arXiv subjects

Gen Chiaki

Publications and source records attributed to Gen Chiaki.

35 records · Page 2Linked to original sources

arXiv2021

Disc fragmentation and oligarchic growth of protostellar systems in low-metallicity gas clouds

We study low-metallicity star formation with a set of high-resolution hydrodynamics simulations for various gas metallicities over a wide range $0$--$10^{-3} \ {\rm Z}_{\bigodot}$. Our simulations follow non-equilibrium chemistry and radiative cooling by adopting realistic elemental abundance and dust size distribution. We examine the condition for the fragmentation of collapsing clouds (cloud fragmentation; CF) and of accretion discs (disc fragmentation; DF). We find that CF is suppressed due to rapid gas heating accompanied with molecular hydrogen formation even with efficient dust cooling for metallicities $\gtrsim 10^{-5} \ {\rm Z}_{\bigodot}$. Instead, DF occurs in almost all runs regardless of metallicity. We also find that, in the accretion discs, the growth of the protostellar systems is overall oligarchic. The primary protostar grows through the accretion of gas, and secondary protostars form through the interaction of spiral arms or the break-up of a rapidly rotating protostar. Despite vigorous fragmentation, a large fraction of secondary protostars are destroyed through mergers or tidal disruption events. For a few hundred years after the first adiabatic core formation, only several protostars survive in the disc, and the total mass of protostars is $0.52$--$3.8 \ {\rm M}_{\bigodot}$.

astro-ph.GA↗

arXiv2021

Amplification of turbulence in contracting prestellar cores in primordial minihalos

We investigate the amplification of turbulence through gravitational contraction of the primordial gas in minihalos. We perform numerical simulations to follow the cloud collapse, assuming polytropic equations of state for different initial turbulent Mach numbers and resolutions. We find that the turbulent velocity is amplified solely by gravitational contraction, and eventually becomes comparable to the sound speed, even for small initial turbulent Mach numbers (${\cal M}_0 \gtrsim 0.05$). We derive an analytic formula for the amplification of turbulent velocity in a collapsing cloud, and find that our numerical results are consistent with the formula. These results suggest that the turbulence can play an important role in collapsing clouds for general cases.

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arXiv2021

Formation of massive stars under protostellar radiation feedback: Very metal-poor stars

We study the formation of very metal-poor stars under protostellar radiative feedback effect. We use cosmological simulations to identify low-mass dark matter halos and star-forming gas clouds within them. We then follow protostar formation and the subsequent long-term mass accretion phase of over one million years using two-dimensional radiation-hydrodynamics simulations. We show that the critical physical process that sets the final mass is formation and expansion of a bipolar HII region. The process is similar to the formation of massive primordial stars, but radiation pressure exerted on dust grains also contributes to halting the accretion flow in the low-metallicity case. We find that the net feedback effect in the case with metallicity $Z = 10^{-2}~Z_{\odot}$ is stronger than in the case with $Z \sim 1~Z_{\odot}$. With decreasing metallicity, the radiation pressure effect becomes weaker, but photoionization heating of the circumstellar gas is more efficient owing to the reduced dust attenuation. In the case with $Z = 10^{-2}~Z_{\odot}$, the central star grows as massive as 200 solar-masses, similarly to the case of primordial star formation. We conclude that metal-poor stars with a few hundred solar masses can be formed by gas accretion despite the strong radiative feedback.

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arXiv2020

Does the structure of Pop III supernova ejecta affect the elemental abundance of extremely metal-poor stars?

The first generation of metal-free (Pop III) stars are crucial for the production of heavy elements in the earliest phase of structure formation. Their mass scale can be derived from the elemental abundance pattern of extremely metal-poor (EMP) stars, which are assumed to inherit the abundances of uniformly mixed supernova (SN) ejecta. If the expanding ejecta maintains its initial stratified structure, the elemental abundance pattern of EMP stars might be different from that from uniform ejecta. In this work we perform numerical simulations of the metal enrichment from stratified ejecta for normal core-collapse SNe (CCSNe) with a progenitor mass $25 \ {\rm M}_{\bigodot}$ and explosion energies 0.7--10 B ($1 \ {\rm B} = 10^{51}$ erg). We find that SN shells fall back into the central minihalo in all models. In the recollapsing clouds, the abundance ratio ${\rm [M/Fe]}$ for stratified ejecta is different from the one for uniform ejecta only within $\pm 0.4$ dex for any element M. We also find that, for the largest explosion energy (10 B), a neighboring halo is also enriched. Only the outer layers containing Ca or lighter elements reach the halo, where ${\rm [C/Fe]} = 1.49$. This means that C-enhanced metal-poor (CEMP) stars can form from the CCSN even with an average abundance ratio ${\rm [C/Fe]} = -0.65$.

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arXiv2020

Seeding the second star -- II. CEMP star formation enriched from faint supernovae

Carbon-enhanced metal-poor (CEMP) stars are the living fossils holding records of chemical enrichment from early generations of stars. In this work, we perform a set of numerical simulations of the enrichment from a supernova (SN) of a first generation of metal-free (Pop III) star and the gravitational collapse of the enriched cloud, considering all relevant cooling/heating processes and chemical reactions as well as the growth of dust grains. We adopt faint SN models for the first time with progenitor masses $M_{\rm PopIII} = 13$--$80 \ {\rm M}_{\bigodot}$, which yield C-enhanced abundance patterns (${\rm [C/Fe]} = 4.57$--$4.75$) through mixing and fallback of innermost layers of the ejecta. This model also considers the formation and destruction of dust grains. We find that the metals ejected by the SN can be partly re-accreted by the same dark matter minihalo, and carbon abundance of the enriched cloud $A({\rm C}) = 3.80$--$5.06$ is lower than the abundance range of observed CEMP stars ($A({\rm C}) \gtrsim 6$) because the mass of the metals ejected by faint SNe is smaller than normal core-collapse SNe due to extensive fallback. We also find that cloud fragmentation is induced by gas cooling from carbonaceous grains for $M_{\rm PopIII} = 13 \ {\rm M}_{\bigodot}$ even with the lowest iron abundance ${\rm [Fe/H]} \sim -9$. This leads to the formation of low-mass stars, and these ``giga metal-poor'' stars can survive until the present-day Universe and may be found by future observations.

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arXiv2018

Seeding the Second Star: enrichment from population III, dust evolution, and cloud collapse

We investigate the formation of extremely metal-poor (EMP) stars that are observed in the Galactic halo and neighboring ultra-faint dwarf galaxies. Their low metal abundances (${\rm [Fe/H]} < -3$) indicate that their parent clouds were enriched by a single or several supernovae (SNe) from the first (Pop III) stars. In this study, we perform numerical simulations of the entire formation sequence of a EMP star through the feedback effects of photo-ionization and metal-enrichment by a Pop III SN. We for the first time employ a metal/dust properties calculated consistently with the progenitor model, and solve all relevant radiative cooling processes and chemical reactions including metal molecular formation and grain growth until the protostar formation. In a minihalo (MH) with mass $1.77\times 10^{6} \ {\rm M}_{\bigodot}$, a Pop III star with mass $13 \ {\rm M}_{\bigodot}$ forms at redshift $z=12.1$. After its SN explosion, the shocked gas falls back into the central MH internally enriching itself. The metallicity in the recollapsing region is $2.6\times 10^{-4} \ {\rm Z}_{\bigodot}$ (${\rm [Fe/H]} = -3.42$). The recollapsing cloud undergoes cooling by HD, CO, and OH molecules and heating along with H$_2$ formation. Eventually by grain growth and dust cooling, knotty filaments appear in the central 100 au region with the help of turbulence driven by the SN, leading to the formation of low-mass EMP stars surviving until the present day.

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arXiv2018

Gravitational collapse and the thermal evolution of low-metallicity gas clouds in the early Universe

We study gravitational collapse of low-metallicity gas clouds and the formation of protostars by three-dimensional hydrodynamic simulations. Grain growth, non-equilibrium chemistry, molecular cooling, and chemical heating are solved in a self-consistent manner for the first time. We employ the realistic initial conditions for the abundances of metal and dust, and the dust size distribution obtained from recent Population III supernova calculations. We also introduce the state-of-the-art particle splitting method based on the Voronoi tessellation and achieve an extremely high mass resolution of ~10^{-5} Msun (10 earth masses) in the central region. We follow the thermal evolution of several early clouds with various metallicities. We show that the condition for cloud fragmentation depends not only on the gas metallicity but also on the collapse timescale. In many cases, the cloud fragmentation is prevented by the chemical heating owing to molecular hydrogen formation even though dust cooling becomes effective. Meanwhile, in several cases, efficient OH and H2O cooling promotes the cloud elongation, and then cloud "filamentation" is driven by dust thermal emission as a precursor of eventual fragmentation. While the filament fragmentation is driven by rapid gas cooling with >10^{-5} Zsun, fragmentation occurs in a different manner by the self-gravity of a circumstellar disk with <10^{-5} Zsun. We use a semi-analytic model to estimate the number fraction of the clouds which undergo the filament fragmentation to be a several percents with 10^{-5}--10^{-4} Zsun. Overall, our simulations show a viable formation path of the recently discovered Galactic low-mass stars with extremely small metallicities.

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arXiv2018

Metal-poor star formation triggered by the feedback effects from Pop III stars

Metal enrichment by the first-generation (Pop III) stars is the very first step of the matter cycle in the structure formation and it is followed by the formation of extremely metal-poor (EMP) stars. To investigate the enrichment process by the Pop III stars, we carry out a series of numerical simulations including the feedback effects of photoionization and supernovae (SNe) of Pop III stars with a range of masses of minihaloes (MHs), M_halo , and Pop III stars, M_PopIII . We find that the metal-rich ejecta reaches neighbouring haloes and external enrichment (EE) occurs when the halo binding energy is sufficiently below the SN explosion energy, E_SN . The neighbouring haloes are only superficially enriched, and the metallicity of the clouds is [Fe/H] < -5. Otherwise, the SN ejecta falls back and recollapses to form enriched cloud, i.e. internal enrichment (IE) process takes place. In case that a Pop III star explodes as a core-collapse SNe (CCSNe), MHs undergo IE, and the metallicity in the recollapsing region is -5 < [Fe/H] < -3 in most cases. We conclude that IE from a single CCSN can explain the formation of EMP stars. For pair-instability SNe (PISNe), EE takes place for all relevant mass range of MHs, consistent with no observational sign of PISNe among EMP stars.

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arXiv2017

Classification of extremely metal-poor stars: absent region in A(C)-[Fe/H] plane and the role of dust cooling

Extremely metal-poor (EMP) stars are the living fossils with records of chemical enrichment history at the early epoch of galaxy formation. By the recent large observation campaigns, statistical samples of EMP stars have been obtained. This motivates us to reconsider their classification and formation conditions. From the observed lower-limits of carbon and iron abundances of Acr(C) ~ 6 and [Fe/H]cr ~ -5 for C-enhanced EMP (CE-EMP) and C-normal EMP (CN-EMP) stars, we confirm that gas cooling by dust thermal emission is indispensable for the fragmentation of their parent clouds to form such low-mass, i.e., long-lived stars, and that the dominant grain species are carbon and silicate, respectively. We constrain the grain radius r_i^{cool} of a species i and condensation efficiency f_{ij} of a key element j as r_C^{cool}/f_{C, C} = 10 um and r_Sil^{cool}/f_{Sil, Mg} = 0.1 um to reproduce Acr(C) and [Fe/H]cr, which give a universal condition 10^{[C/H]-2.30} + 10^[Fe/H] > 10^{-5.07} for the formation of every EMP star. Instead of the conventional boundary [C/Fe] = 0.7 between CE- and CN-EMP stars, this condition suggests a physically meaningful boundary [C/Fe]_{b} = 2.30 above and below which carbon and silicate grains are dominant coolants, respectively.

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arXiv2017

Blocking metal accretion onto population III stars by stellar wind

Low-mass population III (PopIII) stars of $\lesssim 0.8 M_{\odot}$ could survive up until the present. Non-detection of low-mass PopIII stars in our Galaxy has already put a stringent constraint on the initial mass function (IMF) of PopIII stars, suggesting that PopIII stars have a top-heavy IMF. On the other hand, some claims that the lack of such stars stems from metal enrichment of their surface by accretion of heavy elements from interstellar medium (ISM). We investigate effects of the stellar wind on the metal accretion onto low-mass PopIII stars because accretion of the local ISM onto the Sun is prevented by the solar wind even for neutrals. The stellar wind and radiation of low-mass PopIII stars are modeled based on knowledge of nearby low-mass stellar systems including our Sun. We find that low-mass PopIII stars traveling across the Galaxy forms the stellar magnetosphere in most of their life. Once the magnetosphere is formed, most of neutral interstellar particles are photoionized before reaching to the stellar surface and are blown away by the wind. Especially, the accretion abundance of iron will be reduced by a factor of $< 10^{-12}$ compared with Bondi-Hoyle-Lyttleton accretion. The metal accretion can enhance iron abundance [Fe/H] only up to $\sim -14$. This demonstrates that low-mass PopIII stars remain pristine and will be found as metal free stars and that further searches for them are valuable to constrain the IMF of PopIII stars.

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arXiv2015

Particle splitting in smoothed particle hydrodynamics based on Voronoi diagram

We present a novel method for particle splitting in smoothed particle hydrodynamics simulations. Our method utilizes the Voronoi diagram for a given particle set to determine the position of fine daughter particles. We perform several test simulations to compare our method with a conventional splitting method in which the daughter particles are placed isotropically over the local smoothing length. We show that, with our method, the density deviation after splitting is reduced by a factor of about two compared with the conventional method. Splitting would smooth out the anisotropic density structure if the daughters are distributed isotropically, but our scheme allows the daughter particles to trace the original density distribution with length scales of the mean separation of their parent. We apply the particle splitting to simulations of the primordial gas cloud collapse. The thermal evolution is accurately followed to the hydrogen number density of 10^12 /cc. With the effective mass resolution of ~10^-4 Msun after the multi-step particle splitting, the protostellar disk structure is well resolved. We conclude that the method offers an efficient way to simulate the evolution of an interstellar gas and the formation of stars.

astro-ph.IM↗

arXiv2014

Supernova dust formation and the grain growth in the early universe: The critical metallicity for low-mass star formation

We investigate the condition for the formation of low-mass second-generation stars in the early universe. It has been proposed that gas cooling by dust thermal emission can trigger fragmentation of a low-metallicity star-forming gas cloud. In order to determine the critical condition in which dust cooling induces the formation of low-mass stars, we follow the thermal evolution of a collapsing cloud by a one-zone semi-analytic collapse model. Earlier studies assume the dust amount in the local universe, where all refractory elements are depleted onto grains, and/or assume the constant dust amount during gas collapse. In this paper, we employ the models of dust formation and destruction in early supernovae to derive the realistic dust compositions and size distributions for multiple species as the initial conditions of our collapse calculations. We also follow accretion of heavy elements in the gas phase onto dust grains, i.e., grain growth, during gas contraction. We find that grain growth well alters the fragmentation property of the clouds, and that this still does not approach to the value in the local universe. The critical conditions can be written by the gas metallicity Zcr and the initial depletion efficiency fdep,0 of gas-phase metal onto grains, or dust-to-metal mass ratio, as (Zcr/10^{-5.5} Zsun) = (fdep,0/0.18)^{-0.44} with small scatters in the range of Zcr = [0.06--3.2]x10^{-5} Zsun. We also show that the initial dust composition and size distribution are important to determine Zcr.

astro-ph.SR↗

arXiv2014

The origin of the most iron-poor star

We investigate the origin of carbon-enhanced metal-poor (CEMP) stars starting from the recently discovered $\rm [Fe/H]<-7.1$ star SMSS J031300 (Keller et al. 2014). We show that the elemental abundances observed on the surface of SMSS J031300 can be well fit by the yields of faint, metal free, supernovae. Using properly calibrated faint supernova explosion models, we study, for the first time, the formation of dust grains in such carbon-rich, iron-poor supernova ejecta. Calculations are performed assuming both unmixed and uniformly mixed ejecta and taking into account the partial destruction by the supernova reverse shock. We find that, due to the paucity of refractory elements beside carbon, amorphous carbon is the only grain species to form, with carbon condensation efficiencies that range between (0.15-0.84), resulting in dust yields in the range (0.025-2.25)M$_{\odot}$. We follow the collapse and fragmentation of a star forming cloud enriched by the products of these faint supernova explosions and we explore the role played by fine structure line cooling and dust cooling. We show that even if grain growth during the collapse has a minor effect of the dust-to-gas ratio, due to C depletion into CO molecules at an early stage of the collapse, the formation of CEMP low-mass stars, such as SMSS J031300, could be triggered by dust cooling and fragmentation. A comparison between model predictions and observations of a sample of C-normal and C-rich metal-poor stars supports the idea that a single common pathway may be responsible for the formation of the first low-mass stars.

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arXiv2014

One Hundred First Stars : Protostellar Evolution and the Final Masses

We perform a large set of radiation hydrodynamics simulations of primordial star formation in a fully cosmological context. Our statistical sample of 100 First Stars show that the first generation of stars have a wide mass distribution M_popIII = 10 ~ 1000 M_sun. We first run cosmological simulations to generate a set of primordial star-forming gas clouds. We then follow protostar formation in each gas cloud and the subsequent protostellar evolution until the gas mass accretion onto the protostar is halted by stellar radiative feedback. The accretion rates differ significantly among the primordial gas clouds which largely determine the final stellar masses. For low accretion rates the growth of a protostar is self-regulated by radiative feedback effects and the final mass is limited to several tens of solar masses. At high accretion rates the protostar's outer envelope continues to expand and the effective surface temperature remains low; such protostars do not exert strong radiative feedback and can grow in excess to one hundred solar masses. The obtained wide mass range suggests that the first stars play a variety of roles in the early universe, by triggering both core-collapse supernovae and pair-instability supernovae as well as by leaving stellar mass black holes. We find certain correlations between the final stellar mass and the physical properties of the star-forming cloud. These correlations can be used to estimate the mass of the first star from the properties of the parent cloud or of the host halo, without following the detailed protostellar evolution.

astro-ph.CO↗

arXiv2014

Dust grain growth and the formation of the extremely primitive star SDSS J102915+172927

Dust grains in low-metallicity star-forming regions may be responsible for the formation of the first low-mass stars. The minimal conditions to activate dust-induced fragmentation require the gas to be pre-enriched above a critical dust-to-gas mass ratio Dcr=[2.6--6.3]x10^-9 with the spread reflecting the dependence on the grain properties. The recently discovered Galactic halo star SDSS J102915+172927 has a stellar mass of 0.8 Msun and a metallicity of Z=4.5x10^-5 Zsun and represents an optimal candidate for the dust-induced low-mass star formation. Indeed, for the two most plausible Population III supernova progenitors, with 20 Msun and 35 Msun, the critical dust-to-gas mass ratio can be overcome provided that at least 0.4 Msun of dust condenses in the ejecta, allowing for moderate destruction by the reverse shock. Here we show that even if dust formation in the first supernovae is less efficient or strong dust destruction does occur, grain growth during the collapse of the parent gas cloud is sufficiently rapid to activate dust cooling and likely fragmentation into low-mass and long-lived stars. Silicates and magnetite grains can experience significant grain growth in the density range 10^9 /cc < nH<10^12 /cc by accreting gas-phase species (SiO, SiO2, and Fe) until their gas-phase abundance drops to zero, reaching condensation efficiencies =1. The corresponding increase in the dust-to-gas mass ratio allows dust-induced cooling and fragmentation to be activated at 10^12 /cc < nH < 10^14 /cc, before the collapsing cloud becomes optically thick to continuum radiation. We show that for all the initial conditions that apply to the parent cloud of SDSS J102915+172927, dust-driven fragmentation is able to account for the formation of the star.

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arXiv2013

Growth of dust grains in a low-metallicity gas and its effect on the cloud fragmentation

In a low-metallicity gas, rapid cooling by dust thermal emission is considered to induce cloud fragmentation and play a vital role in the formation of low-mass stars (<~ 1 M_sun) in metal-poor environments. We investigate how the growth of dust grains through accretion of heavy elements in the gas phase onto grain surfaces alters the thermal evolution and fragmentation properties of a collapsing gas cloud. We calculate directly grain growth and dust emission cooling in a self-consistent manner. We show that MgSiO3 grains grow sufficiently at gas densities nH = 10^{10}, 10^{12}, and 10^{14} /cc for metallicities Z = 10^{-4}, 10^{-5}, and 10^{-6} Zsun, respectively, where the cooling of the collapsing gas cloud is enhanced. The condition for efficient dust cooling is insensitive to the initial condensation factor of pre-existing grains within the realistic range of 0.001--0.1, but sensitive to metallicity. The critical metallicity is Zcrit ~ 10^{-5.5} Zsun for the initial grain radius r_{MgSiO3,0} <~ 0.01 um and Zcrit ~ 10^{-4.5} Zsun for r_{MgSiO3,0} >~ 0.1 um. The formation of a recently discovered low-mass star with extremely low metallicity (<= 4.5x10^{-5} Zsun) could have been triggered by grain growth.

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arXiv2012

Low-mass star formation triggered by early supernova explosions

We study the formation of low-mass and extremely metal-poor stars in the early universe. Our study is motivated by the recent discovery of a low-mass (M < 0.8 Msun) and extremely metal-poor (Z <= 4.5 x 10^{-5} Zsun) star in the Galactic halo by Caffau et al. We propose a model that early supernova (SN) explosions trigger the formation of low-mass stars via shell fragmentation. We first perform one-dimensional hydrodynamic simulations of the evolution of an early SN remnant. We show that the shocked shell undergoes efficient radiative cooling and then becomes gravitationally unstable to fragment and collapse in about ten million years. We then follow the thermal evolution of the collapsing fragments using a one-zone code. Our one-zone calculation treats chemistry and radiative cooling self-consistently in low-metallicity gas. The collapsing gas cloud evolves roughly isothermally, until it cools rapidly by dust continuum emission at the density 10^{13}-10^{14} /cc. The cloud core then becomes thermally and gravitationally unstable and fragments. We argue that early SNe can trigger the formation of low-mass stars in the extremely metal-poor environment as Caffau et al. discovered recently.

astro-ph.CO↗