arXiv subjects

Jongsuk Hong

Publications and source records attributed to Jongsuk Hong.

At least 19 recordsLinked to original sources

arXiv2026

Kinematics and Dynamics of the Open Cluster NGC 2302

Open clusters are ideal observational testbeds to understand the dynamics of stellar systems. We present a dynamical study of the young open cluster NGC 2302. The latest Gaia data and $UBVIJHK_s$ photometric data are used in this study. A total of 117 stars are selected as the genuine members using the Gaia data. This cluster is, on average, reddened by $ = 0.24 \pm 0.06$ (s.d.). The ratio of total-to-selective extinction ($R_V$) in the direction of NGC 2302 is $2.8 \pm 0.1$. The cluster distance is determined to be $1.16 \pm 0.08$ kpc using Gaia parallaxes. Theoretical isochrone fitting for $Z = 0.008$ on color-magnitude diagrams yields an age of $80 \pm 20$ Myr. The relative proper motions of individual members show no significant radial expansion or contraction. NGC 2302 contains a total stellar mass of $333 \pm 48 M_{\odot}$. The one-dimensional velocity dispersion is approximately 0.26 km s$^{-1}$, which is comparable to the viral velocity dispersion of 0.27 km s$^{-1}$ derived from its total mass. Its relaxation time is estimated to be approximately 90 Myr, which is similar to the age of the cluster within the uncertainty in age estimation. Finally, we report a pattern of mass segregation in the radial distribution of stellar masses. Our results suggest that NGC 2302 is virialized and currently approaching a state of dynamical relaxation. However, because no definitive evidence of kinetic energy equipartition is found, the possibility of the in-situ formation of high-mass stars within the central region should be carefully considered.

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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 process of young stellar population in Messier 16 from a kinematic perspective

We present a kinematic study of young stars in Messier 16 (M16) using the Gaia Data Release 3 and high-resolution spectra. A total of 345 stars are selected as genuine members using the published lists of X-ray, infrared sources, and early-type stars as well as the Gaia data. There is severe differential reddening across this region and the reddening law of the intracluster medium appears abnormal. The distance to M16, derived from the parallaxes of the members, is about 1.7 kpc. The ages of members, estimated by comparing their color-magnitude diagram with theoretical isochrones, range from 1 Myr to 4 Myr. This star-forming region is composed of an open cluster (NGC 6611) and a distributed population. This cluster shows a clear pattern of expansion and rotation. Some of the distributed population are spatially associated with the gas pillars located at the ridge of H II bubble. In particular, several stars moving away from the cluster are physically associated with the northeastern pillar. In addition, their younger ages support the idea that the formation of these stars was triggered by the feedback from massive stars in NGC 6611. On the other hand, the other stars do not show systematic radial or stream motions; therefore, they likely formed through spontaneous star formation events. We discuss the formation of young stars in the context of cluster expansion, spontaneous star formation, and feedback-driven star formation, and suggest that all of these mechanisms possibly contributed to their formation.

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arXiv2025

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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arXiv2025

Formation and Evolution of Compact Binaries Containing Intermediate Mass Black Holes in Dense Star Clusters`

We investigate the evolution of star clusters containing intermediate-mass black hole (IMBH) of $300$ to $5000\ \mathrm{M}_\odot$, focusing on the formation and evolution of IMBH-stellar mass black holes (SBHs; $M_{\rm BH} \lesssim 10^2\ \mathrm{M}_\odot$) binaries. Dense stellar systems like globular clusters (GCs) or nuclear star clusters offer unique laboratories for studying the existence and impact of IMBHs. IMBHs residing in GCs have been under speculation for decades, with their broad astrophysical implications for the cluster's dynamical evolution, stellar population, GW signatures, among others. While existing GW observatories such as the Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO) target binaries with relatively modest mass ratios, $q \lesssim 10$, future observatories such as the Einstein Telescope (ET) and the Laser Interferometer Space Antenna (LISA) will detect intermediate-mass ratio inspirals (IMRIs) with $q > 10$. This work explores the potential for detecting IMRIs adopting these upcoming telescopes. For our experiments, we perform multiple direct $N$-body simulations with IMBHs utilizing Nbody6++GPU, after implementing the GW merger schemes for IMBHs. We then study the statistical properties of the resulting IMRIs, such as the event rates and orbital properties. Assuming that IMRIs with a signal-to-noise ratio $S/N > 8$ are detectable, we derive the following detection rates for each observatory: $\lesssim 0.02\mathrm{yr}^{-1}$ for aLIGO, $\sim 101 - 355 \mathrm{yr}^{-1}$ for ET, $\sim 186 - 200 \mathrm{yr}^{-1}$ for LISA, $\sim 0.24 - 0.34 \mathrm{yr}^{-1}$ for aSOGRO, and $\sim 3880 - 4890 \mathrm{yr}^{-1}$ for DECIGO. Our result confirms the capability of detecting IMRIs with future GW telescopes.

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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

Probing Intracluster Dynamics and Evolution of Globular Clusters through Cataclysmic Variable Populations

Dynamical interactions in globular clusters (GCs) significantly impact the formation and evolution of binary sources, including cataclysmic variables (CVs). This study investigates the connection between dynamical states of GCs and X-ray luminosity ($L_{x}$) distributions of CV populations through both simulations and actual observations. Utilizing a Monte Carlo simulation tool, MOCCA, we categorize the simulated GCs into three different evolutionary stages which are referred to as Classes I/II/III. Significant differences are found in the $L_{x}$ distributions of the CVs among these three Classes. In observational aspects, we have analyzed 179 CV candidates in 18 GCs observed by the {\it Chandra} X-ray Observatory. By dividing these GCs into three Families of different dynamical ages, namely Families I/II/III, the $L_{x}$ distributions of the CV candidates also show significant differences among these three Families. Both simulations and observational results suggest that CVs in more dynamically evolved clusters (Class/Family III) exhibit brighter X-ray emission. This highlights the influence of the dynamical status of a GC on the properties of its hosted compact binaries. Similar to blue stragglers, CV populations can serve as tracers of a GC's dynamical history. Our findings provide insights for understanding the interplay between intracluster dynamics and the evolution of compact binaries in GCs.

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arXiv2023

Influences of dynamical disruptions on the evolution of pulsars in globular clusters

By comparing the physical properties of pulsars hosted by core-collapsed (CCed) and non-core-collapsed (Non-CCed) globular clusters (GCs), we find that pulsars in CCed GCs rotate significantly slower than their counterparts in Non-CCed GCs. Additionally, radio luminosities at 1.4 GHz in CCed GCs are higher. These findings are consistent with the scenario that dynamical interactions in GCs can interrupt angular momentum transfer processes and surface magnetic field decay during the recycling phase. Our results suggest that such effects in CCed GCs are stronger due to more frequent disruptions of compact binaries. This is further supported by the observation that both estimated disruption rates and the fraction of isolated pulsars are predominantly higher in CCed GCs.

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arXiv2023

The kinematics of young stellar population in the W5 region of the Cassiopeia OB6 association: implication on the formation process of stellar associations

The star-forming region W5 is a major part of the Cassiopeia OB6 association. Its internal structure and kinematics may provide hints of the star formation process in this region. Here, we present a kinematic study of young stars in W5 using the Gaia data and our radial velocity data. A total 490 out of 2,000 young stars are confirmed as members. Their spatial distribution shows that W5 is highly substructured. We identify a total of eight groups using the k-means clustering algorithm. There are three dense groups in the cavities of H II bubbles, and the other five sparse groups are distributed at the ridge of the bubbles. The three dense groups have almost the same ages (5 Myr) and show a pattern of expansion. The scale of their expansion is not large enough to account for the overall structure of W5. The three northern groups are, in fact, 3 Myr younger than the dense groups, which indicates the independent star formation events. Only one group of them shows the signature of feedback-driven star formation as its members move away from the eastern dense group. The other two groups might have formed in a spontaneous way. On the other hand, the properties of two southern groups are not understood as those of a coeval population. Their origins can be explained by dynamical ejection of stars and multiple star formation. Our results suggest that the substructures in W5 formed through multiple star-forming events in a giant molecular cloud.

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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

Long-term evolution of multimass rotating star clusters

We investigate the long-term dynamical evolution of the internal kinematics of multimass rotating star clusters. We have performed a set of N-body simulations to follow the internal evolution of clusters with different degrees of initial rotation and have explored the evolution of the rotational velocity, the degree of energy equipartition, and anisotropy in the velocity distribution. Our simulations show that: 1) as the cluster evolves, the rotational velocity develops a dependence on the stellar mass with more massive stars characterised by a more rapid rotation and a peak in the rotation curve closer to the cluster centre than low-mass stars; 2) the degree of energy equipartition in the cluster's intermediate and outer regions depends on the component of the velocity dispersion measured; for more rapidly rotating clusters, the evolution towards energy equipartition is more rapid in the direction of the rotational velocity; 3) the anisotropy in the velocity distribution is stronger for massive stars; 4) both the degree of mass segregation and energy equipartition are characterised by spatial anisotropy; they have a dependence on both $R$ and $z$, correlated with the flattening in the spatial variation of the cluster's density and velocity dispersion, as shown by 2D maps of the mass segregation and energy equipartition on the ($R$-$z$) meridional plane.

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arXiv2022

A Gaia view on the star formation in the Monoceros OB1 and R1 associations

Stellar kinematics provides the key to understanding star formation process. In this respect, we present a kinematic study of the Monoceros OB1 (Mon OB1) and R1 (Mon R1) associations using the recent Gaia data and radial velocities of stars derived from high-resolution spectroscopy and the literature. A total of 728 members are selected using the criteria based on the intrinsic properties of young stars, parallaxes, and proper motions. The spatial distribution and kinematic properties of members show that these associations have distinct substructures. In Mon OB1, we find one northern group and two southern groups. Mon R1 is composed of three small stellar groups that are spatially and kinematically distinct. Some stars are found in a halo around these two associations. We detect patterns of expansion for most stellar groups in the associations. In addition, two stellar groups in Mon OB1 show the signature of rotation, which provides an important constraint on cluster formation. The star formation history of Mon OB1 is slightly revised. Star formation first occurred in the southern region and subsequently in the northern region. Recent star-forming events ignited deeper into the southern region, while some stars are escaping from Mon OB1, forming a halo. Mon R1 might have formed at the same epoch as the formation of the northern group in Mon OB1. Given that star formation is taking place on different scales along a large arc-like structure, Mon OB1 and Mon R1 may be the results of hierarchical star formation.

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arXiv2021

Disruption of Hierarchical Clustering in the Vela OB2 Complex and the Cluster Pair Collinder 135 and UBC7 with Gaia EDR3: Evidence of Supernova Quenching

We identify hierarchical structures in the Vela OB2 complex and the cluster pair Collinder 135 and UBC 7 with Gaia EDR3 using the neural network machine learning algorithm StarGO. Five second-level substructures are disentangled in Vela OB2, which are referred to as Huluwa 1 (Gamma Velorum), Huluwa 2, Huluwa 3, Huluwa 4 and Huluwa 5. For the first time, Collinder 135 and UBC 7 are simultaneously identified as constituent clusters of the pair with minimal manual intervention. We propose an alternative scenario in which Huluwa 1-5 have originated from sequential star formation. The older clusters Huluwa 1-3 with an age of 10-22 Myr, generated stellar feedback to cause turbulence that fostered the formation of the younger-generation Huluwa 4-5 (7-20 Myr). A supernova explosion located inside the Vela IRAS shell quenched star formation in Huluwa 4-5 and rapidly expelled the remaining gas from the clusters. This resulted in global mass stratification across the shell, which is confirmed by the regression discontinuity method. The stellar mass in the lower rim of the shell is $0.32\pm0.14$ $\rm M_\odot$ higher than in the upper rim. Local, cluster-scale mass segregation is observed in the lowest-mass cluster Huluwa 5. Huluwa 1-5 (in Vela OB2) are experiencing significant expansion, while the cluster pair suffers from moderate expansion. The velocity dispersions suggest that all five groups (including Huluwa 1A and Huluwa 1B) in Vela OB2 and the cluster pair are supervirial and are undergoing disruption, and also that Huluwa 1A and Huluwa 1B may be a coeval young cluster pair. N-body simulations predict that Huluwa 1-5 in Vela OB2 and the cluster pair will continue to expand in the future 100 Myr and eventually dissolve.

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arXiv2021

A kinematic perspective on the formation process of the stellar groups in the Rosette Nebula

Stellar kinematics is a powerful tool for understanding the formation process of stellar associations. Here, we present a kinematic study of the young stellar population in the Rosette nebula using the recent Gaia data and high-resolution spectra. We first isolate member candidates using the published mid-infrared photometric data and the list of X-ray sources. A total of 403 stars with similar parallaxes and proper motions are finally selected as members. The spatial distribution of the members shows that this star-forming region is highly substructured. The young open cluster NGC 2244 in the center of the nebula has a pattern of radial expansion and rotation. We discuss its implication on the cluster formation, e.g., monolithic cold collapse or hierarchical assembly. On the other hand, we also investigate three groups located around the border of the H II bubble. The western group seems to be spatially correlated with the adjacent gas structure, but their kinematics is not associated with that of the gas. The southern group does not show any systematic motion relative to NGC 2244. These two groups might be spontaneously formed in filaments of a turbulent cloud. The eastern group is spatially and kinematically associated with the gas pillar receding away from NGC 2244. This group might be formed by feedback from massive stars in NGC 2244. Our results suggest that the stellar population in the Rosette Nebula may form through three different processes: the expansion of stellar clusters, hierarchical star formation in turbulent clouds, and feedback-driven star formation.

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arXiv2021

Mass loss from massive globular clusters in tidal fields

Massive globular clusters lose stars via internal and external processes. Internal processes include mainly two-body relaxation, while external processes include interactions with the Galactic tidal field. We perform a suite of N-body simulations of such massive clusters using three different direct-summation N-body codes, exploring different Galactic orbits and particle numbers. By inspecting the rate at which a star's energy changes as it becomes energetically unbound from the cluster, we can neatly identify two populations we call kicks and sweeps, that escape through two-body encounters internal to the cluster and the external tidal field, respectively. We find that for a typical halo globular cluster on a moderately eccentric orbit, sweeps are far more common than kicks but the total mass loss rate is so low that these clusters can survive for tens of Hubble times. The different N-body codes give largely consistent results, but we find that numerical artifacts may arise in relation to the time step parameter of the Hermite integration scheme, namely that the value required for convergent results is sensitive to the number of particles.

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arXiv2020

Multiple stellar populations at less evolved stages: detection of chemical variations among main-sequence dwarfs in NGC 1978

Multiple stellar populations (MPs) with different chemical compositions are not exclusive features of old GCs (older than 10 Gyr). Indeed, recent studies reveal that younger clusters ($\sim$2--6 Gyr-old) in the Magellanic Clouds also exhibit star-to-star chemical variations among evolved stars. However, whether MPs are present among less evolved dwarfs of these intermediate-age clusters is still unclear. In this work, we search for chemical variations among GK-type dwarfs in the $\sim$2 Gyr-old cluster NGC 1978, which is the youngest cluster with MPs. We exploit deep ultraviolet and visual observations from the Hubble Space Telescope to constrain the nitrogen (N) and oxygen (O) variations among MS stars. To do this, we compare appropriate photometric diagrams that are sensitive to N and O with synthetic diagrams of simple stellar populations and MPs. We conclude that the G- and K-type MS stars in NGC\,1978 host MPs. Our statistical analysis shows that the fraction of N-rich stars ranges from $\sim$40\% to $\sim$80\%, depending on the detailed distributions of nitrogen and oxygen.

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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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arXiv2020

The origin of a distributed stellar population in the star-forming region W4

Stellar kinematics provides the key to understanding the formation process and dynamical evolution of stellar systems. Here, we present a kinematic study of the massive star-forming region W4 in the Cassiopeia OB6 association using the Gaia Data Release 2 and high-resolution optical spectra. This star-forming region is composed of a core cluster (IC 1805) and a stellar population distributed over 20 pc, which is a typical structural feature found in many OB associations. According to a classical model, this structural feature can be understood in the context of the dynamical evolution of a star cluster. The core-extended structure exhibits internally different kinematic properties. Stars in the core have an almost isotropic motion, and they appear to reach virial equilibrium given their velocity dispersion (0.9 +/- 0.3 km/s) comparable to that in a virial state (~0.8 km/s). On the other hand, the distributed population shows a clear pattern of radial expansion. From the N-body simulation for the dynamical evolution of a model cluster in subvirial state, we reproduce the observed structure and kinematics of stars. This model cluster experiences collapse for the first 2 Myr. Some members begin to radially escape from the cluster after the initial collapse, eventually forming a distributed population. The internal structure and kinematics of the model cluster appear similar to those of W4. Our results support the idea that the stellar population distributed over 20 pc in W4 originate from the dynamical evolution of IC 1805.

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