arXiv subjects

Ataru Tanikawa

Publications and source records attributed to Ataru Tanikawa.

At least 19 recordsLinked to original sources

arXiv2026

Mass and Spin Growth of Very Massive Stars in Star Clusters Potentially Associated with Little Red Dots

Using gravitational $N$-body simulations, we investigate the evolution of mass and spin for very massive stars (VMSs) in dense star clusters, which could subsequently evolve into Little Red Dots (LRDs). Our results show that VMS masses can reach $10^3$--$10^4\,M_\odot$, depending on the initial conditions of the host clusters. Notably, the VMS mass increases by up to a factor of three when accounting for the bloated state at the Hayashi track induced by stellar collisions, provided that this state is maintained at accretion rates exceeding $3 \times 10^{-2}\,M_\odot\,{\rm yr}^{-1}$. In all cases, the spin of the VMS, when normalized to the dimensionless black hole (BH) spin parameter, exceeds $10$, although the mass and spin of VMSs after the post-main sequence phase could be decreased by the stellar evolution process. We nonetheless demonstrate that VMSs formed in dense star clusters can be highly spinning. Such a rapidly spinning VMS is expected to collapse into an intermediate-mass BH surrounded by a massive accretion disk. This BH-disk system could trigger powerful explosions and emit burst gravitational waves, similar to those observed in GW190521 and GW231123, for which the remnant BH masses are estimated to be $\gtrsim 100\,M_\odot$.

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arXiv2026

Investigating the formation channel of GW231123: Population III stars or hierarchical mergers?

The gravitational wave event GW231123, with component black hole masses lying within or above the pair-instability mass gap, poses a significant challenge to current stellar evolution models. In this work, we describe how we investigated its origin by coupling the galaxy formation model GAMESH with the cluster population synthesis code RAPSTER and using two distinct binary population synthesis codes, SEVN and BSEEMP. This framework allowed us, for the first time, to reconstruct the life cycle of GW231123-like candidates within the same cosmological simulation, enabling a self-consistent comparison between different formation channels. Although both population synthesis codes can in principle produce black holes compatible with GW231123, we find that isolated binary evolution fails to reproduce the inferred merger redshift. In SEVN, massive black hole binaries form with semi-major axes > 10^3 Rsun , preventing coalescences within a Hubble time. In BSEEMP, candidates arise only at extremely low metallicities (Z = 10^{-10}), which contribute negligibly to the star formation rate density in our overdense simulated volume. Our results therefore strongly support a dynamical hierarchical origin. The observed black hole masses are naturally reproduced through successive mergers in dense globular clusters. The high dimensionless spins reported by the LIGO-Virgo-KAGRA Collaboration are consistent with this hierarchical population. We found a local merger rate density of 0.78 Gpc^{-3} yr^{-1}, with a peak at z = 4 - 6, tracing the maximum formation rate of globular clusters in metal-poor environments (Z = 0.006). Overall, GW231123 may represent a benchmark event for a robust population of hierarchical black holes formed in the early Universe.

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arXiv2026

Direct N-body simulations of rotating and extremely massive Population III star clusters

Aims. We present eight direct N-body simulations with NBODY6++GPU of extremely massive, initially rotating Population III star clusters with 1.01 x 10^5 stars. Methods. Our models include primordial binaries, a continuous initial mass function, differential rotation, tidal mass loss, updated fitting formulae for extremely massive metal-poor Population III stars, and general-relativistic merger recoil kicks. We assess their impact on cluster dynamics. Results. All runs form black holes below, within, and above the pair-instability gap, with multi-generation growth. Faster-rotating clusters core-collapse earlier; post-collapse clusters host a rotating, axisymmetric subsystem of intermediate-mass black holes (IMBHs) at the centre and an expanding halo of lower-mass objects. Pair-instability supernovae and compact-object formation at ~2-3 Myr sharply reduce total mass and a large fraction of the cluster's angular momentum. All Population III clusters in our simulations have the gravothermal-gravogyro catastrophe phase. Conclusions. We confirm two of the hypothesized formation channels of galactic nucleus seed black holes: gravitational runaway mergers of black holes and of Population III stars, which core-collapse into IMBHs thereafter. A higher initial star cluster bulk rotation correlates with earlier core collapse and, in the event counts reported here, with more coalescences and collisions, as well as lower retained (compact) binary abundances. Initial bulk rotation is a primary control parameter of cluster evolution: faster rotation accelerates early angular-momentum transport, gravothermal collapse, mass segregation, and amplifies post-collapse expansion, which also favours the formation of a compact central IMBH subsystem.

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arXiv2026

Discovery of a compact hierarchical triple main-sequence star system while searching for binary stars with compact objects

We have discovered a compact hierarchical triple main-sequence star system, which is cataloged as Gaia DR3 1010268155897156864 or TIC 21502513. Hereafter, we call it ``G1010''. G1010 consists of a primary (the most massive) star and inner binary that orbit each other. The primary star is a $0.85_{-0.03}^{+0.03}\;{\rm M}_\odot$ main-sequence (MS) star, and the inner binary components are $0.63_{-0.02}^{+0.02}$ and $0.61_{-0.02}^{+0.02}\;{\rm M}_\odot$ MS stars. The outer and inner orbital periods are $277.2_{-1.3}^{+1.6}$ and $\sim 18.26$ days, respectively. G1010 is categorized as a single-lined spectroscopic binary, and its orbital solution indicates that G1010 possibly accompanies a massive compact object, such as a neutron star or massive white dwarf. In order to confirm the presence of a massive compact object, we have performed several-times low signal-to-ratio (SNR) and one-time high SNR spectroscopic observations, and determined the outer orbital parameters. Moreover, we have deeply analyzed the high SNR spectroscopic data, and found that G1010 accompanies not a massive compact object, but an inner binary. We have investigated G1010's light curve in Transiting Exoplanet Survey Satellite (TESS), and concluded that the inner binary is actually an eclipsing binary, not included in TESS Eclipsing Binary Stars. We have obtained the inner orbital parameters from the TESS light curve. G1010 is similar to compact hierarchical triple star systems previously discovered by eclipse timing variation analysis. Our discovery has shown that such triple star systems can be discovered by combination of low- and high-SNR spectroscopic observations with the help of Gaia DR3 and the upcoming Gaia DR4/DR5.

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arXiv2025

Cosmic-Ray Bath in a Past Supernova Gives Birth to Earth-Like Planets

A key question in astronomy is how ubiquitous Earth-like rocky planets are. The formation of terrestrial planets in our solar system was strongly influenced by the radioactive decay heat of short-lived radionuclides (SLRs), particularly $^{26}$Al, likely delivered from nearby supernovae. However, current models struggle to reproduce the abundance of SLRs inferred from meteorite analysis without destroying the protosolar disk. We propose the `immersion' mechanism, where cosmic-ray nucleosynthesis in a supernova shockwave reproduces estimated SLR abundances at a supernova distance ($\sim$1 pc), preserving the disk. We estimate that solar-mass stars in star clusters typically experience at least one such supernova within 1 pc, supporting the feasibility of this scenario. This suggests solar-system-like SLR abundances and terrestrial planet formation are more common than previously thought.

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arXiv2025

On the Formation of GW231123 in Population III Star Clusters

GW231123 is a binary black hole merger whose primary component lies within or above the pair-instability mass gap, while the secondary component falls within this gap. The standard theory of stellar evolution is significantly challenged by this event. We investigate the formation of candidate progenitors of GW231123 in Population III (Pop III) star clusters. We find that they could form through stellar mergers, binary black hole mergers, and mixed mergers. The mass distribution of these candidate progenitors covers the component masses of GW231123. Under our model assumptions, their predicted merger rate density spans the range of $0.001-0.26{\rm Gpc^{-3}yr^{-1}}$, encompassing that of GW231123. These findings suggest that GW231123 may originate from Pop III star clusters. Furthermore, such candidate progenitors are expected to be detectable by future gravitational wave detectors LISA/Taiji/TianQin/DECIGO/Cosmic Explorer/Einstein Telescope, which would provide valuable insights into the formation scenarios of events like GW231123.

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arXiv2025

Two unseen massive white dwarf candidates in close binaries

We report the discovery of two binary systems, each consisting of a slightly bloated G-type main-sequence star and an unseen companion, identified through photometric data from TESS and radial velocity variation from Gaia. High-resolution spectroscopy confirms orbital periods of 1.37 and 2 .67 d with circular orbits. The visible components have masses of $\sim$0.9--1.0$\,M_\odot$, while the minimum masses of the unseen companions are $1.078^{+0.058}_{-0.060}\, M_\odot$ and $0.943^{+0.048}_{-0.049}\, M_\odot$, respectively. Assuming tidal synchronization, we estimate the companion masses to be $1.13^{+0.11}_{-0.08} M_\odot$ and $1.05^{+0.15}_{-0.10} M_\odot$. The absence of detectable spectral features from the companions rules out main-sequence stars of these masses, suggesting that the unseen companions are likely O/Ne or C/O massive white dwarfs. The short orbital periods imply that these systems are post-common envelope binaries. Their subsequent evolution is uncertain, with possible outcomes including cataclysmic variables, Type Ia supernovae, or accretion-induced collapse, depending on the nature of future mass transfer.

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arXiv2025

GW231123 Formation from Population III Stars: Isolated Binary Evolution

GW231123 is a merger of two black holes (BHs) with estimated masses exceeding $100\;{\rm M}_{\odot}$, making them the most massive BHs discovered to date via gravitational wave (GW) observations. We investigate whether GW231123-like events can originate from isolated Population (Pop) III binary stars using binary population synthesis calculations. Our findings indicate that isolated Pop III binaries can produce GW231123-like events at a rate sufficient to explain the discovery of GW231123, provided that three conditions are met: (i) Pop III stars evolve with inefficient convective overshooting, (ii) the $^{12}\text{C}(\alpha, \gamma)^{16}\text{O}$ rate is $2\sigma$ lower than the standard value, and (iii) Pop III binary stars share the same orbital parameters as Pop I/II binary stars at the initial time. In contrast, GW190521 -- the most massive BH merger in the Gravitational Wave Transient Catalog 3 -- can be formed from isolated Pop III binaries even with the standard $^{12}\text{C}(\alpha, \gamma)^{16}\text{O}$ rate. We demonstrate that the discovery of GW231123 is increasingly constraining the parameter ranges of single star evolution models, under the assumption that these GW events originate from isolated binary evolution.

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arXiv2025

Double Compact Binary Merger Rate Density in Open Star Clusters: Black Holes, Neutron Stars, and White Dwarfs

Studying compact-object binary mergers in star clusters is crucial for understanding stellar evolution and dynamical interactions in galaxies. Open clusters in particular are more abundant over cosmic time than globular clusters, however, previous research on low-mass clusters with $\lesssim 10^3~\textrm{M}_{\odot}$ has focused on binary black holes (BBHs) or black hole-neutron star (BH-NS) binaries. Binary mergers of other compact objects, such as white dwarfs (WDs), are also crucial as progenitors of transient phenomena such as Type Ia supernovae and Fast Radio Bursts. We present simulations of three types of open clusters with masses of $10^2$, $10^3$, and $10^4~\mathrm{M}_{\odot}$. In massive clusters with $\gtrsim 10^4~\textrm{M}_{\odot}$, BBHs are dynamically formed, however, less massive compact binaries such as WD-WD and WD-NS are perturbed inside the star clusters, causing them to evolve into other objects. We further find BH-NS mergers only in $10^3~\textrm{M}_{\odot}$ clusters. Considering star clusters with a typical open cluster mass, we observe that WD-WD merger rates slightly increase for $10^3~\textrm{M}_{\odot}$ clusters but decrease for $10^2~\textrm{M}_{\odot}$ clusters. Since the host clusters are tidally disrupted, most of them merge outside of the clusters. Our WD-WD merger results have further implications for two classes of transients. Super-Chandrasekhar WD-WD mergers are present in our simulations, demonstrating potential sources of Fast Radio Bursts at a rate of 70-780 Gpc$^{-3}$yr$^{-1}$, higher than the rate estimated for globular clusters. Additionally, we find that Carbon-Oxygen WD-WD mergers in our open clusters (34-640 $\textrm{Gpc}^{-3}$yr$^{-1}$) only account for 0.14-2.6% of the observed Type Ia supernova rate in our local Universe.

astro-ph.SR↗

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

Pair-Instability Gap Black Holes in Population III Star Clusters: Pathways, Dynamics, and Gravitational Wave Implications

The detection of the gravitational wave (GW) event GW190521 raises questions about the formation of black holes within the pair-instability mass gap (PIBHs). We propose that Population III (Pop III) star clusters significantly contribute to events similar to GW190521. We perform $N$-body simulations and find that PIBHs can form from stellar collisions or binary black hole (BBH) mergers, with the latter accounting for 90\% of the contributions. Due to GW recoil during BBH mergers, approximately 10-50% of PIBHs formed via BBH mergers escape from clusters, depending on black hole spins and cluster escape velocities. The remaining PIBHs can participate in secondary and multiple BBH formation events, contributing to GW events. Assuming Pop III stars form in massive clusters (initially 100,000 $M_\odot$) with a top-heavy initial mass function, the average merger rates for GW events involving PIBHs with 0% and 100% primordial binaries are $0.005$ and $0.017$ $\text{yr}^{-1} \text{Gpc}^{-3}$, respectively, with maximum values of $0.030$ and $0.106$ $\text{yr}^{-1} \text{Gpc}^{-3}$. If Pop III stars form in low-mass clusters (initial mass of $1000M_\odot$ and $10000 M_\odot$), the merger rate is comparable with a 100% primordial binary fraction but significantly lower without primordial binaries. We also calculate the characteristic strains of the GW events in our simulations and find that about 43.4% (LISA) 97.8% (Taiji) and 66.4% (Tianqin) of these events could potentially be detected by space-borne detectors, including LISA, Taiji, and TianQin. The next-generation GW detectors such as DECIGO, ET, and CE can nearly cover all these signals.

astro-ph.HE↗

arXiv2025

Unevolved Li-rich stars at low metallicity: a possible formation pathway through novae

A small fraction of low-mass stars have been found to have anomalously high Li abundances. Although it has been suggested that mixing during the red giant branch phase can lead to Li production, this method of intrinsic Li production cannot explain Li-rich stars that have not yet undergone the first dredge-up. To obtain clues about the origin of such stars, we present a detailed chemical abundance analysis of four unevolved Li-rich stars with $-2.1 < [\mathrm{Fe/H}] < -1.3$ and $2.9<A({\rm Li})<3.6$, $0.7-1.4$ dex higher Li abundance than typical unevolved metal-poor stars. One of the stars, Gaia DR3 6334970766103389824 (D25_6334), was serendipitously found in the stellar stream ED-3, and the other three stars have been reported to have massive ($M\gtrsim 1.3\,\mathrm{M_\odot}$) non-luminous companions. We show that three of the four stars exhibit abundance patterns similar to those of known unevolved Li-rich stars, namely normal abundances in most elements except for Li and Na. These abundance similarities suggest a common origin for the unevolved Li-rich stars and low-mass metal-poor stars with massive compact companions. We also made the first detection of N abundance to unevolved Li-rich stars in D25_6334, and found that it is significantly enhanced ($[\mathrm{N/Fe}]=1.3$). The observed abundance pattern of D25_6334, spanning from C to Si, indicates that its surface has been polluted by an intermediate-mass former companion star or a nova system that involves a massive ONe white dwarf. Using a population synthesis model, we show that the nova scenario can lead to the observed level of Li enhancement and also provide an explanation for Li-rich stars without companions and those with massive compact companions.

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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

Compact Binary Formation in Open Star Clusters III: Probability of Binary Black Holes Hidden in Gaia Black Hole Binary

The Gaia mission and its follow-up observations have discovered a few candidates of non-interacting single black holes (BHs) and visible stars, Gaia BH1, BH2, and BH3, collectively called ``astrometric BH binaries''. This paper investigates whether any of these candidates harbor binary BHs (BBHs), namely, whether any such candidates are previously undiscovered ``astrimetric BBH triples''. Focusing on open star clusters, which are promising formation sites of astrometric BH binaries, we estimate the formation rate of astrometric BBH triples through gravitational $N$-body simulations. We find a competitively high formation efficiency of astrometric BBH triples ($\sim 10^{-6} {\rm M}_\odot^{-1}$ or $\sim 10$\% of astrometric BH binaries) in low-metallicity environments but no astrometric BBH triples in solar-metallicity environments. Most of the astrometric BBH triples in our simulations were dynamically stable for $10$ Gyrs, indicating that $\sim10$\% of astrometric BH binary candidates may indeed harbor inner BBHs if they originate from open star clusters in low-metallicity environments. Astrometric BBH triples can be distinguished from astrometric BH binaries through radial velocity follow-up of the tertiary star. According to the statistics of our simulated samples, a small percent of astrometric BH binary candidates should exhibit detectable radial-velocity modulations generated by inner BBHs. Such candidates preferentially exhibit ``outer'' orbital periods of $\gtrsim 10^3$ days and moderately high ``outer'' orbital eccentricities ($\gtrsim 0.7$). Our current result will strongly motivate the search for astrometric BBH triples in the upcoming Gaia Data Release 4 and Gaia Final Data Release.

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arXiv2024

Simulations predict intermediate-mass black hole formation in globular clusters

Intermediate-mass black holes (IMBHs) are those between 100 and 10$^5$ solar masses ($M_{\odot}$); their formation process is debated. One possible origin is the growth of less massive black holes (BHs) via mergers with stars and compact objects within globular clusters (GCs). However, previous simulations have indicated that this process only produces IMBHs $<500 M_{\odot}$ because the gravitational wave recoil ejects them when they merge with other BHs. We perform star-by-star simulations of GC formation, finding that high-density star formation in a GC's parent giant molecular cloud can produce sufficient mergers of massive stars to overcome that mass threshold. We conclude that GCs can form with IMBHs $\gtrsim 10^3 M_{\odot}$, which is sufficiently massive to be retained within the GC even with the expected gravitational wave recoil.

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arXiv2024

Compact Binary Formation in Open Star Clusters II: Difficulty of Gaia NS formation in low-mass star clusters

Gaia mission offers opportunities to search for compact binaries not involved in binary interactions (hereafter inert compact binaries), and results in the discoveries of binaries containing one black hole (BH) or one neutron star (NS), called "Gaia BHs" and "Gaia NSs", respectively. We have assessed if Gaia BHs and NSs can be formed in open clusters through dynamical interactions. In order to obtain a large number of inert compact binaries similar to Gaia BHs and NSs, we have performed gravitational $N$-body simulations for a large number of open clusters whose total mass is $1.2 \times 10^8 M_\odot$. These clusters have various masses, metallicities, densities, and binary fractions. We have found that open clusters form Gaia BHs ($10^{-6}$-$10^{-5} M_\odot^{-1}$) much more efficiently than Gaia NSs ($\lesssim 10^{-7} M_\odot^{-1}$) for any cluster parameters. This is quite inconsistent with observational results, because the reported numbers of Gaia BHs and NSs are $3$ and $21$, respectively. Additionally, we have switched off NS natal kicks for $10^4$ open clusters each weighing $10^3 M_\odot$ in order to retain a large number of NSs in open clusters. Then, open clusters form inert NS binaries originating from primordial binaries rather than formed through dynamical interactions. This means that Gaia NSs are formed dominantly on isolated fields, not in open clusters, if there is no NS natal kick. We have concluded that Gaia BHs can be dominantly formed in open clusters, however Gaia NSs cannot.

astro-ph.SR↗

arXiv2024

Contribution of Population III Stars to Merging Binary Black Holes

A large number of mergers of binary black holes (BHs) have been discovered by gravitational wave observations since the first detection of gravitational waves 2015. Binary BH mergers are the loudest events in the universe, however their origin(s) have been under debate. There have been many suggestions for merging binary BHs. Isolated binary stars are one of the most promising origins. We have investigated the evolution of isolated binary stars ranging from zero metallicity (Population III stars or Pop III stars) to the solar metallicity by means of so-called rapid binary population synthesis simulation. We have found that binary BHs formed from isolated binary stars reproduce the redshift evolution of the merger rate density and the distribution of primary BH masses and mass ratios inferred by Gravitational-Wave Transient Catalog 3 (GWTC-3). Pop III stars have a crucial role in forming merging binary BHs in so-called the pair instability mass gap. Note that we choose the conventional prescription of pair instability mass loss, based on the standard $^{12}$C($\alpha$,$\gamma$)$^{16}$O reaction rate. Finally, we have shown the redshift evolution of the rate density of pair instability supernovae, and have predicted that a few pair instability supernovae would be discovered in the next few years. The discoveries would validate our results of merging binary BHs.

astro-ph.HE↗

arXiv2023

Merging Hierarchical Triple Black Hole Systems with Intermediate-mass Black Holes in Population III Star Clusters

Theoretical predictions suggest that very massive stars have the potential to form through multiple collisions and eventually evolve into intermediate-mass black holes (IMBHs) within Population III star clusters embedded in mini dark matter haloes. In this study, we investigate the long-term evolution of Population III star clusters, including models with a primordial binary fraction of $f_{\rm b}=0$ and 1, using the $N$-body simulation code PETAR. We comprehensively examine the phenomenon of hierarchical triple black holes in the clusters, specifically focusing on their merging inner binary black holes (BBHs), with post-Newtonian correction, by using the TSUNAMI code. Our findings suggest a high likelihood of the inner BBHs containing IMBHs with masses on the order of $\mathcal{O}(100)M_{\odot}$, and as a result, their merger rate could be up to $0.1{\rm Gpc}^{-3}{\rm yr}^{-3}$. The orbital eccentricities of some merging inner BBHs oscillate over time periodically, known as the Kozai-Lidov oscillation, due to dynamical perturbations. Detectable merging inner BBHs for mHz GW detectors LISA/TianQin/Taiji concentrate within $z<3$. More distant sources would be detectable for CE/ET/LIGO/KAGRA/DECIGO, which are sensitive from $\mathcal{O}(0.1)$Hz to $\mathcal{O}(100)$Hz. Furthermore, compared with merging isolated BBHs, merging inner BBHs affected by dynamical perturbations from tertiary BHs tend to have higher eccentricities, with a significant fraction of sources with eccentricities closing to 1 at mHz bands. GW observations would help constrain formation channels of merging BBHs, whether through isolated evolution or dynamical interaction, by examining eccentricities.

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