dynamical environment of the NSC. High number densities lead to frequent encounters that can destroy binaries (see e.g. Marklund et al. Reference Marklund, Church and Trani2025), and other secular processes such as von Zeipel-Lidov-Kozai (ZLK, von Zeipel Reference von Zeipel1910; Lidov Reference Lidov1962; Kozai Reference Kozai1962) oscillations can drive binaries to near-parabolic orbits where they merge (e.g. Prodan et al. Reference Prodan, Antonini and Perets2015; Stephan et al. Reference Stephan, Naoz and Ghez2019). The outcome and evolution of these binaries depend on many different initial parameters, such as, for example, the initial distribution of the binaries’ orbital elements and the region in which they originate in the NSC. It therefore becomes useful to define the hard/soft boundary (H/SB); where the ratio between a binary’s binding energy and the average kinetic energy of the surrounding stars is unity ( h tilde 1 $h\sim 1$ ). Very soft binaries ( h much less than 1 $h\ll1$ ) inevitably evaporate, and very hard binaries ( h much greater than 1 $h\gg1$ ) eventually merge (Rose et al. Reference Rose, Naoz and Gautam2020), while for binaries h tilde 1 $h\sim1$ , binary evolution is not reliably predictable and requires numerical simulations (Marklund et al. Reference Marklund, Church and Trani2025).

2. Numerical set-up

We perform numerical simulations of three-body encounters using tsunami (Trani and Spera Reference Trani and Spera2023), where in addition we also incorporate the quadrupole approximation for ZLK following the implementation described in Naoz et al. (Reference Naoz, Farr and Lithwick2013) and tidal dissipation as implemented in Mardling and Aarseth (Reference Mardling and Aarseth2001). In total, we evolve 5736 binaries originating at a distance of either 0.1 $0.1$ or 0.3 normal p normal c $0.3~\mathrm{pc}$ from the SMBH, for 10 Gyr or until they are either destroyed (merge or evaporate), unbound from the SMBH (ejected) or the primary evolves due to stellar evolution (this process can dominate the binary’s evolution, and therefore diminishing the role the dynamical processes). The initial population is chosen to be low-mass ( less than or equals 1 normal upper M Subscript circled dot $\le 1\,\mathrm{M}_\odot$ stars) short-period binaries ( less than or equivalent to 1000 $\lesssim 1000$ days) that are found in the Galactic field (Moe and Di Stefano Reference Moe and Di Stefano2017), and are sampled with COSMIC (Breivik et al. Reference Breivik, Coughlin and Zevin2020). A significant fraction of the binaries ( greater than or equivalent to 1 divided by 3 $\gtrsim1/3$ ) lie very near the H/SB.

3. Results

Figures 1 & 2 show the distributions of binaries in the (inner) separation-eccentricity plane at 0.1 $0.1$ and 0.3 normal p normal c $0.3~\mathrm{pc}$ , respectively. The line contours show the initial distribution, while the coloured contours show the final distribution for each respective panel: a) Initial total sample, b) Merged binaries, c) Evaporated binaries, d) Ejected binaries, e) Intact binaries (10 Gyr), and finally f) Evolved binaries. There are two distinct groups; initially softer binaries are exclusively destroyed via mergers and evaporation, while the initially harder binaries are subject to all different outcomes. We find that the kick received in three-body encounters is the deciding factor; an inward migration typically leads to the binaries being destroyed whereas outward migration does the opposite – binaries typically remain intact, with a few unbinding from the SMBH and a few evolving before 10 Gyr. The binaries that evolve either have a primary with an initial mass upper M Subscript asterisk Baseline greater than or equals 0.97 normal upper M Subscript circled dot Baseline $M_* \geq 0.97~\mathrm{M}_\odot$ , or due to stellar collisions with surrounding stars, gain mass and acquire an accelerated stellar evolution.

Figure 1.

Initial (solid line contours) and final distributions (coloured contours) of binaries in the semi-major axis and 1 minus $1 \,-$ eccentricity originating at 0.1 normal p normal c $0.1\,\mathrm{pc}$ from the SMBH: a) initial distribution, b) merged binaries c) evaporated binaries, d) ejected binaries, e.g. unbound to the SMBH, e) intact binaries and f) binaries where primary has evolved owing to stellar evolution. Figure adapted from Marklund et al. (Reference Marklund, Church and Trani2025).

Six contour plots showing distributions of binaries in semi-major axis and eccentricity.

Figure 2.

Same as figure 1 but for binaries originating at 0.3 normal p normal c $0.3\,\mathrm{pc}$ from the SMBH. Figure adapted from Marklund et al. (Reference Marklund, Church and Trani2025).

Multiple contour plots depict the distribution of binary star systems around a supermassive black hole.

Figure 3 shows the average distance of intact binaries as a function of time (red solid lines) for binaries originating at 0.1 $0.1$ and 0.3 normal p normal c $0.3~\mathrm{pc}$ in panels a) and b), respectively. Each panel also show the corresponding position and time of all mergers, and are colour-coded by their total expected lifetime (both pre-merger and post-merger). Notably, a group of binaries originating at 0.1 normal p normal c $0.1~\mathrm{pc}$ have migrated to the inner arcsec ( tilde 0.04 normal p normal c $\sim 0.04~\mathrm{pc}$ ) where they then merge, within a few 10 Superscript 8 $ 10^8$ years. Furthermore, and interestingly enough, there are a lot of binaries that migrate outwards where the timescales for most dynamical processes are longer, allowing them to remain intact for greater than or equivalent to 10 normal upper G normal y normal r $\gtrsim 10~\mathrm{Gyr}$ . A significant fraction of these binaries merge close to present day, and would therefore be observable as either main-sequence or recently evolved BSSs (see also for example Thorsbro et al. Reference Thorsbro, Forsberg and Kordopatis2023).

Figure 3.

Time (y-axis) and outer semi-major axis (x-axis) of mergers colour-coded by total expected lifetime (pre-merger and post-merger). The red solid line(s) show the mean outer semi-major axis of all intact binaries as a function of time; a) binaries originating at 0.1 normal p normal c $0.1\,\mathrm{pc}$ from the SMBH and b) binaries originating 0.3 normal p normal c $0.3\,\mathrm{pc}$ from the SMBH.

Scatter plot showing time and outer semi-major axis of mergers with lifetime color coding.

4. Conclusions

The main conclusion from this study is that binaries may migrate both inward and outward as a consequence of strong kicks from binary-single encounters. This in turn leads to (i) mergers in the inner arcsec of the NSC that occur after greater than or equivalent to 10 Superscript 8 $\gtrsim 10^8$ years, which is too slow to explain the S-stars and G-objects that have estimated ages of less than or equivalent to 10 Superscript 7 $\lesssim10^7$ years (e.g. Schödel et al. Reference Schödel, Nogueras-Lara and Gallego-Cano2020; Peiß ker et al. Reference Peißker, Zajaček and Labadie2024) and (ii) both intact and recently merged binaries (which should appear as BSSs) throughout the NSC. A more detailed discussion of the consequences for these points is included in Marklund et al. (Reference Marklund, Church and Trani2025).

Acknowledgements

The computations and data handling were enabled by resources provided by LUNARC, The Centre for Scientific and Technical Computing at Lund University. AAT acknowledges support from the Horizon Europe research and innovation programs under the Marie Skłodowska-Curie grant agreement no. 101103134.

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Figure 0

Figure 1. Initial (solid line contours) and final distributions (coloured contours) of binaries in the semi-major axis and 1−$1 \,-$ eccentricity originating at 0.1pc$0.1\,\mathrm{pc}$ from the SMBH: a) initial distribution, b) merged binaries c) evaporated binaries, d) ejected binaries, e.g. unbound to the SMBH, e) intact binaries and f) binaries where primary has evolved owing to stellar evolution. Figure adapted from Marklund et al. (2025).

Figure 1

Figure 2. Same as figure 1 but for binaries originating at 0.3pc$0.3\,\mathrm{pc}$ from the SMBH. Figure adapted from Marklund et al. (2025).

Figure 2

Figure 3. Time (y-axis) and outer semi-major axis (x-axis) of mergers colour-coded by total expected lifetime (pre-merger and post-merger). The red solid line(s) show the mean outer semi-major axis of all intact binaries as a function of time; a) binaries originating at 0.1pc$0.1\,\mathrm{pc}$ from the SMBH and b) binaries originating 0.3pc$0.3\,\mathrm{pc}$ from the SMBH.