arXiv subjects

Takayuki R. Saitoh

Publications and source records attributed to Takayuki R. Saitoh.

At least 37 records · Page 2Linked to original sources

arXiv2023

Exploring the Sun's birth radius and the distribution of planet building blocks in the Milky Way galaxy: A multi-zone Galactic chemical evolution approach

We explore the influence of the Milky Way galaxy's chemical evolution on the formation, structure, and habitability of the Solar system. Using a multi-zone Galactic Chemical Evolution (GCE) model, we successfully reproduce key observational constraints, including the age-metallicity ([Fe/H]) relation, metallicity distribution functions, abundance gradients, and [X/Fe] ratio trends for critical elements involved in planetary mineralogy, including C, O, Mg, and Si. Our GCE model suggests that the Sun formed in the inner Galactic disc, $R_{\rm birth,\odot}\approx 5$ kpc. We also combined a stoichiometric model with the GCE model to examine the temporal evolution and spatial distribution of planet building blocks (PBBs) within the Milky Way galaxy, revealing trends in the condensed mass fraction ($f_{\rm cond}$), iron-to-silicon mass fraction ($f_{\rm iron}$), and water mass fraction ($f_{\rm water}$) over time and towards the inner Galactic disc regions. Specifically, our model predicts a higher $f_{\rm cond}$ in the protoplanetary disc within the inner regions of the Milky Way galaxy, as well as an increased $f_{\rm iron}$ and a decreased $f_{\rm water}$ in the inner regions. Based on these findings, we discuss the potential impact of the Sun's birth location on the overall structure and habitability of the Solar System.

astro-ph.GA↗

arXiv2023

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

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.

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arXiv2023

$N$-body simulation of planetary formation through pebble accretion in a radially structured protoplanetary disk

In the conventional theory of planet formation, it is assumed that protoplanetary disks are axisymmetric and have a smooth radial profile. However, recent radio observations of protoplanetary disks have revealed that many of them have complex radial structures. In this study, we perform a series of $N$-body simulations to investigate how planets are formed in protoplanetary disks with radial structures. For this purpose, we consider the effect of continuous pebble accretion onto the discontinuity boundary within the terrestrial planet-forming region ($\sim0.6$ AU). We found that protoplanets grow efficiently at the discontinuity boundary, reaching the Earth mass within $\sim10^4$ years. We confirmed that giant collisions of protoplanets occur universally in our model. Moreover, we found that multiple planet-sized bodies form at regular intervals in the vicinity of the discontinuity boundary. These results indicate the possibility of the formation of solar system-like planetary systems in radially structured protoplanetary disks.

astro-ph.EP↗

arXiv2022

Origin of highly $r$-process-enhanced stars in a cosmological zoom-in simulation of a Milky Way-like galaxy

The $r$-process-enhanced (RPE) stars provide fossil records of the assembly history of the Milky Way and the nucleosynthesis of the heaviest elements. Observations by the $R$-Process Alliance (RPA) and others have confirmed that many RPE stars are associated with chemo-dynamically tagged groups, which likely came from accreted dwarf galaxies of the Milky Way (MW). However, we do not know how RPE stars are formed. Here, we present the result of a cosmological zoom-in simulation of an MW-like galaxy with $r$-process enrichment, performed with the highest resolution in both time and mass. Thanks to this advancement, unlike previous simulations, we find that most highly RPE ($r$-II; [Eu/Fe] $> +0.7$) stars are formed in low-mass dwarf galaxies that have been enriched in $r$-process elements for [Fe/H] $\,<-2.5$, while those with higher metallicity are formed in situ, in locally enhanced gas clumps that were not necessarily members of dwarf galaxies. This result suggests that low-mass accreted dwarf galaxies are the main formation site of $r$-II stars with [Fe/H] $\,<-2.5$. We also find that most low-metallicity $r$-II stars exhibit halo-like kinematics. Some $r$-II stars formed in the same halo show low dispersions in [Fe/H] and somewhat larger dispersions of [Eu/Fe], similar to the observations. The fraction of simulated $r$-II stars is commensurate with observations from the RPA, and the distribution of the predicted [Eu/Fe] for halo $r$-II stars matches that observed. These results demonstrate that RPE stars can be valuable probes of the accretion of dwarf galaxies in the early stages of their formation.

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arXiv2022

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

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.

astro-ph.GA↗

arXiv2021

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

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.

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arXiv2021

Testing the effect of resolution on gravitational fragmentation with Lagrangian hydrodynamic schemes

To study the resolution required for simulating gravitational fragmentation with newly developed Lagrangian hydrodynamic schemes, Meshless Finite Volume method (MFV) and Meshless Finite Mass method (MFM), we have performed a number of simulations of the Jeans test and compared the results with both the expected analytic solution and results from the more standard Lagrangian approach: Smoothed Particle Hydrodynamics (SPH). We find that the different schemes converge to the analytic solution when the diameter of a fluid element is smaller than a quarter of the Jeans wavelength, $λ_\mathrm{J}$. Among the three schemes, SPH/MFV shows the fastest/slowest convergence to the analytic solution. Unlike the well-known behaviour of Eulerian schemes, none of the Lagrangian schemes investigated displays artificial fragmentation when the perturbation wavelength, $λ$, is shorter than $λ_\mathrm{J}$, even at low numerical resolution. For larger wavelengths ($λ> λ_\mathrm{J}$) the growth of the perturbation is delayed when it is not well resolved. Furthermore, with poor resolution, the fragmentation seen with the MFV scheme proceeds very differently compared to the converged solution. All these results suggest that, when unresolved, the ratio of the magnitude of hydrodynamic force to that of self-gravity at the sub-resolution scale is the largest/smallest in MFV/SPH, the reasons for which we discussed in detail. These tests are repeated to investigate the effect of kernels of higher-order than the fiducial cubic spline. Our results indicate that the standard deviation of the kernel is a more appropriate definition of the size of a fluid element than its compact support radius.

astro-ph.IM↗

arXiv2021

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

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.

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arXiv2021

First Star Survivors as Metal-Rich Halo Stars that Experienced Supernova Explosions in Binary Systems

The search for the first stars formed from metal-free gas in the universe is one of the key issues in astronomy because it relates to many fields, such as the formation of stars and galaxies, the evolution of the universe, and the origin of elements. It is not still clear if metal-free first stars can be found in the present universe. These first stars are thought to exist among extremely metal-poor stars in the halo of our Galaxy. Here we propose a new scenario for the formation of low-mass first stars that have survived until today and observational counterparts in our Galaxy. The first stars in binary systems, consisting of massive- and low-mass stars, are examined using stellar evolution models, simulations of supernova ejecta colliding with low-mass companions, and comparisons with observed data. These first star survivors will be observed as metal-rich halo stars in our Galaxy. We may have identified a candidate star in the observational database where elemental abundances and kinematic data are available. Our models also account for the existence of several solar-metallicity stars in the literature having space velocities equivalent to the halo population. The proposed scenario demands a new channel of star formation in the early universe and is a supplementary scenario for the origin of the known metal-poor stars.

astro-ph.SR↗

arXiv2019

Enrichment of Strontium in Dwarf Galaxies

Light trans-iron elements such as Sr serve as the key to understanding the astrophysical sites of heavy elements. Spectroscopic studies of metal-poor stars have revealed large star-to-star scatters in the ratios of [Sr/Ba], which indicates that there are multiple sites for the production of Sr. Here we present the enrichment history of Sr by a series of the $N$-body/smoothed particle hydrodynamics simulations of a dwarf galaxy with a stellar mass of 3 $\times$ 10$^{6}$ $M_{\odot}$. We show that binary neutron star mergers (NSMs) and asymptotic giant branch (AGB) stars contribute to the enrichment of Sr in the metallicity ranges [Fe/H] $\gtrsim$ $-$3 and [Fe/H] $\gtrsim$ $-$1, respectively. It appears insufficient, however, to explain the overall observational trends of Sr by considering only these sites. We find that the models including electron-capture supernovae (ECSNe) and rotating massive stars (RMSs), in addition to NSMs and AGBs, reasonably reproduce the enrichment histories of Sr in dwarf galaxies. The contributions of both ECSNe and NSMs make scatters of $\approx$ 0.2 dex in [Sr/Fe], [Sr/Ba], and [Sr/Zn] as can be seen for observed stars in the metallicity range [Fe/H] $<$ $-2$. We also find that the mass range of ECSN progenitors should be substantially smaller than $1\, M_\odot$ (e.g., 0.1-$0.2\, M_\odot$) to avoid over-prediction of [Sr/Ba] and [Sr/Zn] ratios. Our results demonstrate that NSMs, AGBs, ECSNe, and RMSs all play roles in the enrichment histories of Local Group dwarf galaxies, although more observational data are required to disentangle the relative contributions of these sources.

astro-ph.GA↗

arXiv2018

Radial Velocity Survey of Nearby OB Stars

We report on the current status of the radial velocity monitoring of nearby OB stars to look for binaries with small mass ratios. The combined data of radial velocities using the domestic 1-2 m-class telescopes seems to confirm the variations of radial velocities in a few weeks for four out of ten target single-lined spectroscopic binaries. More data are needed to estimate the exact periods and mass distributions.

astro-ph.SR↗

arXiv2018

Compact object mergers driven by gas fallback

Recently several gravitational wave detections have shown evidence for compact object mergers. However, the astrophysical origin of merging binaries is not well understood. Stellar binaries are typically at much larger separations than what is needed for the binaries to merge due to gravitational wave emission, which leads to the so-called final AU problem. In this letter we propose a new channel for mergers of compact object binaries which solves the final AU problem. We examine the binary evolution following gas expansion due to a weak failed supernova explosion, neutrino mass loss, core disturbance, or envelope instability. In such situations the binary is possibly hardened by ambient gas. We investigate the evolution of the binary system after a shock has propagated by performing smoothed particle hydrodynamics simulations. We find that significant binary hardening occurs when the gas mass bound to the binary exceeds that of the compact objects. This mechanism represents a new possibility for the pathway to mergers for gravitational wave events.

astro-ph.HE↗

arXiv2018

Discovery of a Molecular Collision Front in Interacting Galaxies NGC 4567/4568 with ALMA

We present results of $^{12}$CO(J = 1-0) imaging observations of NGC 4567/4568, a galaxy pair in a close encounter, with Atacama Large Millimetre/submillimetre Array. For the first time, we find clear evidence of a molecular collision front whose velocity dispersion is 16.8$\pm$1.4 km s$^{-1}$ at the overlapping region owing to high spatial and velocity resolution. By integrating over the velocity width that corresponds to the molecular collision front, we find a long filamentary structure with a size of 1800 pc $\times$ 350 pc at the collision front. This filamentary molecular structure spatially coincides with a dark-lane seen in R-band image. We find four molecular clouds in the filament, each with a radius of 30 pc and mass of 10$^{6} M_{\odot}$; the radii matching a typical value for giant molecular clouds and the masses corresponding to those between giant molecular clouds and giant molecular associations. All four clouds are gravitationally bound. The molecular filamentary structure and its physical conditions are similar to the structure expected via numerical simulation. The filament could be a progenitor of super star clusters.

astro-ph.GA↗

arXiv2018

Enrichment of Zinc in galactic chemodynamical evolution models

The heaviest iron-peak element, Zn has been used as an important tracer of cosmic chemical evolution. Spectroscopic observations of the metal-poor stars in Local Group galaxies show that an increasing trend of [Zn/Fe] ratios toward lower metallicity. However, enrichment of Zn in galaxies is not well understood due to the poor knowledge of astrophysical sites of Zn as well as metal mixing in galaxies. Here we show possible explanations for the observed trend by taking into account electron-capture supernovae (ECSNe) as one of the sources of Zn in our chemodynamical simulations of dwarf galaxies. We find that the ejecta from ECSNe contribute to stars with [Zn/Fe] $\gtrsim$ 0.5. We also find that scatters of [Zn/Fe] in higher metallicity originate from the ejecta of type Ia supernovae. On the other hand, it appears difficult to explain the observed trends if we do not consider ECSNe as a source of Zn. These results come from inhomogeneous spatial metallicity distribution due to the inefficiency of metal mixing. We find that the optimal value of scaling factor for metal diffusion coefficient is $\sim$ 0.01 in the shear-based metal mixing model in smoothed particle hydrodynamics simulations. These results suggest that ECSNe can be one of the contributors to the enrichment of Zn in galaxies.

astro-ph.GA↗

arXiv2017

Hydrodynamic simulations of the central molecular zone with realistic Galactic potential

We present hydrodynamic simulations of gas clouds inflowing from the disk to a few hundred parsec region of the Milky Way. A gravitational potential is generated to include realistic Galactic structures by using thousands of multipole expansions that describe 6.4 million stellar particles of a self-consistent Galaxy simulation. We find that a hybrid multipole expansion model, with two different basis sets and a thick disk correction, accurately reproduces the overall structures of the Milky Way. Through non-axisymmetric Galactic structures of an elongated bar and spiral arms, gas clouds in the disk inflow to the nuclear region and form a central molecular zone (CMZ)-like nuclear ring. We find that the size of the nuclear ring evolves into ~240 pc at T~1500 Myr, regardless of the initial size. For most simulation runs, the rate of gas inflow to the nuclear region is equilibrated to ~0.02 M_sun/yr. The nuclear ring is off-centered, relative to the Galactic center, by the lopsided central mass distribution of the Galaxy model, and thus an asymmetric mass distribution of the nuclear ring arises accordingly. The vertical asymmetry of the the Galaxy model also causes the nuclear ring to be tilted along the Galactic plane. During the first ~100 Myr, the vertical frequency of the gas motion is twice that of the orbital frequency, thus the projected nuclear ring shows a twisted, infinity-like shape.

astro-ph.GA↗

arXiv2017

Efficiency of metal mixing in dwarf galaxies

Metal mixing plays critical roles in the enrichment of metals in galaxies. The abundance of elements such as Mg, Fe, and Ba in metal-poor stars help us understand the metal mixing in galaxies. However, the efficiency of metal mixing in galaxies is not yet understood. Here we report a series of $N$-body/smoothed particle hydrodynamics simulations of dwarf galaxies with different efficiencies of metal mixing using turbulence-induced mixing model. We show that metal mixing apparently occurs in dwarf galaxies from Mg and Ba abundance. We find that the scaling factor for metal diffusion larger than 0.01 is necessary to reproduce the observation of Ba abundance in dwarf galaxies. This value is consistent with the value expected from turbulence theory and experiment. We also find that timescale of metal mixing is less than 40 Myr. This timescale is shorter than that of typical dynamical times of dwarf galaxies. We demonstrate that the determination of a degree of scatters of Ba abundance by the observation will help us to constrain the efficiency of metal mixing more precisely.

astro-ph.GA↗

arXiv2017

Imprints of Zero-Age Velocity Dispersions and Dynamical Heating on the Age-Velocity dispersion Relation

Observations of stars in the the solar vicinity show a clear tendency for old stars to have larger velocity dispersions. This relation is called the age-velocity dispersion relation (AVR) and it is believed to provide insight into the heating history of the Milky Way galaxy. Here, in order to investigate the origin of the AVR, we performed smoothed particle hydrodynamic simulations of the self-gravitating multiphase gas disks in the static disk-halo potentials. Star formation from cold and dense gas is taken into account, and we analyze the evolution of these star particles. We find that exponents of simulated AVR and the ratio of the radial to vertical velocity dispersion are close to the observed values. We also find that the simulated AVR is not a simple consequence of dynamical heating. The evolution tracks of stars with different epochs evolve gradually in the age-velocity dispersion plane as a result of: (1) the decrease in velocity dispersion in star forming regions, and (2) the decrease in the number of cold/dense/gas as scattering sources. These results suggest that the AVR involves not only the heating history of a stellar disk, but also the historical evolution of the ISM in a galaxy.

astro-ph.GA↗

arXiv2016

Early chemo-dynamical evolution of dwarf galaxies deduced from enrichment of r-process elements

The abundance of elements synthesized by the rapid neutron-capture process (r-process elements) of extremely metal-poor (EMP) stars in the Local Group galaxies gives us clues to clarify the early evolutionary history of the Milky Way halo. The Local Group dwarf galaxies would have similarly evolved with building blocks of the Milky Way halo. However, how the chemo-dynamical evolution of the building blocks affects the abundance of r-process elements is not yet clear. In this paper, we perform a series of simulations using dwarf galaxy models with various dynamical times and total mass, which determine star-formation histories. We find that galaxies with dynamical times longer than 100 Myr have star formation rates less than $10^{-3} M_{\odot}$ yr$^{-1}$ and slowly enrich metals in their early phase. These galaxies can explain the observed large scatters of r-process abundance in EMP stars in the Milky Way halo regardless of their total mass. On the other hand, the first neutron star merger appears at a higher metallicity in galaxies with a dynamical time shorter than typical neutron star merger times. The scatters of r-process elements mainly come from inhomogeneity of the metals in the interstellar medium whereas the scatters of $α$-elements are mostly due to the difference in the yield of each supernova. Our results demonstrate that the future observations of r-process elements in EMP stars will be able to constrain the early chemo-dynamical evolution of the Local Group galaxies.

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