arXiv subjects

Arkadiusz Hypki

Publications and source records attributed to Arkadiusz Hypki.

At least 19 recordsLinked to original sources

arXiv2025

Multiband gravitational wave observations of eccentric escaping binary black holes from globular clusters

Stellar-mass binary black holes (sBBHs) formed in globular clusters (GCs) are promising sources for multiband gravitational wave (GW) observations, particularly with low- and middle-frequency detectors. These sBBHs can retain detectable eccentricities when they enter the sensitivity bands of low-frequency GW observatories. We study multiband GW observations of eccentric sBBHs that escape from GC models simulated with the MOCCA code, focusing on how low- and middle-frequency detectors can constrain their eccentricities and other parameters. Using Monte Carlo simulations, we generate ten realizations of cosmic sBBHs by combining the MOCCA sample with a cosmological model for GC formation and evolution. We then assess their detectability and the precision of parameter estimation. Our results show that LISA, Taiji, the LISA-Taiji network (LT), and AMIGO could detect $0.8\pm0.7$, $11.6\pm2.0$, $15.4\pm2.7$, and $7.9\pm1.3$ escaping sBBHs, respectively, over four years, while LT-AMIGO could detect $20.6\pm3.0$ multiband sBBHs in the same period. LT and AMIGO can measure initial eccentricities with relative errors of approximately $10^{-6}-2\times10^{-4}$ and $10^{-3}-0.7$, respectively. Joint LT-AMIGO observations have a similar ability to estimate eccentricities as LT alone.

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arXiv2025

Simulations of Globular Cluster Evolution with Multiple Stellar Populations

The formation of stars with light-element abundance variations in globular clusters and the subsequent dynamical evolution of these multiple populations remains an open question. One of the most widely discussed is the AGB scenario, in which chemically processed material from the envelopes of AGB stars mixes with re-accreted primordial gas flowing into the center of the cluster. Based on this scenario, more than two hundred MOCCA simulations of cluster evolution have been carried out, incorporating additional physical processes related to the external environment of globular clusters and the initial properties of multiple stellar populations. Analysis of the simulations shows that most observed properties of multiple stellar populations and the global parameters of Milky Way clusters are well reproduced, with the exception of the correlation between cluster mass and the fraction of second-population stars. We present a speculative scenario of globular cluster evolution that may account for the observed properties of Milky Way clusters, including the correlation between cluster mass and the fraction of enriched stars. The scenario further predicts that, under certain conditions, the pristine first population can be more centrally concentrated than the enriched second population, as observed in some clusters. \end{abstract

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arXiv2025

Formation Channels of Gravitationally Resolvable Double White Dwarf Binaries Inside Globular Clusters

Current gravitational wave detectors are sensitive to coalescing black holes and neutron stars. However, double white dwarfs (DWDs) have long been recognized as promising sources of gravitational waves, and upcoming detectors like LISA will be able to observe these systems in abundance. DWDs are expected to be the dominant gravitational wave (GW) sources in parts of the LISA frequency range, making it crucial to understand their formation for future detections. The Milky Way contains many white dwarfs (WDs) in both the field and star clusters, promising a rich population of DWDs for LISA. However, the large number of sources may make it difficult to resolve individual binaries, and DWDs in the field and clusters often have similar properties, complicating the identification of their origins from GW signals alone. In this work, we focus on eccentric and tight DWDs, which cannot form in the field, but require dynamical interactions in dense clusters to increase their eccentricity after circularization through mass transfer phases and common-envelope evolution during binary evolution. These binaries may also form in three- and four-body dynamical interactions where a DWD binary may directly form with high eccentricity and low separation. Our results show that we should expect eccentric and tight DWDs in clusters that can provide meaningful GW signal, however, the number is low; with an upper limit of 10-15 in the MW. These can be used to independently obtain distances of their host cluster.

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arXiv2025

Rapid formation of a very massive star >50000 $M_\odot$ and subsequently an IMBH from runaway collisions. Direct N-body and Monte Carlo simulations of dense star clusters

Context. We present simulations of a massive young star cluster using \textsc{Nbody6++GPU} and \textsc{MOCCA}. The cluster is initially more compact than previously published models, with one million stars, a total mass of $5.86 \times 10^5~\mathrm{M}_{\odot}$, and a half-mass radius of $0.1~\mathrm{pc}$. Aims. We analyse the formation and growth of a very massive star (VMS) through successive stellar collisions and investigate the subsequent formation of an intermediate-mass black hole (IMBH) in the core of a dense star cluster. Methods. We use both direct \textit{N}-body and Monte Carlo simulations, incorporating updated stellar evolution prescriptions (SSE/BSE) tailored to massive stars and VMSs. These include revised treatments of stellar radii, rejuvenation, and mass loss during collisions. While the prescriptions represent reasonable extrapolations into the VMS regime, the internal structure and thermal state of VMSs formed through stellar collisions remain uncertain, and future work may require further refinement. Results. We find that runaway stellar collisions in the cluster core produce a VMS exceeding $5 \times 10^4~\mathrm{M}_{\odot}$ within 5 Myr, which subsequently collapses into an IMBH. Conclusions. Our model suggests that dense stellar environments may enable the formation of very massive stars and massive black hole seeds through runaway stellar collisions. These results provide a potential pathway for early black hole growth in star clusters and offer theoretical context for interpreting recent JWST observations of young, compact clusters at high redshift.

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arXiv2025

Ultraluminous X-ray sources in Globular Clusters

This paper investigates the formation, populations, and evolutionary paths of UltraLuminous X-ray Sources (ULXs) within Globular Clusters (GCs). ULXs, characterised by their extreme X-ray luminosities, present a challenge to our understanding of accretion physics and compact object formation. While previous studies have largely focused on field populations, this research examines the unique environment of GCs, where dynamical interactions play a significant role. Using the MOCCA Monte Carlo code, we explore how dynamics influences ULX populations within these dense stellar clusters. Our findings reveal that dynamical processes, such as binary hardening and exchanges, can both facilitate and impede ULX formation in GCs. The study explores the impact of parameters including the initial binary fraction, tidal filling, and multiple stellar populations on the evolution of ULXs. We find that non-tidally filling clusters exhibit significantly larger ULX populations compared to tidally filling ones. The results indicate that the apparent scarcity of ULXs in GCs may be related to the older stellar populations of GCs relative to the field. Furthermore, the study identifies a population of "escaper" ULXs, which originate in GCs but are ejected and emit X-rays outside the cluster. These escapers may significantly contribute to the observed field ULX population.

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arXiv2024

MOCCA: Effects of pristine gas accretion and cluster migration on globular cluster evolution, global parameters, and multiple stellar populations

Using the MOCCA code, we study the evolution of globular clusters (GCs) with multiple stellar populations. For this purpose, the MOCCA code has been significantly extended to take into account the formation of an enriched population of stars from re-accreted gas with a time delay after the formation of the pristine population of stars. The possibility of cluster migration in the host galaxy and the fact that the pristine population can be described by a model not in virial equilibrium are also taken into account. Gas re-accretion and cluster migration have a decisive impact on the observational parameters of clusters and the ratio of the number of objects between the pristine and enriched populations. The obtained results, together with observational data, suggest a speculative refinement of the AGB scenario that makes it possible to explain some observational data, such as the ratio of the pristine to the enriched populations, the observational fact that for some GCs the pristine population is more concentrated than the enriched one, and possibly a correlation between the ratio of the number of enriched stars to the total number of stars and the mass of the cluster. In this scenario, it is important to take into account the environment in which the cluster lives, the conditions in the galaxy when it formed, and the fact that a significant part of the GCs associated with the Galaxy come from dwarf galaxies that merged with the Milky Way. The initial conditions of GCs in our simulations differ from the widely used typical models, as they require GCs to fill the Roche lobe rather than being highly concentrated within it, imposing strong constraints on their formation locations within the galaxy.

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arXiv2024

Gravitational Wave Astronomy With TianQin

The opening of the gravitational wave window has significantly enhanced our capacity to explore the universe's most extreme and dynamic sector. In the mHz frequency range, a diverse range of compact objects, from the most massive black holes at the farthest reaches of the Universe to the lightest white dwarfs in our cosmic backyard, generate a complex and dynamic symphony of gravitational wave signals. Once recorded by gravitational wave detectors, these unique fingerprints have the potential to decipher the birth and growth of cosmic structures over a wide range of scales, from stellar binaries and stellar clusters to galaxies and large-scale structures. The TianQin space-borne gravitational wave mission is scheduled for launch in the 2030s, with an operational lifespan of five years. It will facilitate pivotal insights into the history of our universe. This document presents a concise overview of the detectable sources of TianQin, outlining their characteristics, the challenges they present, and the expected impact of the TianQin observatory on our understanding of them.

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arXiv2024

MOCCA: Global properties of tidally filling and underfilling globular star clusters with multiple stellar populations

We explore the evolution of various properties of multiple-population globular clusters (GCs) for a broad range of initial conditions. We simulated over 200 GC models using the MOCCA Monte Carlo code and find that present-day properties (core and half-light radii, ratio of the number of second-generation (SG) stars to the total number of stars, NSG/NTOT) of these models cover the observed values of these quantities for Milky Way GCs. Starting with a relatively small value of the SG fraction (NSG/NTOT ~ 0.25) and a SG system concentrated in the inner regions of the cluster, we find, in agreement with previous studies, that systems in which the first-generation (FG) is initially tidally filling or slightly tidally underfilling best reproduce the observed ratios of NSG/NTOT and have values of the core and half-light radii typical of those of many Galactic globular clusters. Models in which the FG is initially tidally underfilling retain values of NSG/NTOT close to their initial values. These simulations expand previous investigations and serve to further constrain the viable range of initial parameters and better understand their influence on present-day GC properties. The results of this investigation also provide the basis for our future survey aimed at building specific models to reproduce the observed trends (or lack thereof) between the properties of multiple stellar populations and other clusters properties.

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arXiv2024

Double white dwarf binary population in MOCCA star clusters -- Comparisons with observations of close and wide binaries

There could be a significant population of double white dwarf binaries (DWDs) inside globular clusters (GCs), however, these are often too faint to be individually observed. We have utilized a large number GC models evolved with the Monte Carlo Cluster Simulator (MOCCA) code, to create a large statistical dataset of DWDs. These models include multiple-stellar populations, resulting in two distinct initial populations: one dense and another less dense. Due to the lower density of one population, a large number of objects escape during the early GC evolution, leading to a high mass-loss rate. In this dataset we have analysed three main groups of DWDs, namely in-cluster binaries, escaped binaries, and isolated evolution of primordial binaries. We compared the properties of these groups to observations of close and wide binaries. We find that the number of escaping DWDs is significantly larger than the number of in-cluster binaries and those that form via the isolated evolution of all promiridial binaries in our GC models. This suggests that dynamics play an important role in the formation of DWDs. For close binaries, we found a good agreement in the separations of escaped binaries and isolated binaries, but in-cluster binaries showed slight differences. We could not reproduce the observed extremely low mass WDs due to the limitations of our stellar and binary evolution prescriptions. For wide binaries, we also found a good agreement in the separations and masses, after accounting for observational selection effects. We conclude that, even though the current observational samples of DWDs are extremely biased and incomplete, our results compare reasonably well with observations.

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arXiv2024

New Parameters for Star Cluster Dynamics: the role of clusters initial conditions

We recently introduced three new parameters that describe the shape of the normalized cumulative radial distribution (nCRD) of the innermost stars in globular clusters and trace the clusters dynamical evolution. Here we extend our previous investigations to the case of a large set of Monte Carlo simulations of globular clusters, started from a broad range of initial conditions. All the models are analyzed at the same age of 13 Gyr, when they have reached different evolutionary phases. The sample of models is well representative of the structural properties of the observed population of Galactic globular clusters. We confirm that the three nCRD parameters are powerful tools to distinguish systems in early stages of dynamical evolution, from those that already experienced core collapse. They might also help disentangle clusters hosting a low-mass intermediate-mass black hole of a few hundred solar masses, from cases with large concentrations of dark remnants in their centers. With respect to other dynamical indicators, the nCRD parameters offer the advantage of being fully empirical and easier to measure from observational data.

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arXiv2022

MOCCA: Dynamics and evolution of binary stars of multiple stellar populations in tidally filling and underfilling globular star clusters

We present an upgraded version of the \MOCCA code for the study of dynamical evolution of globular clusters (GCs) and its first application to the study of evolution of multiple stellar populations. We explore initial conditions spanning different structural parameters for the first (FG) and second generation of stars (SG) and we analyze their effect on the binary dynamics and survival. Here, we focus on the number ratio of FG and SG binaries, its spatial variation, and the way their abundances are affected by various cluster initial properties. We find that present-day SG stars are more abundant in clusters that were initially tidally filling. Conversely, FG stars stay more abundant in clusters that were initially tidally underfilling. We find that the ratio between binary fractions is not affected by the way we calculate these fractions (e.g. only main-sequence binaries (MS) or observational binaries, i.e. MS stars $> 0.4 M_{\odot}$ mass ratios $> 0.5$). This implies that the MS stars themselves are a very good proxy for probing entire populations of FG and SG. We also discuss how it relates to the observations of Milky Way GCs. We show that \MOCCA models are able to reproduce the observed range of SG fractions for Milky Way GCs for which we know these fractions. We show how the SG fractions depend on the initial conditions and provide some constraints for the initial conditions to have more numerous FG or SG stars at the Hubble time.

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arXiv2022

MOCCA-SURVEY Database II -- Properties of Intermediate Mass Black Holes escaping from star clusters

In this work we investigate properties of intermediate-mass black holes (IMBHs) that escape from star clusters due to dynamical interactions. The studied models were simulated as part of the preliminary second survey carried out using the MOCCA code (MOCCA-SURVEY Database II), which is based on the Monte Carlo N-body method and does not include gravitational wave recoil kick prescriptions of the binary black hole merger product. We have found that IMBHs are more likely to be formed and ejected in models where both initial central density and central escape velocities have high values. Most of our studied objects escape in a binary with another black hole (BH) as their companion and have masses between $100$ and $140\: M_{\odot}$. Escaping IMBHs tend to build-up mass most effectively through repeated mergers in a binary with BHs due to gravitational wave emission. Binaries play a key role in their ejection from the system as they allow these massive objects to gather energy needed for escape. The binaries in which IMBHs escape tend to have very high binding energy at the time of escape and the last interaction is strong but does not involve a massive intruder. These IMBHs gain energy needed to escape the cluster gradually in successive dynamical interactions. We present specific examples of the history of IMBH formation and escape from star cluster models. We also discuss the observational implications of our findings as well as the potential influence of the gravitational wave recoil kicks on the process.

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arXiv2020

MOCCA SURVEY Database I: Binary Black Hole Mergers from Globular Clusters with Intermediate Mass Black Holes

The dynamical formation of black hole binaries in globular clusters that merge due to gravitational waves occurs more frequently in higher stellar density. Meanwhile, the probability to form intermediate mass black holes (IMBHs) also increases with the density. To explore the impact of the formation and growth of IMBHs on the population of stellar mass black hole binaries from globular clusters, we analyze the existing large survey of Monte-Carlo globular cluster simulation data (MOCCA SURVEY Database I). We show that the number of binary black hole mergers agrees with the prediction based on clusters' initial properties when the IMBH mass is not massive enough or the IMBH seed forms at a later time. However, binary black hole formation and subsequent merger events are significantly reduced compared to the prediction when the present-day IMBH mass is more massive than $\sim10^4 \rm M_{\odot}$ or the present-day IMBH mass exceeds about 1 per cent of cluster's initial total mass. By examining the maximum black hole mass in the system at the moment of black hole binary escaping, we find that $\sim$ 90 per cent of the merging binary black holes escape before the formation and growth of the IMBH. Furthermore, large fraction of stellar mass black holes are merged into the IMBH or escape as single black holes from globular clusters in cases of massive IMBHs, which can lead to the significant under-population of binary black holes merging with gravitational waves by a factor of 2 depending on the clusters' initial distributions.

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arXiv2019

MOCCA Survey Database I: Dissolution of tidally filling star clusters harbouring BH subsystems

We investigate the dissolution process for dynamically evolving star clusters embedded in an external tidal field by exploring the MOCCA Survey Database I, with focus on the presence and evolution of a stellar-mass black hole subsystem. We argue that the presence of a black hole subsystem can lead to the dissolution of tidally filling star clusters and this can be regarded as a third type of cluster dissolution mechanism (in addition to well-known mechanisms connected with strong mass loss due to stellar evolution and mass loss connected with the relaxation process). This third process is characterized by abrupt cluster dissolution connected with the loss of dynamical equilibrium. The abrupt dissolution is powered by strong energy generation from a stellar-mass black hole subsystem accompanied by tidal stripping. Additionally, we argue that such a mechanism should also work for even tidally under-filling clusters with top-heavy initial mass function. Observationally, star clusters which undergo dissolution powered by the third mechanism would look as a 'dark cluster' i.e. composed of stellar mass black holes surrounded by an expanding halo of luminous stars (Banerjee & Kroupa 2011), and they should be different from 'dark clusters' harbouring intermediate mass black holes as discussed by Askar et al. (2017a). An additional observational consequence of an operation of the third dissolution mechanism should be a larger than expected abundance of free floating black holes in the Galactic halo.

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arXiv2019

MOCCA-SURVEY Database I: Dissolution of tidally filling star clusters harboring black hole subsystem

We investigate the dissolution process of star clusters embedded in an external tidal field and harboring a subsystem of stellar-mass black hole. For this purpose we analyzed the MOCCA models of real star clusters contained in the Mocca Survey Database I. We showed that the presence of a stellar-mass black hole subsystem in tidally filling star cluster can lead to abrupt cluster dissolution connected with the loss of cluster dynamical equilibrium. Such cluster dissolution can be regarded as a third type of cluster dissolution mechanism. We additionally argue that such a mechanism should also work for tidally under-filling clusters with a top-heavy initial mass function.

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arXiv2016

MOCCA-SURVEY Database I: Is NGC 6535 a Dark Star Cluster Harboring an IMBH?

We describe the dynamical evolution of a unique type of dark star cluster model in which the majority of the cluster mass at Hubble time is dominated by an intermediate-mass black hole (IMBH). We analyzed results from about 2000 star cluster models (Survey Database I) simulated using the Monte-Carlo code MOCCA and identified these dark star cluster models. Taking one of these models, we apply the method of simulating realistic "mock observations" by utilizing the COCOA and SISCO codes to obtain the photometric and kinematic observational properties of the dark star cluster model at 12 Gyr. We find that the perplexing Galactic globular cluster NGC 6535 closely matches the observational photometric and kinematic properties of the dark star cluster model presented in this paper. Based on our analysis and currently observed properties of NGC 6535, we suggest that this globular cluster could potentially harbour an IMBH. If it exists, the presence of this IMBH can be detected robustly with proposed kinematic observations of NGC 6535.

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arXiv2016

MOCCA-SURVEY database I. Accreting white dwarf binary systems in globular clusters -- I. cataclysmic variables -- present-day population

In this paper, which is the first in a series of papers associated with cataclysmic variables and related objects, we introduce the CATUABA code, a numerical machinery written for analysis of the MOCCA simulations, and show some first results by investigating the present-day population of cataclysmic variables in globular clusters. Emphasis was given on their properties and the observational selection effects when observing and detecting them. In this work we analysed in this work six models, including three with Kroupa distributions of the initial binaries. We found that for models with Kroupa initial distributions, considering the standard value of the efficiency of the common envelope phase adopted in BSE, no single cataclysmic variable was formed only via binary stellar evolution, i. e., in order to form them, strong dynamical interactions have to take place. We show and explain why this is inconsistent with observational and theoretical results. Our results indicate that the population of cataclysmic variables in globular clusters is, mainly, in the last stage of their evolution and observational selection effects can drastically change the expected number of observed cataclysmic variables. We show that the probability of observing them during the outbursts is extremely small and conclude that the best way of looking for cataclysmic variables in globular clusters is by searching for variabilities during quiescence, instead of during outbursts. For that, one would need a very deep observation which could reach magnitudes $\gtrsim$ 27 mag. Finally, we argue that cataclysmic variables in globular clusters are not necessarily magnetic.

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arXiv2016

MOCCA Code for Star Cluster Simulation: Comparison with Optical Observations using COCOA

We introduce and present preliminary results from COCOA (Cluster simulatiOn Comparison with ObservAtions) code for a star cluster after 12 Gyrs of evolution simulated using the MOCCA code. The COCOA code is being developed to quickly compare results of numerical simulations of star clusters with observational data. We use COCOA to obtain parameters of the projected cluster model. For comparison, a FITS file of the projected cluster was provided to observers so that they could use their observational methods and techniques to obtain cluster parameters. The results show that the similarity of cluster parameters obtained through numerical simulations and observations depends significantly on the quality of observational data and photometric accuracy.

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