Showing 1–50 of 95 results for author: Fujii, M

Bursty star formation, chemical enrichment, and star cluster formation in numerical analogues of GN-z11

Authors: Takayuki R. Saitoh, Yutaka Hirai, Michiko S. Fujii, Yuki Isobe

Abstract: The James Webb Space Telescope reveals anomalous nitrogen enrichment (high N/O ratios) in compact, star-forming galaxies, such as GN-z11 at $z\sim10$. The origin of this chemical signature provides an insight into the early star and galaxy formation processes, yet remains unclear. We performed high-resolution cosmological zoom-in simulations of massive galaxies at high redshift ($z\sim10$) in rare… ▽ More The James Webb Space Telescope reveals anomalous nitrogen enrichment (high N/O ratios) in compact, star-forming galaxies, such as GN-z11 at $z\sim10$. The origin of this chemical signature provides an insight into the early star and galaxy formation processes, yet remains unclear. We performed high-resolution cosmological zoom-in simulations of massive galaxies at high redshift ($z\sim10$) in rare density peaks, incorporating various chemical evolution channels including stellar winds, core-collapse, Type Ia supernovae, and asymptotic giant branch stars. Our simulations reproduce several key features of high-redshift galaxies: (1) stars form with high efficiencies ($>0.1$) at the center of rare peak halos, creating very compact galaxies similar to GN-z11; (2) high N/O ratios emerge during the first 10-20 Myr of intense starburst, before being diluted by CCSNe; (3) multiple star clusters form in and around the galaxy with high efficiency ($\sim20\%$), some of which exhibit high N/O ratios and sodium-oxygen anti-correlations similar to those observed in local globular clusters. Although our simulations can reproduce the high log(N/O) values (up to -0.61, exceeding the solar value by 0.25 dex), they remain below the observational lower limits of GN-z11, indicating room for improvement through additional chemical evolution channels, such as supermassive stars. △ Less

Submitted 28 November, 2025; originally announced November 2025.

Comments: 25 pages, 22 figures, submitted to PASJ

The First Star-by-star $N$-body/Hydrodynamics Simulation of Our Galaxy Coupling with a Surrogate Model

Authors: Keiya Hirashima, Michiko S. Fujii, Takayuki R. Saitoh, Naoto Harada, Kentaro Nomura, Kohji Yoshikawa, Yutaka Hirai, Tetsuro Asano, Kana Moriwaki, Masaki Iwasawa, Takashi Okamoto, Junichiro Makino

Abstract: A major goal of computational astrophysics is to simulate the Milky Way Galaxy with sufficient resolution down to individual stars. However, the scaling fails due to some small-scale, short-timescale phenomena, such as supernova explosions. We have developed a novel integration scheme of $N$-body/hydrodynamics simulations working with machine learning. This approach bypasses the short timesteps ca… ▽ More A major goal of computational astrophysics is to simulate the Milky Way Galaxy with sufficient resolution down to individual stars. However, the scaling fails due to some small-scale, short-timescale phenomena, such as supernova explosions. We have developed a novel integration scheme of $N$-body/hydrodynamics simulations working with machine learning. This approach bypasses the short timesteps caused by supernova explosions using a surrogate model, thereby improving scalability. With this method, we reached 300 billion particles using 148,900 nodes, equivalent to 7,147,200 CPU cores, breaking through the billion-particle barrier currently faced by state-of-the-art simulations. This resolution allows us to perform the first star-by-star galaxy simulation, which resolves individual stars in the Milky Way Galaxy. The performance scales over $10^4$ CPU cores, an upper limit in the current state-of-the-art simulations using both A64FX and X86-64 processors and NVIDIA CUDA GPUs. △ Less

Submitted 27 October, 2025; originally announced October 2025.

Comments: 12 pages, 7 figures, 7 tables, IEEE/ACM Supercomputing Conference (SC25)

Report number: RIKEN-iTHEMS-Report-25

On the Formation of GW231123 in Population III Star Clusters

Authors: Shuai Liu, Long Wang, Ataru Tanikawa, Weiwei Wu, Michiko S. Fujii

Abstract: 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 st… ▽ More 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. △ Less

Submitted 2 November, 2025; v1 submitted 7 October, 2025; originally announced October 2025.

Comments: 8 pages, 4 figures, and 1 table. Published in ApJL

Journal ref: ApJL, 993 (2025) L30

Little Red Dots Are Nurseries of Massive Black Holes

Authors: Fabio Pacucci, Lars Hernquist, Michiko Fujii

Abstract: The James Webb Space Telescope (JWST) has revealed a previously unknown population of compact, red galaxies at $z \sim 5$, known as "Little Red Dots" (LRDs). With effective radii of $\sim 100$ pc and stellar masses of $10^9-10^{11} \, M_\odot$, a purely stellar interpretation implies extreme central densities, $ρ_\star\sim10^4-10^5 \, M_\odot \, \mathrm{pc}^{-3}$ and in some cases up to… ▽ More The James Webb Space Telescope (JWST) has revealed a previously unknown population of compact, red galaxies at $z \sim 5$, known as "Little Red Dots" (LRDs). With effective radii of $\sim 100$ pc and stellar masses of $10^9-10^{11} \, M_\odot$, a purely stellar interpretation implies extreme central densities, $ρ_\star\sim10^4-10^5 \, M_\odot \, \mathrm{pc}^{-3}$ and in some cases up to $\sim 10^9 \, M_\odot \, \mathrm{pc}^{-3}$, far exceeding those of globular clusters. At such densities, the dynamical friction time for $10 \, M_\odot$ stars in the central $0.1$ pc is $< 0.1$ Myr, driving rapid mass segregation. We investigate the dynamical consequences of such an environment using: (i) a Fokker-Planck analysis of long-term core evolution, (ii) an analytical model for the collisional growth of a very massive star (VMS), and (iii) direct $N$-body simulations. All approaches show that runaway collisions produce a VMS with mass $9\times10^3 < M_{\rm VMS} \, [M_\odot] < 5\times10^4$ within $<1$ Myr. Once the supply of massive stars is depleted, the VMS contracts on a $\sim 8000$ yr Kelvin-Helmholtz timescale and undergoes a general relativistic collapse, leaving a massive black hole of mass $M_\bullet \sim 10^4 \, M_\odot$. We conclude that LRDs are natural nurseries for the formation of heavy black hole seeds via stellar-dynamical processes. This pathway produces seed number densities that far exceed those expected from direct collapse models, and, owing to the dense residual stellar core, can sustain high rates of tidal disruption events. △ Less

Submitted 21 October, 2025; v1 submitted 2 September, 2025; originally announced September 2025.

Comments: Accepted for publication in The Astrophysical Journal. 12 pages, 4 figures

Journal ref: The Astrophysical Journal, 2025, Volume 994, Number 40

GW231123 Formation from Population III Stars: Isolated Binary Evolution

Authors: Ataru Tanikawa, Shuai Liu, WeiWei Wu, Michiko S. Fujii, Long Wang

Abstract: GW231123 is a merger of two black holes (BHs) whose inferred masses exceed $100\;{\rm M}_\odot$ typically; they are the most massive BHs among those discovered by gravitational wave (GW) observations. We examine if GW231123-like events can be formed from isolated Population (Pop) III binary stars by means of binary population synthesis calculations. We find that Pop III isolated binary stars can c… ▽ More GW231123 is a merger of two black holes (BHs) whose inferred masses exceed $100\;{\rm M}_\odot$ typically; they are the most massive BHs among those discovered by gravitational wave (GW) observations. We examine if GW231123-like events can be formed from isolated Population (Pop) III binary stars by means of binary population synthesis calculations. We find that Pop III isolated binary stars can create GW231123-like events at a rate large enough to explain the discovery of GW231123, if two conditions are satisfied. First, Pop III stars evolve with inefficient convective overshooting, and second the $^{12}{\rm C}(α,γ)^{16}{\rm O}$ rate is $2σ$ lower than the standard value. On the other hand, GW190521, which is the most massive BHs in Gravitational Wave Transient Catalog 3, can be formed from isolated Pop III binary stars even if the $^{12}{\rm C}(α,γ)^{16}{\rm O}$ rate is the standard value. We reveal that the discovery of GW231123 is progressively putting constraints on possible parameter ranges of single star evolution models, assuming that all the GW events are formed through isolated binary evolution. △ Less

Submitted 1 August, 2025; originally announced August 2025.

Comments: 7 pages, 3 figures, 1 table

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

Authors: Savannah Cary, Michiko Fujii, Long Wang, Ataru Tanikawa

Abstract: 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) binari… ▽ More 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. △ Less

Submitted 21 August, 2025; v1 submitted 27 June, 2025; originally announced June 2025.

Comments: 14 pages, 7 figures

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

Authors: Weiwei Wu, Long Wang, Shuai Liu, Yining Sun, Ataru Tanikawa, Michiko Fujii

Abstract: 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 l… ▽ More 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. △ Less

Submitted 28 April, 2025; originally announced April 2025.

Comments: 19 pages, 13 figures, accepted for publication in ApJ

Dynamics of Star Cluster Formation: The Effects of Ongoing Star Formation and Stellar Feedback

Authors: Jeremy Karam, Michiko S. Fujii, Alison Sills

Abstract: We perform a high resolution zoom-in simulation of star cluster assembly including the merger of two sub-clusters with initial conditions taken from previous large scale giant molecular cloud (GMC) simulations. We couple hydrodynamics to N-body dynamics to simulate the individual stars themselves, and the gas-rich environment in which they evolve. We include prescriptions for star formation and st… ▽ More We perform a high resolution zoom-in simulation of star cluster assembly including the merger of two sub-clusters with initial conditions taken from previous large scale giant molecular cloud (GMC) simulations. We couple hydrodynamics to N-body dynamics to simulate the individual stars themselves, and the gas-rich environment in which they evolve. We include prescriptions for star formation and stellar feedback and compare directly to previous simulations of the same region without these prescriptions to determine their role in shaping the dynamics inherited from the cluster assembly process. The stellar mass of the cluster grows through star formation within the cluster and accretion of new stars and star forming gas from a nearby filament. This growth results in an enhancement in the cluster's rotation and anisotropic expansion compared to simulations without star formation. We also analyze the internal kinematics of the cluster once it has lost most of its gas and find that the rotational velocity and the velocity anisotropy profiles are qualitatively similar to those expected of clusters that have undergone violent relaxation. As well, rotation and anisotropic expansion are still present by the time of gas removal. This implies that evolution within the GMC was unable to completely erase the kinematics inherited by the merger. △ Less

Submitted 2 July, 2025; v1 submitted 27 March, 2025; originally announced March 2025.

Comments: Accepted for publication in The Astrophysical Journal, 14 pages, 11 figures

Spectral evolution of the narrow emission line components in optical during the 2022 nova eruption of U Scorpii

Authors: Katsuki Muraoka, Naoto Kojiguchi, Junpei Ito, Daisaku Nogami, Taichi Kato, Yusuke Tampo, Kenta Taguchi, Keisuke Isogai, Arthur Leduc, Hamish Barker, Terry Bohlsen, Raul Bruzzone, Forrest Sims, James Foster, Mitsugu Fujii, Keith Shank, Pavol A. Dubovsky, Paolo Cazzato, Stéphane Charbonnel, Olivier Garde, Pascal le Dû, Lionel Mulato, Thomas Petit

Abstract: There remains debate over whether the accretion disk survives or is entirely disrupted after the nova eruption. In our previous paper, Muraoka et al. (2024, PASJ, 76, 293) have photometrically demonstrated that the surviving accretion disk was expanded close to the L1 point during the optical plateau stage and then drastically shrank to the tidal truncation radius after the optical plateau stage e… ▽ More There remains debate over whether the accretion disk survives or is entirely disrupted after the nova eruption. In our previous paper, Muraoka et al. (2024, PASJ, 76, 293) have photometrically demonstrated that the surviving accretion disk was expanded close to the L1 point during the optical plateau stage and then drastically shrank to the tidal truncation radius after the optical plateau stage ended. To approach the clarification of the physical mechanism that drives these structural changes, we have then conducted systematic analyses of the spectral evolution of the narrow emission line components in optical over 22 d following the optical peak during the 2022 nova eruption of U Scorpii (U Sco). Additionally, we present its optical spectrum in quiescence 794 d after the 2022 nova eruption. We find that the single-peaked narrow components of H$α$ and He II 4686 appeared almost simultaneously between roughly days 6 and 8, preceding the onset of the disk eclipses observed after day 11. This finding suggests that the nova wind near the binary system may be the primary origin of these narrow components and even remained active several days after the nova eruption with a velocity of approximately 1000 km s$^{-1}$, likely driving the expansion of the accretion disk until the end of the optical plateau stage. While the contribution of the rotating accretion disk might be dominated by that of the nova wind in the H$α$ line profile, the outward surface flow from the expanded disk might also contribute to these narrow features during the optical plateau stage, making the single-peaked narrow line profiles more pronounced. △ Less

Submitted 26 March, 2025; originally announced March 2025.

Comments: 11 pages, 6 figures, 2 tables, accepted for publication in PASJ

MSC Class: 85-11

Ripples spreading across the Galactic disc. Interplay of direct and indirect effects of the Sagittarius dwarf impact

Authors: Tetsuro Asano, Michiko S. Fujii, Junichi Baba, Simon Portegies Zwart, Jeroen Bédorf

Abstract: Gaia data have revealed vertically asymmetric phase-space structures in the Milky Way (MW) disc, such as phase spirals, indicating vertical oscillations. These oscillations exhibit two distinct modes: the bending mode and the breathing mode, associated with one-arm and two-arm phase spirals, respectively. This study aims to explore the excitation mechanisms of the bending and breathing modes and t… ▽ More Gaia data have revealed vertically asymmetric phase-space structures in the Milky Way (MW) disc, such as phase spirals, indicating vertical oscillations. These oscillations exhibit two distinct modes: the bending mode and the breathing mode, associated with one-arm and two-arm phase spirals, respectively. This study aims to explore the excitation mechanisms of the bending and breathing modes and their subsequent evolution in the MW disc, focusing on the interplay between direct perturbations from the Sagittarius dwarf galaxy and indirect contributions from tidally induced spiral arms. We perform high-resolution $N$-body simulations to model the interaction between an MW-like disc galaxy and a Sagittarius dwarf-like satellite. These simulations resolve fine phase-space structures, enabling analysis of the bending and breathing modes at both macroscopic (global bending and breathing waves) and microscopic (local phase spirals) scales. Our simulations demonstrate that the satellite's perturbation directly excites the bending mode and induces spiral arms in the galactic disc. These spiral arms excite the breathing mode, making it an indirect consequence of the satellite interaction. Initially, the bending mode dominates, but it rapidly decays due to horizontal mixing. In contrast, the breathing mode persists for a longer duration, sustained by the spiral arms, leading to a transition from a bending-dominated to a breathing-dominated state. This transition progresses faster in the inner galaxy than in the outer regions. The simulations reproduce the one-arm phase spiral observed in the solar neighbourhood and reveal two-arm phase spirals, particularly in the inner galaxy, associated with spiral arm-induced breathing modes. Our findings highlight the combined effects of direct satellite perturbations and indirect spiral arm dynamics in shaping the vertical structure of the MW disc. △ Less

Submitted 21 July, 2025; v1 submitted 21 January, 2025; originally announced January 2025.

Comments: 23 pages, 16 figures, A&A accepted

Journal ref: A&A 700, A109 (2025)

Discovery of dual "little red dots" indicates excess clustering on kilo-parsec scales

Authors: Takumi S. Tanaka, John D. Silverman, Kazuhiro Shimasaku, Junya Arita, Hollis B. Akins, Kohei Inayoshi, Xuheng Ding, Masafusa Onoue, Zhaoxuan Liu, Caitlin M. Casey, Erini Lambrides, Vasily Kokorev, Shuowen Jin, Andreas L. Faisst, Nicole Drakos, Yue Shen, Junyao Li, Mingyang Zhuang, Qinyue Fei, Kei Ito, Wenke Ren, Suin Matsui, Makoto Ando, Shun Hatano, Michiko S. Fujii , et al. (22 additional authors not shown)

Abstract: ``Little Red Dots'' (LRDs) are an abundant high-redshift population newly discovered by the James Webb Space Telescope (JWST). They are characterized by a red color in the rest-frame optical band, compact morphology, and broad Balmer emission lines (${\rm FWHM} \gtrsim 1000~{\rm km\,s^{-1}}$) that suggest an AGN nature. Using a method of pixel-by-pixel color selection and relaxing the compactness… ▽ More ``Little Red Dots'' (LRDs) are an abundant high-redshift population newly discovered by the James Webb Space Telescope (JWST). They are characterized by a red color in the rest-frame optical band, compact morphology, and broad Balmer emission lines (${\rm FWHM} \gtrsim 1000~{\rm km\,s^{-1}}$) that suggest an AGN nature. Using a method of pixel-by-pixel color selection and relaxing the compactness criteria, we identify three of the first dual LRD candidates in the COSMOS-Web survey with projected separations of $0.\!\!^{\prime\prime}2-0.\!\!^{\prime\prime}4$ (1-2 pkpc at their photometric redshifts). A comparison between existing LRD samples and mock data reveals that the projected separations of these dual LRD candidates are unlikely to result from chance projections of objects at different redshifts. In one case (CW-B5-15958), the dual LRD includes two bright sources ($m_{\rm F444W}=24.3$ and $24.8$) with characteristic V-shape spectral energy distribution (SEDs) and photometric redshifts consistent with each other. We find that CW-B5-15958 has a faint off-centered component and a companion galaxy. In the other two dual systems, the brighter LRD exhibits a V-shape SED, while the fainter LRD ($m_{\rm F444W}\gtrsim26$) is undetected in both F115W and F150W. These discoveries suggest that the angular auto-correlation function (ACF) of LRDs exhibits a significant excess ($\sim3\times10^2$ times) on sub-arcsec (kilo-parsec) separations compared to the extrapolation of a power-law ACF of JWST-found AGNs measured over $10^{\prime\prime}-100^{\prime\prime}$. Follow-up spectroscopic confirmation of their redshifts and the construction of a larger sample are essential to advance our understanding of the evolution of supermassive black holes and the importance of mergers in the early universe. △ Less

Submitted 18 December, 2024; originally announced December 2024.

Comments: 12 pages, 5 figures, and 1 table. Comments are welcome

SIRIUS: Identifying Metal-poor Stars Enriched by a Single Supernova in a Dwarf Galaxy Cosmological Zoom-in Simulation Resolving Individual Massive Stars

Authors: Yutaka Hirai, Takayuki R. Saitoh, Michiko S. Fujii, Katsuhiro Kaneko, Timothy C. Beers

Abstract: Metal-poor stars enriched by a single supernova (mono-enriched stars) are direct proof (and provide valuable probes) of supernova nucleosynthesis. Photometric and spectroscopic observations have shown that metal-poor stars have a wide variety of chemical compositions; the star's chemical composition reflects the nucleosynthesis process(es) that occurred before the star's formation. While the ident… ▽ More Metal-poor stars enriched by a single supernova (mono-enriched stars) are direct proof (and provide valuable probes) of supernova nucleosynthesis. Photometric and spectroscopic observations have shown that metal-poor stars have a wide variety of chemical compositions; the star's chemical composition reflects the nucleosynthesis process(es) that occurred before the star's formation. While the identification of mono-enriched stars enables us to study the ejecta properties of a single supernova, the fraction of mono-enriched stars among metal-poor stars remains unknown. Here we identify mono-enriched stars in a dwarf galaxy cosmological zoom-in simulation resolving individual massive stars. We find that the fraction of mono-enriched stars is higher for lower metallicity, stars with [Fe/H] $< -2.5$. The percentages of mono-enriched stars are 11% at [Fe/H] = $-$5.0 and 1% at [Fe/H] = $-$2.5, suggesting that most metal-poor stars are affected by multiple supernovae. We also find that mono-enriched stars tend to be located near the center of the simulated dwarf. Such regions will be explored in detail in upcoming surveys such as the Prime Focus Spectrograph (PFS) on the Subaru telescope. △ Less

Submitted 27 January, 2025; v1 submitted 27 November, 2024; originally announced November 2024.

Comments: 8 pages, 5 figures, accepted for publication in ApJL

ASURA-FDPS-ML: Star-by-star Galaxy Simulations Accelerated by Surrogate Modeling for Supernova Feedback

Authors: Keiya Hirashima, Kana Moriwaki, Michiko S. Fujii, Yutaka Hirai, Takayuki R. Saitoh, Junnichiro Makino, Ulrich P. Steinwandel, Shirley Ho

Abstract: We introduce new high-resolution galaxy simulations accelerated by a surrogate model that reduces the computation cost by approximately 75 percent. Massive stars with a Zero Age Main Sequence mass of more than about 10 $\mathrm{M_\odot}$ explode as core-collapse supernovae (CCSNe), which play a critical role in galaxy formation. The energy released by CCSNe is essential for regulating star formati… ▽ More We introduce new high-resolution galaxy simulations accelerated by a surrogate model that reduces the computation cost by approximately 75 percent. Massive stars with a Zero Age Main Sequence mass of more than about 10 $\mathrm{M_\odot}$ explode as core-collapse supernovae (CCSNe), which play a critical role in galaxy formation. The energy released by CCSNe is essential for regulating star formation and driving feedback processes in the interstellar medium (ISM). However, the short integration timesteps required for SNe feedback have presented significant bottlenecks in astrophysical simulations across various scales. Overcoming this challenge is crucial for enabling star-by-star galaxy simulations, which aim to capture the dynamics of individual stars and the inhomogeneous shell's expansion within the turbulent ISM. To address this, our new framework combines direct numerical simulations and surrogate modeling, including machine learning and Gibbs sampling. The star formation history and the time evolution of outflow rates in the galaxy match those obtained from resolved direct numerical simulations. Our new approach achieves high-resolution fidelity while reducing computational costs, effectively bridging the physical scale gap and enabling multi-scale simulations. △ Less

Submitted 7 May, 2025; v1 submitted 30 October, 2024; originally announced October 2024.

Comments: 22 pages, 15 figures, 3 tables, accepted for publication in ApJ

Report number: RIKEN-iTHEMS-Report-25

Gravitational Wave Astronomy With TianQin

Authors: En-Kun Li, Shuai Liu, Alejandro Torres-Orjuela, Xian Chen, Kohei Inayoshi, Long Wang, Yi-Ming Hu, Pau Amaro-Seoane, Abbas Askar, Cosimo Bambi, Pedro R. Capelo, Hong-Yu Chen, Alvin J. K. Chua, Enrique Condés-Breña, Lixin Dai, Debtroy Das, Andrea Derdzinski, Hui-Min Fan, Michiko Fujii, Jie Gao, Mudit Garg, Hongwei Ge, Mirek Giersz, Shun-Jia Huang, Arkadiusz Hypki , et al. (28 additional authors not shown)

Abstract: 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 sig… ▽ More 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. △ Less

Submitted 2 December, 2024; v1 submitted 29 September, 2024; originally announced September 2024.

Comments: TianQin Gravitational Wave Whitepaper, 72 pages, 30 figures

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

Authors: Ataru Tanikawa, Long Wang, Michiko S. Fujii, Alessandro A. Trani, Toshinori Hayashi, Yasushi Suto

Abstract: 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… ▽ More 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. △ Less

Submitted 20 June, 2025; v1 submitted 4 July, 2024; originally announced July 2024.

Comments: 25 pages, 22 figures, accepted for publication in OJAp

Simulations predict intermediate-mass black hole formation in globular clusters

Authors: Michiko S. Fujii, Long Wang, Ataru Tanikawa, Yutaka Hirai, Takayuki R. Saitoh

Abstract: 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 gravitatio… ▽ More 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. △ Less

Submitted 10 June, 2024; originally announced June 2024.

Comments: Published online on 30 May 2024 in Science. Main (15 pages, 4 figures) and Supplementary materials (19 pages, 10 figures and 4 tables). The accepted version

Unravelling the asphericities in the explosion and multi-faceted circumstellar matter of SN 2023ixf

Authors: Avinash Singh, R. S. Teja, T. J. Moriya, K. Maeda, K. S. Kawabata, M. Tanaka, R. Imazawa, T. Nakaoka, A. Gangopadhyay, M. Yamanaka, V. Swain, D. K. Sahu, G. C. Anupama, B. Kumar, R. M. Anche, Y. Sano, A. Raj, V. K. Agnihotri, V. Bhalerao, D. Bisht, M. S. Bisht, K. Belwal, S. K. Chakrabarti, M. Fujii, T. Nagayama , et al. (11 additional authors not shown)

Abstract: We present a detailed investigation of photometric, spectroscopic, and polarimetric observations of the Type II SN 2023ixf. Earlier studies have provided compelling evidence for a delayed shock breakout from a confined dense circumstellar matter (CSM) enveloping the progenitor star. The temporal evolution of polarization in SN~2023ixf revealed three distinct peaks in polarization evolution at 1.4… ▽ More We present a detailed investigation of photometric, spectroscopic, and polarimetric observations of the Type II SN 2023ixf. Earlier studies have provided compelling evidence for a delayed shock breakout from a confined dense circumstellar matter (CSM) enveloping the progenitor star. The temporal evolution of polarization in SN~2023ixf revealed three distinct peaks in polarization evolution at 1.4 d, 6.4 d, and 79.2 d, indicating an asymmetric dense CSM, an aspherical shock front and clumpiness in the low-density extended CSM, and an aspherical inner ejecta/He-core. SN 2023ixf displayed two dominant axes, one along the CSM-outer ejecta and the other along the inner ejecta/He-core, showcasing the independent origin of asymmetry in the early and late evolution. The argument for an aspherical shock front is further strengthened by the presence of a high-velocity broad absorption feature in the blue wing of the Balmer features in addition to the P-Cygni absorption post 16 d. Hydrodynamical light curve modeling indicated a progenitor of 10 solar mass with a radius of 470 solar radii and explosion energy of 2e51 erg, along with 0.06 solar mass of 56-Ni, though these properties are not unique due to modeling degeneracies. The modeling also indicated a two-zone CSM: a confined dense CSM extending up to 5e14 cm, with a mass-loss rate of 1e-2 solar mass per year, and an extended CSM spanning from 5e14 cm to at least 1e16cm with a mass-loss rate of 1e-4 solar mass per year, both assuming a wind-velocity of 10 km/s. The early nebular phase observations display an axisymmetric line profile of [OI], red-ward attenuation of the emission of Halpha post 125 days, and flattening in the Ks-band, marking the onset of dust formation. △ Less

Submitted 3 September, 2024; v1 submitted 31 May, 2024; originally announced May 2024.

Comments: 32 pages, 15 figures, 1 Table, Accepted in the Astrophysical Journal

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

Authors: Ataru Tanikawa, Long Wang, Michiko S. Fujii

Abstract: 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 obt… ▽ More 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. △ Less

Submitted 16 May, 2024; v1 submitted 2 April, 2024; originally announced April 2024.

Comments: 11 pages, 8 figures, accepted for publication in The Open Journal of Astrophysics

Clues to growth and disruption of two neighbouring spiral arms of the Milky Way

Authors: Natsuki Funakoshi, Noriyuki Matsunaga, Daisuke Kawata, Junichi Baba, Daisuke Taniguchi, Michiko Fujii

Abstract: Studying the nature of spiral arms is essential for understanding the formation of the intricate disc structure of the Milky Way. The European Space Agency's Gaia mission has provided revolutionary observational data that have uncovered detailed kinematical features of stars in the Milky Way. However, so far the nature of spiral arms continues to remain a mystery. Here we present that the stellar… ▽ More Studying the nature of spiral arms is essential for understanding the formation of the intricate disc structure of the Milky Way. The European Space Agency's Gaia mission has provided revolutionary observational data that have uncovered detailed kinematical features of stars in the Milky Way. However, so far the nature of spiral arms continues to remain a mystery. Here we present that the stellar kinematics traced by the classical Cepheids around the Perseus and Outer spiral arms in the Milky Way shows strikingly different kinematical properties from each other: the radial and azimuthal velocities of Cepheids with respect to the Galactic centre show positive and negative correlations in the Perseus and Outer arms, respectively. We also found that the dynamic spiral arms commonly seen in an N-body/hydrodynamics simulation of a Milky Way-like galaxy can naturally explain the observed kinematic trends. Furthermore, a comparison with such a simulation suggests that the Perseus arm is being disrupted while the Outer arm is growing. Our findings suggest that two neighbouring spiral arms in distinct evolutionary phases - growing and disrupting phases - coexist in the Milky Way. △ Less

Submitted 27 August, 2024; v1 submitted 23 January, 2024; originally announced January 2024.

Comments: 10 pages, 10 figures; accepted for publication in MNRAS

Surrogate Modeling for Computationally Expensive Simulations of Supernovae in High-Resolution Galaxy Simulations

Authors: Keiya Hirashima, Kana Moriwaki, Michiko S. Fujii, Yutaka Hirai, Takayuki R. Saitoh, Junichiro Makino, Shirley Ho

Abstract: Some stars are known to explode at the end of their lives, called supernovae (SNe). The substantial amount of matter and energy that SNe release provides significant feedback to star formation and gas dynamics in a galaxy. SNe release a substantial amount of matter and energy to the interstellar medium, resulting in significant feedback to star formation and gas dynamics in a galaxy. While such fe… ▽ More Some stars are known to explode at the end of their lives, called supernovae (SNe). The substantial amount of matter and energy that SNe release provides significant feedback to star formation and gas dynamics in a galaxy. SNe release a substantial amount of matter and energy to the interstellar medium, resulting in significant feedback to star formation and gas dynamics in a galaxy. While such feedback has a crucial role in galaxy formation and evolution, in simulations of galaxy formation, it has only been implemented using simple {\it sub-grid models} instead of numerically solving the evolution of gas elements around SNe in detail due to a lack of resolution. We develop a method combining machine learning and Gibbs sampling to predict how a supernova (SN) affects the surrounding gas. The fidelity of our model in the thermal energy and momentum distribution outperforms the low-resolution SN simulations. Our method can replace the SN sub-grid models and help properly simulate un-resolved SN feedback in galaxy formation simulations. We find that employing our new approach reduces the necessary computational cost to $\sim$ 1 percent compared to directly resolving SN feedback. △ Less

Submitted 14 November, 2023; originally announced November 2023.

Comments: 11 pages, 9 figures, Accepted for the NeurIPS 2023 AI4Science Workshop

Growing Local arm inferred by the breathing motion

Authors: Tetsuro Asano, Daisuke Kawata, Michiko S. Fujii, Junichi Baba

Abstract: Theoretical models of spiral arms suggest that the spiral arms provoke a vertical bulk motion in disc stars. By analysing the breathing motion, a coherent asymmetric vertical motion around the mid-plane of the Milky Way disc, with $\textit{Gaia}$ DR3, we found that a compressing breathing motion presents along the Local arm. On the other hand, with an $N$-body simulation of an isolated Milky Way-l… ▽ More Theoretical models of spiral arms suggest that the spiral arms provoke a vertical bulk motion in disc stars. By analysing the breathing motion, a coherent asymmetric vertical motion around the mid-plane of the Milky Way disc, with $\textit{Gaia}$ DR3, we found that a compressing breathing motion presents along the Local arm. On the other hand, with an $N$-body simulation of an isolated Milky Way-like disc galaxy, we found that the transient and dynamic spiral arms induce compressing breathing motions when the arms are in the growth phase, while the expanding breathing motion appears in the disruption phase. The observed clear alignment of the compressing breathing motion with the Local arm is similar to what is seen in the growth phase of the simulated spiral arms. Hence, we suggest that the Local arm's compressing breathing motion can be explained by the Local arm being in the growth phase of a transient and dynamic spiral arm. We also identified the tentative signatures of the expanding breathing motion associated with the Perseus arm and also the Outer arm coinciding with the compressing breathing motion. This may infer that the Perseus and Outer arms are in the disruption and growth phases, respectively. △ Less

Submitted 28 December, 2023; v1 submitted 3 October, 2023; originally announced October 2023.

Comments: 7 pages, 4 figures, published in MNRAS Letters

JASMINE: Near-Infrared Astrometry and Time Series Photometry Science

Authors: Daisuke Kawata, Hajime Kawahara, Naoteru Gouda, Nathan J. Secrest, Ryouhei Kano, Hirokazu Kataza, Naoki Isobe, Ryou Ohsawa, Fumihiko Usui, Yoshiyuki Yamada, Alister W. Graham, Alex R. Pettitt, Hideki Asada, Junichi Baba, Kenji Bekki, Bryan N. Dorland, Michiko Fujii, Akihiko Fukui, Kohei Hattori, Teruyuki Hirano, Takafumi Kamizuka, Shingo Kashima, Norita Kawanaka, Yui Kawashima, Sergei A. Klioner , et al. (64 additional authors not shown)

Abstract: Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency. JASMINE has two main science goals. One is the Galactic archaeology with Galactic Center Survey, which aims to reveal the Milky Way's central core structure and formation history from Gaia-level… ▽ More Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency. JASMINE has two main science goals. One is the Galactic archaeology with Galactic Center Survey, which aims to reveal the Milky Way's central core structure and formation history from Gaia-level (~25 $μ$as) astrometry in the Near-Infrared (NIR) Hw-band (1.0-1.6 $μ$m). The other is the Exoplanet Survey, which aims to discover transiting Earth-like exoplanets in the habitable zone from NIR time-series photometry of M dwarfs when the Galactic center is not accessible. We introduce the mission, review many science objectives, and present the instrument concept. JASMINE will be the first dedicated NIR astrometry space mission and provide precise astrometric information of the stars in the Galactic center, taking advantage of the significantly lower extinction in the NIR. The precise astrometry is obtained by taking many short-exposure images. Hence, the JASMINE Galactic center survey data will be valuable for studies of exoplanet transits, asteroseismology, variable stars and microlensing studies, including discovery of (intermediate mass) black holes. We highlight a swath of such potential science, and also describe synergies with other missions. △ Less

Submitted 4 March, 2024; v1 submitted 11 July, 2023; originally announced July 2023.

Comments: 50 pages, 22 figures, accepted for publication in PASJ

Bright Type II Supernova 2023ixf in M101: A Quick Analysis of the Early-Stage Spectra and Near-Infrared Light Curves

Authors: Masayuki Yamanaka, Mitsugu Fujii, Takahiro Nagayama

Abstract: We present early-stage analyses of low-resolution ($R=1000$) optical spectra and near-infrared light curves of the bright Type II supernova (SN II) 2023ixf in the notable nearby face-on spiral galaxy M101, which were obtained from $t=1.7$ to $8.0$ d. Our first spectrum showed remarkable emission features of Balmer series, He~{\sc ii}, N~{\sc iii}, C~{\sc iv}, and N~{\sc iv} with a strong blue cont… ▽ More We present early-stage analyses of low-resolution ($R=1000$) optical spectra and near-infrared light curves of the bright Type II supernova (SN II) 2023ixf in the notable nearby face-on spiral galaxy M101, which were obtained from $t=1.7$ to $8.0$ d. Our first spectrum showed remarkable emission features of Balmer series, He~{\sc ii}, N~{\sc iii}, C~{\sc iv}, and N~{\sc iv} with a strong blue continuum. Compared with the SNe II showing flash-ionized features, we suggest that this SN could be categorized into the high-luminosity SNe II with a nitrogen/helium-rich circumstellar material (CSM), e.g., SNe 2014G, 2017ahn, and 2020pni. The H~{$α$} emission line can be tentatively explained by a narrower component with a velocity of $<300$ km~s$^{-1}$ and a broader one with $\sim2200$ km~s$^{-1}$. The near-infrared light curves were well consistent with those of the another luminous SN 2017ahn, and its absolute magnitudes locate on the bright end in the luminosity distribution of SNe II. These observational facts support that SN 2023ixf is well consistent with high-luminosity SNe II showing evidences of a dense nitrogen/helium-rich CSM. △ Less

Submitted 21 July, 2023; v1 submitted 31 May, 2023; originally announced June 2023.

Comments: 5 pages, 4 figures, accepted for publication in the PASJ Letters

Compact Binary Formation in Open Star Clusters I: High Formation Efficiency of Gaia BHs and Their Multiplicities

Authors: Ataru Tanikawa, Savannah Cary, Minori Shikauchi, Long Wang, Michiko S. Fujii

Abstract: Gaia BHs, black hole (BH) binaries discovered from database of an astrometric telescope Gaia, pose a question to the standard binary evolution model. We have assessed if Gaia BHs can be formed through dynamical capture in open clusters rather than through isolated binary evolution. We have performed gravitational $N$-body simulations of $100$ open clusters with $10^5 M_\odot$ in total for each met… ▽ More Gaia BHs, black hole (BH) binaries discovered from database of an astrometric telescope Gaia, pose a question to the standard binary evolution model. We have assessed if Gaia BHs can be formed through dynamical capture in open clusters rather than through isolated binary evolution. We have performed gravitational $N$-body simulations of $100$ open clusters with $10^5 M_\odot$ in total for each metallicity $Z=0.02$, $0.01$, and $0.005$. We have discovered one Gaia BH-like binary escaping from an open cluster, and found that the formation efficiency of Gaia BHs in open clusters ($\sim 10^{-5} M_\odot^{-1}$) is larger than in isolated binaries ($\sim 10^{-8} M_\odot^{-1}$) by 3 orders of magnitude. The Gaia BH-like binary is the inner binary of a triple star system. Gaia BHs can have tertiary stars frequently, if they are formed in open clusters. Combining additional $N$-body simulations with 8000 open clusters with $8 \times 10^6 M_\odot$, we have estimated the number of Gaia BHs in the Milky Way disk to $10^4 - 10^5$ (depending on the definitions of Gaia BHs), large enough for the number of Gaia BHs discovered so far. Our results indicate that the discoveries of Gaia BHs do not request the reconstruction of the standard binary evolution model, and that Gaia BHs are a probe for the dynamics of open clusters already evaporated. △ Less

Submitted 24 October, 2023; v1 submitted 10 March, 2023; originally announced March 2023.

Comments: 9 pages, 10 figures, 1 table, MNRAS accepted

3D-Spatiotemporal Forecasting the Expansion of Supernova Shells Using Deep Learning toward High-Resolution Galaxy Simulations

Authors: Keiya Hirashima, Kana Moriwaki, Michiko S. Fujii, Yutaka Hirai, Takayuki R. Saitoh, Junichiro Makino

Abstract: Supernova (SN) plays an important role in galaxy formation and evolution. In high-resolution galaxy simulations using massively parallel computing, short integration timesteps for SNe are serious bottlenecks. This is an urgent issue that needs to be resolved for future higher-resolution galaxy simulations. One possible solution would be to use the Hamiltonian splitting method, in which regions req… ▽ More Supernova (SN) plays an important role in galaxy formation and evolution. In high-resolution galaxy simulations using massively parallel computing, short integration timesteps for SNe are serious bottlenecks. This is an urgent issue that needs to be resolved for future higher-resolution galaxy simulations. One possible solution would be to use the Hamiltonian splitting method, in which regions requiring short timesteps are integrated separately from the entire system. To apply this method to the particles affected by SNe in a smoothed-particle hydrodynamics simulation, we need to detect the shape of the shell on and within which such SN-affected particles reside during the subsequent global step in advance. In this paper, we develop a deep learning model, 3D-MIM, to predict a shell expansion after a SN explosion. Trained on turbulent cloud simulations with particle mass $m_{\rm gas}=1$M$_\odot$, the model accurately reproduces the anisotropic shell shape, where densities decrease by over 10 per cent by the explosion. We also demonstrate that the model properly predicts the shell radius in the uniform medium beyond the training dataset of inhomogeneous turbulent clouds. We conclude that our model enables the forecast of the shell and its interior where SN-affected particles will be present. △ Less

Submitted 17 September, 2023; v1 submitted 31 January, 2023; originally announced February 2023.

Comments: 14 pages, 14 figures, 3 tables, accepted for MNRAS

Gravitational wave of intermediate-mass black holes in Population III star clusters

Authors: Long Wang, Ataru Tanikawa, Michiko Fujii

Abstract: Previous theoretical studies suggest that the Population III (Pop3) stars tend to form in extremely metal poor gas clouds with approximately $10^5 M_\odot$ embedded in mini dark matter halos. Very massive stars can form via multiple collisions in Pop3 star clusters and eventually evolve to intermediate-mass black holes (IMBHs). In this work, we conduct star-by-star $N$-body simulations for modelli… ▽ More Previous theoretical studies suggest that the Population III (Pop3) stars tend to form in extremely metal poor gas clouds with approximately $10^5 M_\odot$ embedded in mini dark matter halos. Very massive stars can form via multiple collisions in Pop3 star clusters and eventually evolve to intermediate-mass black holes (IMBHs). In this work, we conduct star-by-star $N$-body simulations for modelling the long-term evolution of Pop3 star clusters. We find that if the mini dark matter halos can survive today, these star clusters can avoid tidal disruption by the galactic environment and can efficiently produce IMBH-BH mergers among a wide range of redshift from 0 to 20. The average gravitational wave event rate is estimated to be $0.1-0.8~\mathrm{yr}^{-1} \mathrm{Gpc}^{-3}$, and approximately $40-80$ percent of the mergers occur at high redshift ($z>6$). The characteristic strain shows that a part of low-redshift mergers can be detected by LISA, TianQin, and Taiji, whereas most mergers can be covered by DECIGO and advanced LIGO/VIRGO/Kagra. Mergers with pair-instability BHs have a rate of approximately $0.01-0.15$~yr$^{-1}$~Gpc$^{-3}$, which can explain the GW190521-like events. △ Less

Submitted 19 July, 2022; originally announced July 2022.

Comments: 15 pages, 11 figures, 4 tables, accepted for MNRAS

SIRIUS Project. V. Formation of off-center ionized bubbles associated with Orion Nebula Cluster

Authors: Michiko S. Fujii, Kohei Hattori, Long Wang, Yutaka Hirai, Jun Kumamoto, Yoshito Shimajiri, Takayuki R. Saitoh

Abstract: Massive stars born in star clusters terminate star cluster formation by ionizing the surrounding gas. This process is considered to be prevalent in young star clusters containing massive stars. The Orion Nebula is an excellent example associated with a forming star cluster including several massive stars (the Orion Nebula Cluster; ONC) and a 2-pc size H{\sc ii} region (ionized bubble) opening towa… ▽ More Massive stars born in star clusters terminate star cluster formation by ionizing the surrounding gas. This process is considered to be prevalent in young star clusters containing massive stars. The Orion Nebula is an excellent example associated with a forming star cluster including several massive stars (the Orion Nebula Cluster; ONC) and a 2-pc size H{\sc ii} region (ionized bubble) opening toward the observer; however, the other side is still covered with dense molecular gas. Recent astrometric data acquired by the Gaia satellite revealed the stellar kinematics in this region. By comparing this data with star cluster formation simulation results, we demonstrate that massive stars born in the ONC center were ejected via three-body encounters. Further, orbit analysis indicates that $θ^2$ Ori A, the second massive star in this region, was ejected from the ONC center toward the observer and is now returning to the cluster center. Such ejected massive stars can form a hole in the dense molecular cloud and contribute to the formation of the 2-pc bubble. Our results demonstrate that the dynamics of massive stars are essential for the formation of star clusters and H{\sc ii} regions that are not always centered by massive stars. △ Less

Submitted 9 June, 2022; originally announced June 2022.

Comments: 13 pages, 12 figures, accepted for MNRAS

Dippers from TESS Full-frame Images. II. Spectroscopic Characterization of Four Young Dippers

Authors: Yui Kasagi, Takayuki Kotani, Hajime Kawahara, Tomoyuki Tajiri, Takayuki Muto, Masataka Aizawa, Michiko S. Fujii, Kohei Hattori, Kento Masuda, Munetake Momose, Ryou Ohsawa, Satoshi Takita

Abstract: Photometric monitoring by the Transiting Exoplanet Survey Satellite (TESS) has discovered not only periodic signals by transiting exoplanets but also episodic or quasi-periodic dimming around young stellar objects. The mechanisms of the dimming of these objects, so-called "dippers", are thought to be related to the property of the accretion or the structure of protoplanetary disks especially in re… ▽ More Photometric monitoring by the Transiting Exoplanet Survey Satellite (TESS) has discovered not only periodic signals by transiting exoplanets but also episodic or quasi-periodic dimming around young stellar objects. The mechanisms of the dimming of these objects, so-called "dippers", are thought to be related to the property of the accretion or the structure of protoplanetary disks especially in regions close to the host star. Recently, we have created the catalog of dippers from the one year of TESS Full Frame Image (FFI) data. In this paper, we report spectral features of four newly found dippers in that catalog and show that they potentially shed light on the dimming mechanisms. We found that all of the targets exhibit the H alpha emission line, which is an indicator of an ongoing accretion. Based on its line profiles and/or their variability, we characterized the properties of the disks of each source, which can support the dimming mechanisms due to a dusty disk wind or an accretion warp. Also, we found an interesting dipper (TIC 317873721), "close-in binary dipper," showing the complex variability of the line profile and the large radial velocity variation. Since the dimming intervals are similar to the orbital period of the binary, we suggest that the dips are caused by dust in the warp of accretion from a circumbinary disk onto stars. Such a close-in ($<$ 0.1 au) binary dipper is rarely reported so far, further investigation will reveal the new aspect of the disk evolution and planetary formation. △ Less

Submitted 24 January, 2022; originally announced January 2022.

Comments: 15 pages, 10 figures, accepted for publication in ApJS

Spectroscopic Observations of V455 Andromedae Superoutburst in 2007: the Most Exotic Spectral Features in Dwarf Nova Outbursts

Authors: Yusuke Tampo, Daisaku Nogami, Taichi Kato, Kazuya Ayani, Hiroyuki Naito, Norio Narita, Mitsugu Fujii, Osamu Hashimoto, Kenzo Kinugasa Satoshi Honda, Hidenori Takahashi, Shin-ya Narusawa, Makoto Sakamoto, Akira Imada

Abstract: We present our spectroscopic observations of V455 Andromedae during the 2007 superoutburst. Our observations cover this superoutburst from around the optical peak of the outburst to the post-superoutburst stage. During the early superhump phase, the emission lines of Balmer series, He I, He II, Bowen blend, and C IV / N IV blend were detected. He II 4686 line exhibited a double-peaked emission pro… ▽ More We present our spectroscopic observations of V455 Andromedae during the 2007 superoutburst. Our observations cover this superoutburst from around the optical peak of the outburst to the post-superoutburst stage. During the early superhump phase, the emission lines of Balmer series, He I, He II, Bowen blend, and C IV / N IV blend were detected. He II 4686 line exhibited a double-peaked emission profile, where Balmer emission lines were single-peaked, which is unexpected from its high inclination. In the ordinary superhump phase, Balmer series transitioned to double-peaked emission profiles, and high-ionization lines were significantly weakened. These transitions of the line profiles should be related to the structural transformation of the accretion disk, as expected between the early and ordinary superhump transition in the thermal-tidal instability model. The Doppler map of H$α$ during the early superhump phase exhibits a compact blob centered at the primary white dwarf. In analogy to SW Sex-type cataclysmic variables, this feature could emerge from the disk wind and/or the mass accretion column onto the magnetized white dwarf. The Doppler map of He II 4686 Å~ is dominated by the ring-like structure and imposed two flaring regions with the velocity of $\sim$300 km/s, which is too slow for a Keplerian accretion disk. The phase of the flaring regions was coincident with the inner spiral arm structure identified during the early superhump phase. Our disk wind model with the enhanced emission from the inner arm structure successfully reproduced the observed properties of He II 4686 Å. Therefore, V455 And is the first case in dwarf nova outbursts that the presence of the disk wind is inferred from an optical spectrum. △ Less

Submitted 22 January, 2022; originally announced January 2022.

Comments: 19 pages, 11 figures, 3 tables, accepted for publication in PASJ

Impact of bar resonances in the velocity-space distribution of the solar neighbourhood stars in a self-consistent $N$-body Galactic disc simulation

Authors: Tetsuro Asano, Michiko S. Fujii, Junichi Baba, Jeroen Bédorf, Elena Sellentin, Simon Portegies Zwart

Abstract: The velocity-space distribution of the solar neighbourhood stars shows complex substructures. Most of the previous studies use static potentials to investigate their origins. Instead we use a self-consistent $N$-body model of the Milky Way, whose potential is asymmetric and evolves with time. In this paper, we quantitatively evaluate the similarities of the velocity-space distributions in the $N$-… ▽ More The velocity-space distribution of the solar neighbourhood stars shows complex substructures. Most of the previous studies use static potentials to investigate their origins. Instead we use a self-consistent $N$-body model of the Milky Way, whose potential is asymmetric and evolves with time. In this paper, we quantitatively evaluate the similarities of the velocity-space distributions in the $N$-body model and that of the solar neighbourhood, using Kullback-Leibler divergence (KLD). The KLD analysis shows the time evolution and spatial variation of the velocity-space distribution. The KLD fluctuates with time, which indicates the velocity-space distribution at a fixed position is not always similar to that of the solar neighbourhood. Some positions show velocity-space distributions with small KLDs (high similarities) more frequently than others. One of them locates at $(R,φ)=(8.2\;\mathrm{kpc}, 30^{\circ})$, where $R$ and $φ$ are the distance from the galactic centre and the angle with respect to the bar's major axis, respectively. The detection frequency is higher in the inter-arm regions than in the arm regions. In the velocity maps with small KLDs, we identify the velocity-space substructures, which consist of particles trapped in bar resonances. The bar resonances have significant impact on the stellar velocity-space distribution even though the galactic potential is not static. △ Less

Submitted 23 May, 2022; v1 submitted 1 December, 2021; originally announced December 2021.

Comments: 9 pages, 11 figures. Accepted by MNRAS

SIRIUS Project. IV. The formation history of the Orion Nebula Cluster driven by clump mergers

Authors: Michiko S. Fujii, Long Wang, Yutaka Hirai, Yoshito Shimajiri, Jun Kumamoto, Takayuki Saitoh

Abstract: The Orion Nebula Cluster (ONC) is an excellent example for understanding the formation of star clusters. Recent studies have shown that ONC has three distinct age populations and anisotropy in velocity dispersions, which are key characteristics for understanding the formation history of the ONC. In this study, we perform a smoothed-particle hydrodynamics/$N$-body simulation of star cluster formati… ▽ More The Orion Nebula Cluster (ONC) is an excellent example for understanding the formation of star clusters. Recent studies have shown that ONC has three distinct age populations and anisotropy in velocity dispersions, which are key characteristics for understanding the formation history of the ONC. In this study, we perform a smoothed-particle hydrodynamics/$N$-body simulation of star cluster formation from a turbulent molecular cloud. In this simulation, stellar orbits are integrated using a high-order integrator without gravitational softening; therefore, we can follow the collisional evolution of star clusters. We find that hierarchical formation causes episodic star formation that is observed in the ONC. In our simulation, star clusters evolve due to mergers of subclumps. The mergers bring cold gas with the clumps into the forming cluster. This enhances the star formation in the cluster centre. The dense cold gas in the cluster centre continues to form stars until the latest time. This explains the compact distribution of the youngest stars observed in the ONC. Subclump mergers also contribute to the anisotropy in the velocity dispersions and the formation of runaway stars. However, the anisotropy disappears within 0.5 Myr. The virial ratio of the cluster also increases after a merger due to the runaways. These results suggest that the ONC recently experienced a clump merger. We predict that most runaways originated from the ONC have already been found, but walkaways have not. △ Less

Submitted 31 May, 2022; v1 submitted 30 November, 2021; originally announced November 2021.

Comments: 15 pages, 21 figures, and 3 tables, accepted for MNRAS

The impact of primordial binary on the dynamical evolution of intermediate massive star clusters

Authors: Long Wang, Ataru Tanikawa, Michiko S. Fujii

Abstract: Observations found that star clusters contain a large fraction of binaries. Tight binaries are an important heating source that influences the long-term dynamical evolution of star clusters. However, due to the limitation of $N$-body tool, previous theoretical modelling for globular clusters (GCs) by using direct $N$-body simulations have not investigated how a large fraction of primordial binarie… ▽ More Observations found that star clusters contain a large fraction of binaries. Tight binaries are an important heating source that influences the long-term dynamical evolution of star clusters. However, due to the limitation of $N$-body tool, previous theoretical modelling for globular clusters (GCs) by using direct $N$-body simulations have not investigated how a large fraction of primordial binaries affect their long-term evolution. In this work, by using the high-performance $N$-body code, PeTar, we carry out star-by-star models for intermediate massive GCs ($N=100000$) with the primordial binary fraction varying from 0 to 1. We find that when a stellar-mass black hole (BH) subsystem exists, the structural evolution of GCs (core and half-mass radii) only depends on the properties of massive primordial binaries, because they affect the number of BH binaries (BBHs), which dominate the binary heating process. Low-mass binaries including double white dwarf binaries (BWDs) have almost no influence on the dynamics. Meanwhile, only gravitational wave (GW) mergers from BBHs are strongly affected by dynamical interactions, while low-mass mergers from BWDs show no difference in the isolated environment (field) and in GCs. Low-mass binaries become important only after most BHs escape and the core collapse of light stars occurs. Our result suggests that for $N$-body modelling of GCs with a black hole subsystem dominating binary heating, it is not necessary to include low-mass binaries. These binaries can be studied separately by using standalone binary stellar evolution codes. This way can significantly reduce the computing cost. △ Less

Submitted 29 October, 2021; originally announced October 2021.

Comments: 10 pages, 6 figures, submitted to MNRAS

Gravitational Wave Physics and Astronomy in the nascent era

Authors: Makoto Arimoto, Hideki Asada, Michael L. Cherry, Michiko S. Fujii, Yasushi Fukazawa, Akira Harada, Kazuhiro Hayama, Takashi Hosokawa, Kunihito Ioka, Yoichi Itoh, Nobuyuki Kanda, Koji S. Kawabata, Kyohei Kawaguchi, Nobuyuki Kawai, Tsutomu Kobayashi, Kazunori Kohri, Yusuke Koshio, Kei Kotake, Jun Kumamoto, Masahiro N. Machida, Hideo Matsufuru, Tatehiro Mihara, Masaki Mori, Tomoki Morokuma, Shinji Mukohyama , et al. (28 additional authors not shown)

Abstract: The detections of gravitational waves (GW) by LIGO/Virgo collaborations provide various possibilities to physics and astronomy. We are quite sure that GW observations will develop a lot both in precision and in number owing to the continuous works for the improvement of detectors, including the expectation to the newly joined detector, KAGRA, and the planned detector, LIGO-India. In this occasion,… ▽ More The detections of gravitational waves (GW) by LIGO/Virgo collaborations provide various possibilities to physics and astronomy. We are quite sure that GW observations will develop a lot both in precision and in number owing to the continuous works for the improvement of detectors, including the expectation to the newly joined detector, KAGRA, and the planned detector, LIGO-India. In this occasion, we review the fundamental outcomes and prospects of gravitational wave physics and astronomy. We survey the development focusing on representative sources of gravitational waves: binary black holes, binary neutron stars, and supernovae. We also summarize the role of gravitational wave observations as a probe of new physics. △ Less

Submitted 6 April, 2021; originally announced April 2021.

Comments: 89pages, Prog. Theor. Exp. Phys. accepted

SIRIUS Project. III. Star-by-star simulations of star cluster formation using a direct N-body integrator with stellar feedback

Authors: Michiko S. Fujii, Takayuki R. Saitoh, Yutaka Hirai, Long Wang

Abstract: One of the computational challenges of cluster formation simulations is resolving individual stars and simulating massive clusters with masses of more than $10^4 M_{\odot}$ without gravitational softening. Combining direct $N$-body code with smoothed-particle hydrodynamics (SPH) code, we have developed a new code, \textsc{ASURA+BRIDGE}, in which we can integrate stellar particles without softening… ▽ More One of the computational challenges of cluster formation simulations is resolving individual stars and simulating massive clusters with masses of more than $10^4 M_{\odot}$ without gravitational softening. Combining direct $N$-body code with smoothed-particle hydrodynamics (SPH) code, we have developed a new code, \textsc{ASURA+BRIDGE}, in which we can integrate stellar particles without softening. We add a feedback model for \HII regions into this code, in which thermal and momentum feedback is given within the Str{ö}mgren radius. We perform $N$-body/SPH simulations of star cluster formation. Without softening, a portion of massive stars are ejected from the forming clusters. As a result, the stellar feedback works outside the clusters. This enhances/suppresses the star formation in initially sub-virial/super-virial clouds. We find that the formed star clusters are denser than currently observed open clusters, but the mass--density relation is consistent with or even higher than that is estimated as an initial cluster density. We also find that some clusters have multiple peaks in their stellar age distribution as a consequence of their hierarchical formation. △ Less

Submitted 5 July, 2021; v1 submitted 3 March, 2021; originally announced March 2021.

Comments: 25 pages, 29 figures, accepted for publication in PASJ

Spin distribution of binary black holes formed in open clusters

Authors: Jun Kumamoto, Michiko S. Fujii, Alessandro A. Trani, Ataru Tanikawa

Abstract: We performed direct N-body simulations of open clusters with four different metallicities. To investigate the effective spins of merging binary black holes (BBHs) originated from these open clusters, we calculated the spin evolution of Wolf-Rayet (WR) stars with close companion stars (BBH progenitors), taking into account stellar wind mass loss and tidal spin-up of the WR stars. We found that BBH… ▽ More We performed direct N-body simulations of open clusters with four different metallicities. To investigate the effective spins of merging binary black holes (BBHs) originated from these open clusters, we calculated the spin evolution of Wolf-Rayet (WR) stars with close companion stars (BBH progenitors), taking into account stellar wind mass loss and tidal spin-up of the WR stars. We found that BBH progenitors with smaller semi-major axes evolve to merging BBHs with greater effective spins because of strong tidal forces. In the local Universe, about 16% of merging BBHs get effective spins larger than 0.1 even if BHs and their progenitors do not get spin angular momenta other than tidal forces exerted by their companion stars. If we assume that WR stars have flat and isotropic distribution of dimensionless spins just after common envelope phases, the effective spin distribution of merging BBHs is similar to that inferred from gravitational wave observations with LIGO and Virgo. △ Less

Submitted 18 February, 2021; originally announced February 2021.

Comments: 8 pages, 10 figures, submitted to MNRAS

Spin misalignment of black hole binaries from young star clusters: implications for the origin of gravitational waves events

Authors: Alessandro Alberto Trani, Ataru Tanikawa, Michiko S. Fujii, Nathan W. C. Leigh, Jun Kumamoto

Abstract: Recent studies indicate that the progenitors of merging black hole (BH) binaries from young star clusters can undergo a common envelope phase just like isolated binaries. If the stars emerge from the common envelope as naked cores, tidal interactions can efficiently synchronize their spins before they collapse into BHs. Contrary to the isolated case, these binary BHs can also undergo dynamical int… ▽ More Recent studies indicate that the progenitors of merging black hole (BH) binaries from young star clusters can undergo a common envelope phase just like isolated binaries. If the stars emerge from the common envelope as naked cores, tidal interactions can efficiently synchronize their spins before they collapse into BHs. Contrary to the isolated case, these binary BHs can also undergo dynamical interactions with other BHs in the cluster before merging. The interactions can tilt the binary orbital plane, leading to spin-orbit misalignment. We estimate the spin properties of merging binary BHs undergoing this scenario by combining up-to-date binary population synthesis and accurate few-body simulations. We show that post-common envelope binary BHs are likely to undergo only a single encounter, due to the high binary recoil velocity and short coalescence times. Adopting conservative limits on the binary-single encounter rates, we obtain a local BH merger rate density of ~6.6 yr^-1 Gpc^-3. Assuming low (<0.2) natal BH spins, this scenario reproduces the trends in the distributions of effective spin Xeff and precession parameters Xp inferred from GWTC-2, including the peaks at (Xeff, Xp) ~ (0.1, 0.2) and the tail at negative Xeff values. △ Less

Submitted 2 April, 2021; v1 submitted 2 February, 2021; originally announced February 2021.

Comments: 10 pages, 6 figures, accepted for publication in MNRAS

Impact of initial mass functions on the dynamical channel of gravitational wave sources

Authors: Long Wang, Michiko S. Fujii, Ataru Tanikawa

Abstract: Dynamically formed black hole (BH) binaries (BBHs) are important sources of gravitational waves (GWs). Globular clusters (GCs) provide a major environment to produce such BBHs, but the total mass of the known GCs is small compared to that in the Galaxy; thus, the fraction of BBHs formed in GCs is also small. However, this assumes that GCs contain a canonical initial mass function (IMF) similar to… ▽ More Dynamically formed black hole (BH) binaries (BBHs) are important sources of gravitational waves (GWs). Globular clusters (GCs) provide a major environment to produce such BBHs, but the total mass of the known GCs is small compared to that in the Galaxy; thus, the fraction of BBHs formed in GCs is also small. However, this assumes that GCs contain a canonical initial mass function (IMF) similar to that of field stars. This might not be true because several studies suggest that extreme dense and metal-poor environment can result in top-heavy IMFs, where GCs may originate. Although GCs with top-heavy IMFs were easily disrupted or have become dark clusters, the contribution to the GW sources can be significant. Using a high-performance and accurate $N$-body code, \textsc{petar}, we investigate the effect of varying IMFs by carrying out four star-by-star simulations of dense GCs with the initial mass of $5\times10^5 M_\odot$ and the half-mass radius of $2$~pc. We find that the BBH merger rate does not monotonically correlate with the slope of IMFs. Due to a rapid expansion, top-heavy IMFs lead to less efficient formation of merging BBHs. The formation rate continuously decreases as the cluster expands because of the dynamical heating caused by BHs. However, in star clusters with a top-heavier IMF, the total number of BHs is larger, and therefore, the final contribution to merging BBHs can still be more than from clusters with the standard IMF, if the initial cluster mass and density is higher than those used in our model. △ Less

Submitted 21 April, 2021; v1 submitted 22 January, 2021; originally announced January 2021.

Comments: 10 pages, 12 figures, accepted for publication in MNRAS

SIRIUS project II: a new tree-direct hybrid code for smoothed particle hydrodynamics/N-body simulations of star clusters

Authors: M. S. Fujii, T. R. Saitoh, L. Wang, Y. Hirai

Abstract: Star clusters form via clustering star formation inside molecular clouds. In order to understand the dynamical evolution of star clusters in their early phase, in which star clusters are still embedded in their surrounding gas, we need an accurate integration of individual stellar orbits without gravitational softening in the systems including both gas and stars, as well as modeling individual sta… ▽ More Star clusters form via clustering star formation inside molecular clouds. In order to understand the dynamical evolution of star clusters in their early phase, in which star clusters are still embedded in their surrounding gas, we need an accurate integration of individual stellar orbits without gravitational softening in the systems including both gas and stars, as well as modeling individual stars with a realistic mass function. We develop a new tree-direct hybrid smoothed particle hydrodynamics/N-body code, ASURA+BRIDGE, in which stars are integrated using a direct N-body scheme or PeTar, a particle-particle particle-tree scheme code, without gravitational softening. In ASURA+BRIDGE, stars are assumed to have masses randomly drawn from a given initial mass function. With this code, we perform star-cluster formation simulations starting from molecular clouds without gravitational softening. We find that artificial dense cores in star-cluster centers due to the softening disappear when we do not use softening. We further demonstrate that star clusters are built up via mergers of smaller clumps. Star clusters formed in our simulations include some dynamically formed binaries with the minimum semi-major axes of a few au, and the binary fraction is higher for more massive stars. △ Less

Submitted 15 April, 2021; v1 submitted 14 January, 2021; originally announced January 2021.

Comments: 17 pages, 18 figures, 3 tables, accepted for publication in PASJ

Radial-velocity search and statistical studies for short-period planets in the Pleiades open cluster

Authors: Takuya Takarada, Bun'ei Sato, Masashi Omiya, Yasunori Hori, Michiko S. Fujii

Abstract: We report radial-velocity search for short-period planets in the Pleiades open cluster. We observed 30 Pleiades member stars at the Okayama Astrophysical Observatory (OAO) with High Dispersion Echelle Spectrograph (HIDES). To evaluate and mitigate the effects of stellar activity on radial-velocity measurements, we computed four activity indicators (FWHM, $V_{\rm span}$, $W_{\rm span}$ and… ▽ More We report radial-velocity search for short-period planets in the Pleiades open cluster. We observed 30 Pleiades member stars at the Okayama Astrophysical Observatory (OAO) with High Dispersion Echelle Spectrograph (HIDES). To evaluate and mitigate the effects of stellar activity on radial-velocity measurements, we computed four activity indicators (FWHM, $V_{\rm span}$, $W_{\rm span}$ and $S_{\rm Hα}$). Among our sample, no short-period planet candidates were detected. Stellar intrinsic RV jitter was estimated to be ${\rm 52\ m\,s^{-1}}$, ${\rm 128\ m\,s^{-1}}$ and ${\rm 173\ m\,s^{-1}}$ for stars with $v\sin i$ of ${\rm 10\ km\,s^{-1}}$, ${\rm 15\ km\,s^{-1}}$ and ${\rm 20\ km\,s^{-1}}$, respectively. We determined the planet occurrence rate from our survey and set the upper limit to 11.4\% for the planets with masses 1--13 $M_{\rm JUP}$ and period 1--10 days. To set a more stringent constraint on the planet occurrence rate, we combined the result of our survey with those of other surveys targeting open clusters with ages between 30--300 Myr. As a result, the planet occurrence rate in young open clusters was found to be less than 7.4\%, 2.9\% and 1.9\% for the planets with an orbital period of three days and masses between 1--5, 5--13, and 13--80 $M_{\rm JUP}$, respectively. △ Less

Submitted 22 October, 2020; originally announced October 2020.

Comments: 54 pages, 39 figures, accepted for publication in PASJ

Dippers from the TESS Full-Frame Images I: The Results of the first 1 year data and Discovery of A Runaway dipper

Authors: Tomoyuki Tajiri, Hajime Kawahara, Masataka Aizawa, Michiko S. Fujii, Kohei Hattori, Yui Kasagi, Takayuki Kotani, Kento Masuda, Munetake Momose, Takayuki Muto, Ryou Ohsawa, Satoshi Takita

Abstract: We present a comprehensive catalog of the dippers---young stellar objects that exhibit episodic dimming---derived from the one year's worth of data of Transiting Exoplanet Survey Satellite ($ TESS$) full-frame images. In the survey, we found 35 dippers using the convolutional neural network, most of them newly discovered. Although these dippers are widely distributed over the first half-hemisphere… ▽ More We present a comprehensive catalog of the dippers---young stellar objects that exhibit episodic dimming---derived from the one year's worth of data of Transiting Exoplanet Survey Satellite ($ TESS$) full-frame images. In the survey, we found 35 dippers using the convolutional neural network, most of them newly discovered. Although these dippers are widely distributed over the first half-hemisphere that $TESS$ surveyed, we identified the majority's membership with the nearest association Scorpius--Centaurus, Velorum OB2, and nearby Orion molecular cloud complex. However, several dippers are likely to be located in the field. We also found three old dippers whose age exceeds ten million year, which is considered as the disk dissipation time. The color-color diagram indicates that these old dippers are likely to have an extreme debris disk. In particular, we found a runaway old dipper having a large three-dimensional velocity of $72 \mathrm{km\ s}^{-1}$. The dippers in the field, which were probably escaping from their birth molecular clouds or were born outside the current area of star forming regions, are more common than previously considered. △ Less

Submitted 27 September, 2020; originally announced September 2020.

Comments: 18 pages, 20 figures; accepted for publication in The Astrophysical Journal Supplement

Trimodal structure of Hercules stream explained by originating from bar resonances

Authors: Tetsuro Asano, Michiko S. Fujii, Junichi Baba, Jeroen Bédorf, Elena Sellentin, Simon Portegies Zwart

Abstract: Gaia Data Release 2 revealed detailed structures of nearby stars in phase space. These include the Hercules stream, whose origin is still debated. Most of the previous numerical studies conjectured that the observed structures originate from orbits in resonance with the bar, based on static potential models for the Milky Way. We, in contrast, approach the problem via a self-consistent, dynamic, an… ▽ More Gaia Data Release 2 revealed detailed structures of nearby stars in phase space. These include the Hercules stream, whose origin is still debated. Most of the previous numerical studies conjectured that the observed structures originate from orbits in resonance with the bar, based on static potential models for the Milky Way. We, in contrast, approach the problem via a self-consistent, dynamic, and morphologically well-resolved model, namely a full $N$-body simulation of the Milky Way. Our simulation comprises about 5.1 billion particles in the galactic stellar bulge, bar, disk, and dark-matter halo and is evolved to 10 Gyr. Our model's disk component is composed of 200 million particles, and its simulation snapshots are stored every 10 Myr, enabling us to resolve and classify resonant orbits of representative samples of stars. After choosing the Sun's position in the simulation, we compare the distribution of stars in its neighborhood with Gaia's astrometric data, thereby establishing the role of identified resonantly trapped stars in the formation of Hercules-like structures. From our orbital spectral-analysis we identify multiple, especially higher order resonances. Our results suggest that the Hercules stream is dominated by the 4:1 and 5:1 outer Lindblad and corotation resonances. In total, this yields a trimodal structure of the Hercules stream. From the relation between resonances and ridges in phase space, our model favored a slow pattern speed of the Milky-Way bar (40--45 $\mathrm{km \; s^{-1} \; kpc^{-1}}$). △ Less

Submitted 15 September, 2020; v1 submitted 28 May, 2020; originally announced May 2020.

Comments: 11 pages, 9 figures, MNRAS accepted

SIRIUS project. I. Star formation models for star-by-star simulations of star clusters and galaxy formation

Authors: Yutaka Hirai, Michiko S. Fujii, Takayuki R. Saitoh

Abstract: Most stars are formed as star clusters in galaxies, which then disperse into galactic disks. Upcoming exascale supercomputational facilities will enable performing simulations of galaxies and their formation by resolving individual stars (star-by-star simulations). This will substantially advance our understanding of star formation in galaxies, star cluster formation, and assembly histories of gal… ▽ More Most stars are formed as star clusters in galaxies, which then disperse into galactic disks. Upcoming exascale supercomputational facilities will enable performing simulations of galaxies and their formation by resolving individual stars (star-by-star simulations). This will substantially advance our understanding of star formation in galaxies, star cluster formation, and assembly histories of galaxies. In previous galaxy simulations, a simple stellar population approximation was used. It is, however, difficult to improve the mass resolution with this approximation. Therefore, a model for forming individual stars that can be used in simulations of galaxies must be established. In this first paper of a series of the SIRIUS (SImulations Resolving IndividUal Stars) project, we demonstrate a stochastic star formation model for star-by-star simulations. An assumed stellar initial mass function (IMF) is randomly assigned to newly formed stars. We introduce a maximum search radius to assemble the mass from surrounding gas particles to form star particles. In this study, we perform a series of N-body/smoothed particle hydrodynamics simulations of star cluster formations from turbulent molecular clouds and ultra-faint dwarf galaxies as test cases. The IMF can be correctly sampled if a maximum search radius that is larger than the value estimated from the threshold density for star formation is adopted. In small clouds, the formation of massive stars is highly stochastic because of the small number of stars. We confirm that the star formation efficiency and threshold density do not strongly affect the results. We find that our model can naturally reproduce the relationship between the most massive stars and the total stellar mass of star clusters. Herein, we demonstrate that our models can be applied to simulations varying from star clusters to galaxies for a wide range of resolutions. △ Less

Submitted 17 April, 2021; v1 submitted 25 May, 2020; originally announced May 2020.

Comments: 23 pages, 22 figures, accepted for publication in PASJ

Gaia's Detectability of Black Hole-Main Sequence Star Binaries Formed in Open Clusters

Authors: Minori Shikauchi, Jun Kumamoto, Ataru Tanikawa, Michiko S. Fujii

Abstract: Black hole-main sequence star (BH-MS) binaries are one of the targets of the future data releases of the astrometric satellite {\it Gaia}. They are supposed to be formed in two main sites: a galactic field and star clusters. However, previous work has never predicted the number of BH-MS binaries originating in the latter site. In this paper, we estimate the number of BH-MS binaries formed in open… ▽ More Black hole-main sequence star (BH-MS) binaries are one of the targets of the future data releases of the astrometric satellite {\it Gaia}. They are supposed to be formed in two main sites: a galactic field and star clusters. However, previous work has never predicted the number of BH-MS binaries originating in the latter site. In this paper, we estimate the number of BH-MS binaries formed in open clusters and detectable with {\it Gaia} based on the results of {\it N}-body simulations. By considering interstellar extinction in the Milky Way (MW) and observational constraints, we predict $\sim 10$ BH-MS binaries are observable. We also find that chemical abundance patterns of companion MSs will help us to identify the origin of the binaries as star clusters. Such MSs are not polluted by outflows of the BH progenitors, such as stellar winds and supernova ejecta. Chemical anomalies might be a good test to confirm the origin of binaries with relatively less massive MSs ($\lesssim 5M_{\odot}$), orbital periods ($\sim 1.5\;$year) and higher eccentricities ($e \gtrsim 0.1$). △ Less

Submitted 23 March, 2020; v1 submitted 30 January, 2020; originally announced January 2020.

Comments: 10 pages, 9 figures, submitted to PASJ

Merger rate density of binary black holes formed in open clusters

Authors: Jun Kumamoto, Michiko S. Fujii, Ataru Tanikawa

Abstract: Several binary black holes (BBHs) have been observed using gravitational wave detectors. For the formation mechanism of BBHs, two main mechanisms, isolated binary evolution and dynamical formation in dense star clusters, have been suggested. Future observations are expected to provide more information about BBH distributions, and it will help us to distinguish the two formation mechanisms. For the… ▽ More Several binary black holes (BBHs) have been observed using gravitational wave detectors. For the formation mechanism of BBHs, two main mechanisms, isolated binary evolution and dynamical formation in dense star clusters, have been suggested. Future observations are expected to provide more information about BBH distributions, and it will help us to distinguish the two formation mechanisms. For the star cluster channel, globular clusters have mainly been investigated. However, recent simulations have suggested that BBH formation in open clusters is not negligible. We estimate a local merger rate density of BBHs originated from open clusters using the results of our $N$-body simulations of open clusters with four different metallicities. We find that the merger rate per cluster is the highest for our 0.1 solar metallicity model. Assuming a cosmic star formation history and a metallicity evolution with dispersion, we estimate the local merger rate density of BBHs originated from open clusters to be $\sim 70~{\rm yr}^{-1} {\rm Gpc}^{-3}$. This value is comparable to the merger rate density expected from the first and second observation runs of LIGO and Virgo. In addition, we find that BBH mergers obtained from our simulations can reproduce the distribution of primary mass and mass ratio of merging BBHs estimated from the LIGO and Virgo observations. △ Less

Submitted 20 May, 2020; v1 submitted 28 January, 2020; originally announced January 2020.

Comments: 12 pages, 9 figures, accepted for publication in MNRAS

Formation rate of LB-1-like systems through dynamical interactions

Authors: Ataru Tanikawa, Tomoya Kinugawa, Jun Kumamoto, Michiko S. Fujii

Abstract: We estimate formation rates of LB-1-like systems through dynamical interactions in the framework of the theory of stellar evolution before the discovery of the LB-1 system. The LB-1 system contains $\sim 70M_\odot$ black hole (BH), so-called pair instability (PI)-gap BH, and B-type star with solar metallicity, and has nearly zero eccentricity. The most efficient formation mechanism is as follows.… ▽ More We estimate formation rates of LB-1-like systems through dynamical interactions in the framework of the theory of stellar evolution before the discovery of the LB-1 system. The LB-1 system contains $\sim 70M_\odot$ black hole (BH), so-called pair instability (PI)-gap BH, and B-type star with solar metallicity, and has nearly zero eccentricity. The most efficient formation mechanism is as follows. In an open cluster, a naked helium (He) star (with $\sim 20M_\odot$) collides with a heavy main-sequence (MS) star (with $\sim 50M_\odot$) which has a B-type companion. The collision results in a binary consisting of the collision product and B-type star with a high eccentricity. The binary can be circularized through the dynamical tide with radiative damping of the collision-product envelope. Finally, the collision product collapses to a PI-gap BH, avoiding pulsational pair instability and pair instability supernovae because its He core is as massive as the pre-colliding naked He star. We find that the number of LB-1-like systems in the Milky Way galaxy is $\sim 0.01 (ρ_{\rm oc} / 10^4 M_\odot \mbox{pc}^{-3})$, where $ρ_{\rm oc}$ is the initial mass densities of open clusters. If we take into account LB-1-like systems with O-type companion stars, the number increases to $\sim 0.03 (ρ_{\rm oc} / 10^4 M_\odot \mbox{pc}^{-3})$. This mechanism can form LB-1-like systems at least 10 times more efficiently than the other mechanisms: captures of B-type stars by PI-gap BHs, stellar collisions between other type stars, and stellar mergers in hierarchical triple systems. We conclude that no dynamical mechanism can explain the presence of the LB-1 system. △ Less

Submitted 1 March, 2020; v1 submitted 10 December, 2019; originally announced December 2019.

Comments: 9 pages, 1 figure, accepted for publication in PASJ

The Keplerian three-body encounter II. Comparisons with isolated encounters and impact on gravitational wave merger timescales

Authors: Alessandro A. Trani, Mario Spera, Nathan W. C. Leigh, Michiko S. Fujii

Abstract: We investigate the role of the Keplerian tidal field generated by a supermassive black hole (SMBH) on the three-body dynamics of stellar mass black holes. We consider two scenarios occurring close to the SMBH: the breakup of unstable triples and three-body encounters between a binary and a single. These two cases correspond to the hard and soft binary cases, respectively. The tidal field affects t… ▽ More We investigate the role of the Keplerian tidal field generated by a supermassive black hole (SMBH) on the three-body dynamics of stellar mass black holes. We consider two scenarios occurring close to the SMBH: the breakup of unstable triples and three-body encounters between a binary and a single. These two cases correspond to the hard and soft binary cases, respectively. The tidal field affects the breakup of triples by tidally limiting the system, so that the triples break earlier with lower breakup velocity, leaving behind slightly larger binaries (relative to the isolated case). The breakup direction becomes anisotropic and tends to follow the shape of the Hill region of the triple, favouring breakups in the radial direction. Furthermore, the tidal field can torque the system, leading to angular momentum exchanges between the triple and its orbit about the SMBH. This process changes the properties of the final binary, depending on the initial angular momentum of the triple. Finally, the tidal field also affects binary-single encounters: binaries tend to become both harder and more eccentric with respect to encounters that occur in isolation. Consequently, single-binary scattering in a deep Keplerian potential produces binaries with shorter gravitational wave merger timescales. △ Less

Submitted 24 September, 2019; v1 submitted 16 April, 2019; originally announced April 2019.

Comments: 15 pages, 11 Figures, 4 tables, expanded and revised version, accepted for publication in ApJ

Kinematics of Subclusters in Star Cluster Complexes: Imprint of their Parental Molecular Clouds

Authors: M. S. Fujii

Abstract: Star cluster complexes such as the Carina Nebula can have formed in turbulent giant molecular clouds. We perform a series of $N$-body simulations starting from subclustering initial conditions based on hydrodynamic simulations of turbulent molecular clouds. These simulations finally result in the formation of star cluster complexes consisting of several subclusters (clumps). We obtain the inter-cl… ▽ More Star cluster complexes such as the Carina Nebula can have formed in turbulent giant molecular clouds. We perform a series of $N$-body simulations starting from subclustering initial conditions based on hydrodynamic simulations of turbulent molecular clouds. These simulations finally result in the formation of star cluster complexes consisting of several subclusters (clumps). We obtain the inter-clump velocity distribution, the size of the region, and the mass of the most massive cluster in our simulated complex and compare the results with observed ones (the Carina Nebula and NGC 2264). The one-dimensional inter-clump velocity dispersion obtained from our simulations is $2.9\pm0.3$ and $1.4\pm0.4$ km s$^{-1}$ for the Carina- and NGC 2264-like models, respectively, which are consistent with those obtained from Gaia Data Release 2: 2.35 and 0.99 km s$^{-1}$ for the Carina Nebula and NGC 2264, respectively. We estimate that the masses of the parental molecular clouds for the Carina Nebula and the NGC 2264 are $4\times 10^5$ and $4\times 10^4M_{\odot}$, respectively. △ Less

Submitted 10 April, 2019; v1 submitted 5 December, 2018; originally announced December 2018.

Comments: 9 pages, 5 figures, MNRAS accepted

Survival Rates of Planets in Open Clusters: the Pleiades, Hyades, and Praesepe clusters

Authors: M. S. Fujii, Y. Hori

Abstract: In clustered environments, stellar encounters can liberate planets from their host stars via close encounters. Although the detection probability of planets suggests that the planet population in open clusters resembles that in the field, only a few dozen planet-hosting stars have been discovered in open clusters. We explore the survival rates of planets against stellar encounters in open clusters… ▽ More In clustered environments, stellar encounters can liberate planets from their host stars via close encounters. Although the detection probability of planets suggests that the planet population in open clusters resembles that in the field, only a few dozen planet-hosting stars have been discovered in open clusters. We explore the survival rates of planets against stellar encounters in open clusters similar to the Pleiades, Hyades, and Praesepe and embedded clusters. We performed a series of N-body simulations of high-density and low-density open clusters, open clusters that grow via mergers of subclusters, and embedded clusters. We semi-analytically calculated the survival rate of planets in star clusters up to 1Gyr using relative velocities, masses, and impact parameters of intruding stars. Less than 1.5% of close-in planets within 1 AU and at most 7% of planets with 1-10 AU are ejected by stellar encounters in clustered environments after the dynamical evolution of star clusters. If a planet population from 0.01-100 AU in an open cluster initially follows the probability distribution function of exoplanets with semi-major axis ($a_p$) between 0.03-3 AU in the field discovered by RV surveys, the PDF of surviving planets beyond ~10 AU in open clusters can be slightly modified to $\propto a_p^{-0.76}$. The production rate of free-floating planets (FFPs) per star is 0.0096-0.18, where we have assumed that all the stars initially have one giant planet with a mass of 1--13 MJ in a circular orbit. The expected frequency of FFPs is compatible with the upper limit on that of FFPs indicated by recent microlensing surveys. Our survival rates of planets in open clusters suggest that planets within 10 AU around FGKM-type stars are rich in relatively-young (<~10-100 Myr for open clusters and ~1-10 Myr for embedded clusters), less massive open clusters, which are promising targets for planet searches. △ Less

Submitted 11 April, 2019; v1 submitted 21 November, 2018; originally announced November 2018.

Comments: 23 pages, 15 figures, A&A accepted

Journal ref: A&A 624, A110 (2019)

Gravitational-Wave Emission from Binary Black Holes Formed in Open Clusters

Authors: Jun Kumamoto, Michiko S. Fujii, Ataru Tanikawa

Abstract: In order to investigate the formation rate of binary black holes (BBHs) in stellar clusters with a mass comparable to open clusters, we performed a series of direct $N$-body simulations of open clusters with a mass of $2.5\times10^3$ (Model A) and $10^4 M_{\odot}$ (Model B). Since such low-mass clusters would have been more populous than globular clusters when they were born, low-mass clusters are… ▽ More In order to investigate the formation rate of binary black holes (BBHs) in stellar clusters with a mass comparable to open clusters, we performed a series of direct $N$-body simulations of open clusters with a mass of $2.5\times10^3$ (Model A) and $10^4 M_{\odot}$ (Model B). Since such low-mass clusters would have been more populous than globular clusters when they were born, low-mass clusters are also candidates as the origin of BBHs which are the source of the gravitational waves. In model A, most of BBHs merged within 10 Gyr formed via dynamically formed main-sequence binary stars and stable and unstable mass transfer between them since open clusters collapse within the main-sequence life-time of massive stars. These binaries, therefore, have little eccentricities. The fraction of such binaries among all merging BBHs increases as the cluster mass decrease due to the shorter relaxation time. In our simulations, $4.0\times10^{-5}$ and $1.7\times10^{-5}$ BBHs per solar mass merged within 10 Gyr for models A and B, respectively. These values correspond to $\sim$ 20-50% of the number of mergers per solar mass originated from globular clusters with a mass of $10^5$-$10^6M_{\odot}$. Thus, the contribution of BBHs originated from open clusters is not negligible. The estimated mergers rate density in the local universe is about 0.3 yr$^{-1}$ Gpc$^{-3}$ assuming a cluster mass function with a power of $-$2. △ Less

Submitted 16 November, 2018; originally announced November 2018.

Comments: 7 pages, 6 figures, submitted to MNRAS

PENTACLE: Parallelized Particle-Particle Particle-Tree Code for Planet Formation

Authors: Masaki Iwasawa, Shoichi Oshino, Michiko S. Fujii, Yasunori Hori

Abstract: We have newly developed a Parallelized Particle-Particle Particle-tree code for Planet formation, PENTACLE, which is a parallelized hybrid $N$-body integrator executed on a CPU-based (super)computer. PENTACLE uses a 4th-order Hermite algorithm to calculate gravitational interactions between particles within a cutoff radius and a Barnes-Hut tree method for gravity from particles beyond. It also imp… ▽ More We have newly developed a Parallelized Particle-Particle Particle-tree code for Planet formation, PENTACLE, which is a parallelized hybrid $N$-body integrator executed on a CPU-based (super)computer. PENTACLE uses a 4th-order Hermite algorithm to calculate gravitational interactions between particles within a cutoff radius and a Barnes-Hut tree method for gravity from particles beyond. It also implements an open-source library designed for full automatic parallelization of particle simulations, FDPS (Framework for Developing Particle Simulator) to parallelize a Barnes-Hut tree algorithm for a memory-distributed supercomputer. These allow us to handle $1-10$ million particles in a high-resolution $N$-body simulation on CPU clusters for collisional dynamics, including physical collisions in a planetesimal disc. In this paper, we show the performance and the accuracy of PENTACLE in terms of $\tilde{R}_{\rm cut}$ and a time-step $Δt$. It turns out that the accuracy of a hybrid $N$-body simulation is controlled through $Δt / \tilde{R}_{\rm cut}$ and $Δt / \tilde{R}_{\rm cut} \sim 0.1$ is necessary to simulate accurately accretion process of a planet for $\geq 10^6$ years. For all those who interested in large-scale particle simulations, PENTACLE customized for planet formation will be freely available from https://github.com/PENTACLE-Team/PENTACLE under the MIT lisence. △ Less

Submitted 29 October, 2018; originally announced October 2018.

Comments: 12 pages, 14 figures, published in PASJ

Journal ref: Publications of the Astronomical Society of Japan, 69, 81, 2017