arXiv subjects

O. Sobodar

Publications and source records attributed to O. Sobodar.

5 recordsLinked to original sources

arXiv2026

Is the overconcentration of pristine populations in Galactic globular clusters real? An N-body approach to the problem

Recent observations indicate that in some Milky Way globular clusters (GCs) pristine red giant branch (RGB) stars appear more centrally concentrated than enriched ones. This contradicts most multiple stellar population (MSP) formation scenarios, which predict that the enriched (second) population (2P) should initially be more concentrated than the pristine (first) population (1P). Previous MOCCA Monte Carlo simulations suggested that this apparent overconcentration is a transient effect arising in clusters that have lost a large fraction of their initial mass and host an active black hole subsystem (BHS), and is visible only when RGB stars are used as tracers. In this letter, we test this interpretation using tailored NBODY6++GPU models evolved with direct N-body simulations, providing an independent validation that does not rely on a statistical treatment of relaxation. We performed direct N-body simulations with the NBODY6++GPU code, adopting initial conditions designed to reproduce the dynamical regime relevant to the proposed mechanism. The simulations include updated stellar and binary evolution, dynamical interactions, and the Galactic tidal field, enabling a direct comparison with MOCCA results. The simulations confirm that the spatial distributions and kinematics inferred from RGB stars can be strongly affected by stochastic fluctuations and interactions with the BHS. Preferential ejection of 2P RGB and their progenitors from the cluster center leads to a transient apparent overconcentration of 1P RGB stars, in agreement with earlier MOCCA predictions.

astro-ph.GA↗

arXiv2026

How our proto-nuclear star cluster formed and grew due to early globular cluster disruption. I. Case of low masses

We investigate the accretion of globular cluster stars on early cosmological timescales through detailed N-body simulations of theoretical GC models to assess the role of this mechanism in Milky Way-like galaxies. For the dynamical modelling, we used the updated parallel N-body code phi-GPU, including stellar evolution. We prepared three sets of GC models with different half-mass radii (r_hm), each consisting of 50 full N-body GC models, and integrated these models in an external, time-variable MW-like potential taken from the cosmological database IllustrisTNG-100. The simulations cover the time interval from -10 Gyr to -5 Gyr, enabling us to assess the rate of early stellar accretion onto the proto-NSC. We find that GC models with average orbital eccentricities of 0.4-0.5 and orbits oriented perpendicular to the galactic disc contribute most significantly to the mass of the proto-NSC formation. Accretion is especially efficient in the first billion years and in compact GC models with r_hm = 1 pc. In all sets, the dominant accreted stellar population consists of low-mass stars (~0.33 Msun). However, the accreted mass alone is insufficient to fully account for the current NSC mass. Based on our extended set of numerical simulations, we obtained an average lower limit of mass contribution (~6 percent) to the NSC from investigated GCs. The fraction of mass contribution from individual disrupted GCs can significantly vary from 0.1 percent up to 90 percent. Generally, we conclude that the GC stellar accretion channel alone might not be sufficient to ensure the present-day MW galaxy NSC mass budget.

astro-ph.GA↗

arXiv2024

Cosmological insights into the early accretion of r-process-enhanced stars II. Dynamical identification of lost members of Reticulum II

Aims. We identify the possible dynamical connection between individual r-process-enhanced stars and the ultra-faint dwarf galaxy Reticulum II based on the current phase-space information for these stars and the dynamical mass-loss model of Reticulum II during its orbital motion for 11.5 Gyr of lookback time. The dynamical orbital modelling together with the chemical abundance analysis proved to be useful tools for the progenitor identification of the peculiar stars in our Galaxy. Methods. To reproduce the Reticulum II orbital mass loss, we used our high-precision N-body phi-GPU code to integrate almost 1 million stars into the system evolution inside a external Galactic potential. We also investigated the orbits of r-process-enhanced stars using the same code. Results. We present our Reticulum II dynamical modelling results in the context of the stars energies - angular momentum phase-space and phase-space overlapping of the currently observed r-process-enhanced stars with Reticulum II stellar tidal tails. Of the 530 r-stars known today, at least 93 are former members of the Reticulum II dynamical progenitor system.

astro-ph.GA↗

arXiv2024

Dynamical evolution of Milky Way globular clusters on the cosmological timescale I. Mass loss and interaction with the nuclear star cluster

Context. Based on the Gaia DR3, we reconstructed the orbital evolution of the known Milky Way globular clusters and found that six objects, NGC 6681, NGC 6981, Palomar 6, NGC 6642, HP 1, and NGC 1904, very likely interact closely with the nuclear star cluster. Aims. We study the dynamical evolution of selected Milky Way globular clusters and their interactions with the Galactic centre over cosmological timescales. We examine the global dynamical mass loss of these globular cluster systems, their close interactions with the Galactic centre, and the potential capture of stars by the Milky Way nuclear star cluster. Methods. For the dynamical modelling of the clusters, we used the parallel N-body code phi-GPU, which allows star-by-star simulations of the systems. Our current code also enabled us to follow the stellar evolution of individual particles, including the formation of high-mass remnants. The modelling was carried out in a Milky Way-like, time-variable potential (with a dynamically changing mass and scale length), obtained from the IllustrisTNG-100 database, with a full integration time of eight billion years. Results. Based on extensive numerical modelling and analysis, we estimated the mass loss and the global and inner structures of the selected six clusters. Over an evolution of eight billion years, the clusters lost 80% of their initial mass. We analysed the phase-space evolution of the individual unbound stars NGC 6681, NGC 6642, HP 1, and NGC 1904. We found that only NGC 6642 could potentially have been a source for populating the Milky Way nuclear star cluster in the past.

astro-ph.GA↗

arXiv2023

Milky Way globular clusters on cosmological timescales. I. Evolution of the orbital parameters in time-varying potentials

Context. Recent observational data show that the Milky Way (MW) galaxy contains about 170 globular clusters (GCs). A fraction of them is likely formed in dwarf galaxies accreted onto the MW in the past, while the remaining of clusters are formed in-situ. Therefore, different parameters, including orbits, of the globular clusters is a valuable tool for studying the Milky Way evolution. However, since the evolution of the 3D mass distribution of the MW is poorly constrained, the orbits of the clusters are usually calculated in static potentials. Aims. In this work, we study the evolution of the GCs in several external potentials, where we aim to quantify the effects of the evolving galaxy potential on the orbits of the GCs. Methods. For the orbits calculation we used five MW-like potentials from IllustrisTNG-100 simulation. The orbits of 159 GCs were integrated using a high-order N-body parallel dynamic code phi-GPU, with initial conditions obtained from recent Gaia DR3 catalogues. Results. We provide a classification of the GCs orbits according to their 3D shapes and association with different components of the MW (disk, halo, bulge). We also found that the globular clusters in the external potentials have roughly similar energy-angular momentum distributions at the present time. However, both total energy and total angular momentum of the GCs are not conserved due to time-varying nature of the potentials. In some extreme cases, the total energy can change up to 40% (18 objects) over the last 5 Gyr of evolution. We found that the in-situ formed GCs are less affected by the evolution of the TNG potentials as compared to the clusters which are likely formed ex-situ. Therefore, our results suggest that time-varying potentials significantly affect the orbits of the GC, thus making it vital for understanding the formation of the MW.

astro-ph.GA↗