arXiv subjects

Shingo Hirano

Publications and source records attributed to Shingo Hirano.

At least 19 recordsLinked to original sources

arXiv2026

Formation of Heavy Seed Black Holes and Little Red Dots-like Compact Clusters in Metal-enriched Star-forming Regions

The origin of supermassive black holes (SMBHs) in the early Universe remains uncertain. While the classical direct-collapse scenario requires pristine gas exposed to intense Lyman-Werner (LW) radiation, recent JWST observations suggest that early growing BHs may reside in compact, metal-enriched environments. We investigate early BH formation using a cosmological radiation-hydrodynamic simulation that self-consistently follows the formation and evolution of rapidly accreting supermassive stars (SMSs). We find that intense LW radiation develops in clustered star-forming regions; by the time the first heavy seed forms, the mass-weighted internal intensity has reached $J_{21}\sim1000$. Most seeds form in weakly metal-enriched gas with [Z/H]$\sim-3$ to $-2$, because the strongest LW fields occur in regions already enriched by previous stellar feedback. All halos above $10^9M_\odot$ in our zoom-in region host a BH more massive than $10^5M_\odot$. We construct stellar spectra using MIST/PHOENIX libraries for normal stars and $6000~\mathrm{K}$ blackbody spectra for bloated SMSs. Five of eight massive clusters with stellar masses larger than $10^6~M_\odot$ exhibit intrinsically V-shaped spectra similar to those of Little Red Dots (LRDs). In the most prominent cases, luminous SMSs dominate the red optical emission, although Balmer breaks in normal stellar populations also contribute. We also identify off-center LRD-like systems in our simulation resembling those recently reported in strongly lensed observations. Our results suggest that heavy-seed formation, compact-cluster formation, and LRD-like continuum emission are linked outcomes of clustered, weakly metal-enriched, strongly irradiated environments.

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arXiv2026

Pebbles to Gems: Intermediate-mass black holes in the first star clusters

The rapid assembly of supermassive black holes (SMBHs) observed at $z\gtrsim7$ requires efficient seeding mechanisms in the early Universe. Population III (Pop. III) star clusters have recently emerged as a promising pathway that may bridge the gap between traditional light- and heavy-seed scenarios by producing intermediate-mass black holes (IMBHs) with masses up to $\sim10^4\,\rm M_{\odot}$. We investigate the properties and number densities of IMBHs forming in Pop. III star clusters with masses $M_{\rm cl}\sim10^3-4\times10^5\,\rm M_{\odot}$, and hosted in isolated dark matter minihalos, using a suite of direct $N$-body simulations. We adopt cosmologically motivated initial conditions and explore different stellar evolution prescriptions, binary orbital parameter distributions, and cluster dynamical configurations. By $z\sim19$, the IMBH mass function consistently peaks at $m_{\rm IMBH}\sim200\,\rm M_{\odot}$, with number densities of $n_{\rm IMBH}\sim0.2-5\,\rm cMpc^{-3}$. In sufficiently dense and massive clusters, IMBHs with masses $>10^3\,\rm M_{\odot}$ can already form by $z\sim19$, reaching number densities of $n_{\rm IMBH}\sim10^{-4}-10^{-2}\,\rm cMpc^{-3}$. The most massive IMBHs in our models reach $\sim6200\,\rm M_{\odot}$ through the collapse of very massive stars assembled by repeated stellar collisions, a process enhanced in fractal clusters. Lower-mass IMBHs form instead predominantly through single and binary stellar evolution and binary stellar mergers. We find that models combining large stellar radii and tight binaries produce the highest IMBH abundances relative to isolated Pop. III evolution. Owing to the high retention fraction of IMBHs ($\gtrsim88\%$), massive dense Pop. III star clusters can act as efficient incubators of both light and heavy SMBH seeds, even if only a fraction of Pop. III stars formed in such environments.

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arXiv2026

Towards A Universal Analytical Model of Population III Star Formation: A Bridge Between Cosmological Scales and Protostars

We construct an analytical model of Population III star formation that connects the cosmological radiation background to sub-AU protostellar disk fragmentation, a dynamic range inaccessible to any single simulation. Our approach is based on combining separate models of the disparate relevant scales: from the cosmological environment to the host-halo scale, from the halo scale to the scale of the star-forming cloud, and from the cloud scale to the fragmenting, accreting protostellar disk. Individually and collectively, the models agree well with the predictions of state of the art simulations, while remaining computationally inexpensive and physically transparent. As an example of the applicability of the model, we study the effects of varying the Lyman-Werner flux on the Pop. III star formation efficiency. We show that depending on the halo properties and the strength of the dissociating radiation field, the halo-scale Pop. III star formation efficiency varies by more than two orders of magnitude from $\varepsilon_{\rm SFE,H} \approx 10^{-3}$ to $\varepsilon_{\rm SFE, H} \approx 0.5$. The abrupt transitions between hydrogen-deuteride cooling (in low virial temperature mini-halos subjected to low radiation backgrounds), molecular hydrogen cooling (at intermediate temperatures and radiation intensities), and atomic cooling (in higher temperature halos exposed to strong radiation fields) produces sharp features in the halo-scale star formation efficiency as a function of the halo properties. Meanwhile, at the scale of individual star-forming clouds, the star formation efficiency is $\varepsilon_{\rm SFE,c} \gtrsim 0.2$. That is, pristine gas in a halo is converted into unstable clouds at a wide range of efficiencies, and these unstable clouds are efficiently converted into Pop. III stars.

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arXiv2026

Multiple protostellar outflows from a single protostar with a misaligned disk

We investigate how misalignment between the core angular momentum and the large-scale magnetic field affects protostellar outflows, and whether a single protostellar system can drive multiple outflow components. We perform three-dimensional nonideal magnetohydrodynamic simulations of magnetized rotating cores, focusing on the formation of a protostar, a circumstellar disk, and magnetically driven outflows. The initial angle between the core angular-momentum vector and the magnetic field is systematically varied from $0^\circ$ to $90^\circ$. All models launch a classical magnetocentrifugal disk wind (DW) roughly along the local disk normal. For large misalignment, the system also develops a spiralflow (SF) component that propagates parallel to the disk plane. In a representative model with a $60^\circ$ misalignment, the outflow transitions from a DW-dominated to an SF-dominated phase, with the SF becoming more massive and more extended than the DW, and the two components intermittently coexisting. Across the model suite, the maximum mass and size ratios of SF to DW, as well as the relative lifetimes of the two components, increase for misalignment angles $\gtrsim60^\circ$. We propose that secondary, misaligned outflows (or their fossil remnants) observed in some protostellar systems can be interpreted as the SF component, while the main bipolar outflow traces the DW from the same misaligned system.

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arXiv2026

Formation of first star clusters under the supersonic gas flow -- III. Environmental trends and halo-to-halo scatter in the Pop III IMF

The first generations of stars ionised and enriched their host galaxies and seeded the growth of massive black holes. Models often assume that Pop III stellar masses in different minihaloes are stochastic realisations of a single universal initial mass function (IMF). We use 138 cosmological zoom-in hydrodynamics simulations to test this assumption and to map the first-star IMF across redshift, halo mass, and baryon-dark matter streaming velocity (SV). We construct a dense-cloud merger tree and assign first-star masses by mapping the radial gas accretion-rate profile to stellar mass, yielding per-halo stellar mass functions without imposing any a priori IMF. The high-mass tail and multiplicity increase systematically with redshift, halo mass, and SV. Low-mass, low-SV haloes form only one or a few first stars, whereas massive, high-SV haloes host rich first star clusters and commonly produce very massive ($\gtrsim10^3$-$10^4\,{\rm M}_\odot$) first stars. Even in a fixed environment, halo-to-halo scatter remains substantial. Nevertheless, group-averaged IMFs converge to well-defined forms, ruling out a single universal IMF at the halo level across the range of environments probed here. Mapping our seeds onto the redshift-mass plane, we show that high-SV and massive haloes preferentially populate the heavy-seed regime relevant to luminous high-redshift sources. At the same time, low-SV environments are consistent with single/few-event enrichment signatures in metal-poor stars. Our results deliver a practical, physically motivated prescription for per-halo IMF.

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arXiv2026

Twisted Pseudodisk and Asymmetric Mass Accretion on the Circumstellar Disk

We model gas inflow patterns onto circumstellar disks and the evolution of the pseudodisk using three-dimensional resistive MHD simulations. Starting from a prestellar core without turbulence and with a misalignment between the initial magnetic field and rotation axis, the simulations are performed for $\sim10^5$ yr after protostar formation. After disk formation, the magnetic field around the disk becomes significantly distorted due to the disk rotational motion. Consequently, the structure of the pseudodisk also evolves into a complex morphology. As a result, both accretion onto the disk and outflow become asymmetric and anisotropic. Accretion to the disk occurs primarily through narrow-channel flows or streams. The time evolution of the infalling envelope leads to non-steady accretion onto the disk, which in turn causes variability in the mass accretion onto the central protostar. This study demonstrates that complex infalling envelope structures and channelized accretion flows onto the disk naturally arise even without assuming turbulence or external asymmetric inflows.

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arXiv2026

Rapid emergence of overmassive black holes in the early Universe

The origin of supermassive black holes (SMBHs) remains a long-standing problem in astrophysics. Recent JWST observations reveal an unexpectedly abundant population of overmassive black holes at z>4-6, where the BH masses lie far above local scaling relations and not reproduced by current cosmological models. How such overmassive black holes form and rapidly grow within young galaxies has remained unclear. Here we present fully cosmological radiation-hydrodynamic simulations that, for the first time, self-consistently follow the birth, early growth, and emergent observable signatures of SMBHs in proto-cluster environments. We find that heavy seeds of order $10^6 M_\text{sun}$ naturally form, exceeding typical theoretical expectations by an order of magnitude. These seeds rapidly develop dense, optically thick disks whose strong electron scattering produces broad H$\alpha$ emission comparable to that seen in little red dots (LRDs). Sustained super-Eddington accretion then drives fast growth to $\sim 3 \times 10^7 ~M_\text{sun}$ by $z \sim 8$. These results provide a unified physical scenario in which LRDs correspond to a short-lived, enshrouded phase of heavy-seed formation, naturally evolving into the overmassive quasars detected by JWST and ultimately the progenitors of today's SMBHs.

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arXiv2025

Effect of Magnetic Field on the Accretion Phase of Population III Star Formation

We examine the impact of the magnetic field on Population III star formation by varying the magnetic field strength. We perform simulations with magnetic field strengths ranging from $10^{-20}$ G to $10^{-4}$ G, in addition to a model without a magnetic field. The simulations are run for $>1000-1400$ yr after the first protostar forms. In weak-field models, the surrounding disk fragments, forming multiple protostars, and the magnetic field is amplified by the orbital motion and rotation of these protostars. In the model without a magnetic field, frequent fragmentation occurs, and the most massive protostar reaches $\sim200 M_\odot$. However, in models with a magnetic field, once the magnetic field is amplified, the protostars merge to form a single massive protostar, and no further fragmentation occurs except in the model with the strongest magnetic field. Even after the formation of the single protostar, the magnetic field continues to amplify, leading to the formation of a thick disk supported by magnetic pressure and a global spiral pattern. In models with moderate or strong magnetic fields, a rotating disk can form, but fragmentation does not occur, and a strong magnetic field drives an outflow. However, the range of parameters for both disk formation and outflow driving is very narrow, making their appearance under realistic conditions unlikely. Given the weak magnetic field in the early universe, Population III stars are expected to form as single stars, surrounded by a thick disk with a spiral pattern. Thus, the magnetic field, regardless of its strength, plays a crucial role in Population III star formation.

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arXiv2025

Dark Matter Density Profile Around a Newborn First Star

Ambient dark matter (DM) around binary black holes can imprint characteristic signatures on gravitational waves emitted from their merger. The exact signature depends sensitively on the DM density profile around the black holes. We run very high resolution cosmological hydrodynamics simulations of first star formation that follow the collapse of a $3\times10^{5}\,M_\odot$ mini-halo from $z=49$ to $z\simeq22$. Our flagship model achieves a DM particle mass of $3.7\times10^{-4}\,M_\odot$ and resolves the inner-most structure down to $0.02\,$pc. We show that the halo experiences a two-stage gravitational collapse, where a rotating, constant-density core with $r\lesssim3\,$pc is formed first, surrounded by an extended outskirts. Baryonic infall toward the center continues to raise the local Keplerian velocity and promotes adiabatic contraction of DM. The resulting density profile has an approximately power-law shape of $\rho_{\rm dm} \propto r^{-0.6}$ inside $\sim\!1\,$pc.We find that a piecewise power-law fit reproduces the simulation result to better than 10\%, and also find numerical convergence down to $\simeq\!0.01\,$pc. The DM density profile is typical for ordinary Pop~III halos, but our additional simulations reveal that inner slope varies significantly with halo-to-halo scatter, and the effect of Lyman-Werner irradiation and of supersonic baryon-DM streaming velocities, implying a wide distribution of slopes rather than a single universal curve. The large variation should be considered when calculating the predicted DM-induced dephasing of gravitational waves by up to an order of magnitude relative to the classical analytic model of the DM spike.

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arXiv2025

Formation of first star clusters under the supersonic gas flow -- II. Critical halo mass and core mass function

The formation and mass distribution of the first stars depend on various environmental factors in the early universe. We compare 120 cosmological hydrodynamical simulations to explore how the baryonic streaming velocity (SV) relative to dark matter affects the formation of the first stars. We vary SV from zero to three times its cosmic root-mean-square value, $v_{\rm SV}/\sigma_{\rm SV}=0-3$, and identify 20 representative halos from cosmological simulations. For each model, we follow the evolution of a primordial star-forming cloud from the first appearance of a dense core (with gas density > $10^{6}\,{\rm cm^{-3}}$) until 2 Myr later. In each model, higher SV systematically delays the formation of primordial clouds, formed inside more massive halos ($10^{5}-10^{7}\,{\rm M}_\odot$), and promotes cloud-scale fragmentation and multiple-core formation. The number and total mass of dense cores increase with increasing SV. More than half of models with $v_{\rm SV}/\sigma_{\rm SV} \ge 1.5$ form three or more dense cores in a single halo. In extreme cases, up to 25 cores form at once, which leaves a massive first star cluster. On the other hand, models with $v_{\rm SV}/\sigma_{\rm SV} \leq 1$ form only one or two cores in a halo. In addition, HD-cooling is often enabled in models with low SV, especially in low-z, where HD-cooling is enabled in more than 50% of models. This leads to the formation of the low-mass first star. SV shapes the resulting initial mass function of the first stars and plays a critical role in setting the star-forming environment of the first galaxies.

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arXiv2025

Velocity Structure of Circumstellar Environment around Class 0/I Protostars: Uncertainty in the Protostellar Mass Estimation Using Circumstellar Velocities

Recent high-resolution observations have enabled detailed investigations of the circumstellar environments around Class 0/I protostars. Several studies have reported that the infall velocity of the envelope is a few times smaller than the free-fall velocity inferred from protostellar masses estimated via the observed rotational velocity of their Keplerian disks. To explore the physical origins of the slow infall, we perform a set of three-dimensional resistive magnetohydrodynamic simulations of the star formation process, extending to $10^5$ yr after protostar formation. Our simulations show that the infall velocity decreases markedly at the outer edge of the pseudo-disk (at radii of $\sim\!100-1000$ au) and is much slower than the expected free-fall velocity. The degree of this reduction depends on (1) the initial magnetic field strength, (2) the alignment between the initial field and the rotation axis, and (3) the evolutionary stage of the system. Across our parameter space, the ratio of the infall velocity to the free-fall velocity is as small as $0.2-0.5$, which is consistent with the observations. We further examine the reliability of protostellar mass estimates derived from infall and rotational velocities. While the mass derived from disk rotation closely matches the true value, deviation by a factor of $0.3-2$ is found for the estimates using the infall velocity; it is underestimated due to slow infall, but could also be overestimated due to the contribution of disk mass. These findings underscore the critical role of magnetic fields in shaping star formation dynamics and highlight the uncertainties associated with protostellar mass estimates.

astro-ph.SR↗

arXiv2025

A Semi-analytic Framework of Population III and Subsequent Galaxy Formation on Cosmological N-body Simulations

We develop a new semi-analytic framework of Population (Pop) III and subsequent galaxy formation designed to run on dark matter halo merger trees. In our framework, we consider the effect of the Lyman-Werner flux from Pop III and II stars and the dark matter baryon streaming velocity on the critical halo mass for the Pop III formation. Our model incorporates the Lyman-Werner feedback in a self-consistent way, therefore, the spatial variation of Lyman-Werner feedback naturally emerges. The Pop III mass depends on the properties of a halo as reproducing radiative hydrodynamical simulation results. We perform statistical studies of Pop III stars by applying this framework to high-resolution cosmological N-body simulations with a maximum box size of 16 Mpc/h and enough mass resolution to resolve Pop III-forming halos. A top-heavy initial mass function emerges and two peaks corresponding to the H$_2$ ($20 \lesssim z \lesssim 25$) and atomic cooling halos ($z \lesssim 15$) exist in the distribution. Supermassive stars can be formed in the atomic cooling halos, and the fractions of such supermassive stars increase with the value of streaming velocity. At least an 8 Mpc/h simulation box and the self-consistent model for the Lyman-Werner feedback are necessary to correctly model the Pop III formation in the atomic cooling halos. Our model predicts one supermassive star per halo with several $10^9$ Msun at z=7.5, which is enough to reproduce a high redshift quasar.

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arXiv2024

An Analytic Model of Gravitational Collapse Induced by Radiative Cooling: Instability Scale, Infall Velocity, and Accretion Rate

We present an analytic description of the spherically symmetric gravitational collapse of radiatively cooling gas clouds, which illustrates the mechanism by which radiative cooling induces gravitational instability at a characteristic mass scale determined by the microphysics of the gas. The approach is based on developing the "one-zone" density-temperature relationship of the gas into a full dynamical model. We convert this density-temperature relationship into a barotropic equation of state, which we use to calculate the density and velocity profiles of the gas. From these quantities, we calculate the time-dependent mass accretion rate onto the center of the cloud. The approach clarifies the mechanism by which radiative cooling induces gravitational instability. In particular, we distinguish the rapid, quasi-equilibrium contraction of a cooling gas core to high central densities from the legitimate instability this contraction establishes in the envelope. We develop a refined criterion for the mass scale of this instability, based only on the chemical-thermal evolution in the core. We explicate our model in the context of a primordial mini-halo cooled by molecular hydrogen, and then provide two further examples, a delayed collapse with hydrogen deuteride cooling and the collapse of an atomic cooling halo. In all three cases, we show that our results agree well with full hydrodynamical treatments.

astro-ph.GA↗

arXiv2024

Towards a universal analytical model for Population III star formation: interplay between feedback and fragmentation

JWST has brought us new insights into Cosmic Dawn with tentative detection of the unique signatures of metal-free Population III (Pop III) stars, such as strong HeII emission, extremely blue UV spectrum, and enhanced nitrogen abundance. Self-consistent theoretical predictions of the formation rates, sites, and masses of Pop III stars are crucial for interpreting the observations, but are challenging due to complex physical processes operating over the large range of length scales involved. One solution is to combine analytical models for the small-scale star formation process with cosmological simulations that capture the large-scale physics such as structure formation, radiation backgrounds, and baryon-dark matter streaming motion that regulate the conditions of Pop III star formation. We build an analytical model to predict the final masses of Pop III stars/clusters from the properties of star-forming clouds, based on the key results of small-scale star formation simulations and stellar evolution models. Our model for the first time considers the interplay between feedback and fragmentation and covers different modes of Pop III star formation ranging from ordinary small ($\sim 10-2000\ \rm M_\odot$) clusters in molecular-cooling clouds to massive ($\gtrsim 10^{4}\ \rm M_\odot$) clusters containing supermassive ($\sim 10^{4}-3\times 10^{5}\ \rm M_\odot$) stars under violent collapse of atomic-cooling clouds. As an example, the model is applied to the Pop III star-forming clouds in the progenitors of typical haloes hosting high-$z$ luminous quasars, which shows that formation of Pop III massive clusters is common ($\sim 20-70\%$) in such biased ($\sim4\sigma$) regions, and the resulting heavy black hole seeds from supermassive stars can account for a significant fraction of observed luminous ($\gtrsim 10^{46}\ \rm erg\ s^{-1}$) quasars at $z\sim 6$.

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arXiv2023

Low-mass Pop III star formation due to the HD-cooling induced by weak Lyman-Werner radiation

Lyman-Werner (LW) radiation photodissociating molecular hydrogen (H$_2$) influences the thermal and dynamical evolution of the Population III (Pop III) star-forming gas cloud. The effect of powerful LW radiation has been well investigated in the context of supermassive black hole formation in the early universe. However, the average intensity in the early universe is several orders of magnitude lower. For a comprehensive study, we investigate the effects of LW radiation at $18$ different intensities, ranging from $J_{\rm LW}/J_{21}=0$ (no radiation) to $30$ (H-cooling cloud), on the primordial star-forming gas cloud obtained from a three-dimensional cosmological simulation. The overall trend with increasing radiation intensity is a gradual increase in the gas cloud temperature, consistent with previous works. Due to the HD-cooling, on the other hand, the dependence of gas cloud temperature on $J_{\rm LW}$ deviates from the aforementioned increasing trend for a specific range of intensities ($J_{\rm LW}/J_{21}=0.025-0.09$). In HD-cooling clouds, the temperature remained below $200$ K during $10^5$ yr after the first formation of the high-density region, maintaining a low accretion rate. Finally, the HD-cooling clouds have only a low-mass dense core (above $10^8\,{\rm cm^{-3}}$) with about $1-16\, M_{\odot}$, inside which a low-mass Pop III star with $\leq\!0.8\,M_{\odot}$ (so-called "surviving star") could form. The upper limit of star formation efficiency $M_{\rm core}/M_{\rm vir, gas}$ significantly decreases from $10^{-3}$ to $10^{-5}$ as HD-cooling becomes effective. This tendency indicates that, whereas the total gas mass in the host halo increases with the LW radiation intensity, the total Pop III stellar mass does not increase similarly.

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arXiv2023

Secondary outflow driven by the protostar Ser-emb 15 in Serpens

We present the detection of a secondary outflow associated with a Class I source, Ser-emb 15, in the Serpens Molecular Cloud. We reveal two pairs of molecular outflows consisting of three lobes, namely primary and secondary outflows, using ALMA 12CO and SiO line observations at a resolution of 318 au. The secondary outflow is elongated approximately perpendicular to the axis of the primary outflow in the plane of the sky. We also identify two compact structures, Sources A and B, within an extended structure associated with Ser-emb 15 in the 1.3 mm continuum emission at a resolution of 40 au. The projected sizes of Sources A and B are 137 au and 60 au, respectively. Assuming a dust temperature of 20 K, we estimate the dust mass to be 0.0024 Msun for Source A and 0.00033 Msun for Source B. C18O line data imply the existence of rotational motion around the extended structure, however, cannot resolve rotational motion in Source A and/or B, due to insufficient angular and frequency resolutions. Therefore, we cannot conclude whether Ser-emb 15 is a single or binary system. Thus, either Source A or B could drive the secondary outflow. We discuss two scenarios to explain the driving mechanism of the primary and secondary outflows: the Ser-emb 15 system is (1) a binary system composed of Source A and B or (2) a single star system composed of only Source A. In either case, the system could be a suitable target for investigating the disk and/or binary formation processes in complicated environments. Detecting these outflows should contribute to understanding complex star-forming environments, which may be common in the star-formation processes.

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arXiv2023

Formation of first star clusters under the supersonic gas flow -- I. Morphology of the massive metal-free gas cloud

We performed $42$ simulations of the first star formation with initial supersonic gas flows relative to the dark matter at the cosmic recombination era. Increasing the initial streaming velocities led to delayed halo formation and increased halo mass, enhancing the mass of the gravitationally shrinking gas cloud. For more massive gas clouds, the rate of temperature drop during contraction, in other words, the structure asymmetry, becomes more significant. When the maximum and minimum gas temperature ratios before and after contraction exceed about ten, the asymmetric structure of the gas cloud prevails, inducing fragmentation into multiple dense gas clouds. We continued our simulations until $10^5$ years after the first dense core formation to examine the final fate of the massive star-forming gas cloud. Among the $42$ models studied, we find the simultaneous formation of up to four dense gas clouds, with a total mass of about $2254\,M_\odot$. While the gas mass in the host halo increases with increasing the initial streaming velocity, the mass of the dense cores does not change significantly. The star formation efficiency decreases by more than one order of magnitude from $ε_{\rm III} \sim 10^{-2}$ to $10^{-4}$ when the initial streaming velocity, normalised by the root mean square value, increases from 0 to 3.

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arXiv2023

Formation of Massive and Wide First-star Binaries in Radiation Hydrodynamics Simulations

We study the formation of Pop III stars by performing radiation hydrodynamics simulations for three different initial clouds extracted from cosmological hydrodynamics simulations. Starting from the cloud collapse stage, we follow the growth of protostars by accretion for $\sim 10^5$ yr until the radiative feedback from the protostars suppresses the accretion and the stellar properties are nearly fixed. We find that the Pop III stars form in massive and wide binaries/small-multiple stellar systems, with masses $>30\,M_\odot$ and separations $>2000$ au. We also find that the properties of the final stellar system correlate with those of the initial clouds: the total mass increases with the cloud-scale accretion rate, and the angular momentum of the binary orbit matches that of the initial cloud. While the total mass of the system in our simulations is consistent with our previous single-star formation simulations, individual masses are lower due to mass sharing, suggesting potential modification in the extent of feedback from Pop III stars in the subsequent evolution of the Universe. We also identify such systems as mini-binaries embedded in a wider outer multiple-star system, which could evolve into progenitors for observed gravitational wave events.

astro-ph.CO↗