Papers - SUSA Hajime
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Miura H, Nakamoto T, Susa H.
Icarus 160 ( 2 ) 258 - 270 2002.12
Joint Work
A shock-wave heating model is one of the possible models for chondrule formation. We examine, within the framework of a shock-wave heating model, the effects of evaporation on the heating of chondrule precursor particles and the stability of their molten state in the postshock flow. We numerically simulate the heating process in the flow taking into account evaporation. We find that the melting criterion and the minimum radius criterion do not change significantly. However, if the latent heat cooling due to the evaporation dominates the radiative cooling from the precursor particle, the peak temperature of the precursor particle is suppressed by a few hundred Kelvins. We also find that the total gas pressure (ram plus static) acting on the precursor particle exceeds the vapor pressure of the molten precursor particle. Therefore, it is possible to form chondrules in the shock-wave heating model if the precursor temperature increases up to the melting point....
Other Link： http://orcid.org/0000-0002-3380-5302
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On the maximal size of chondrules in shock wave heating model
Susa H, Nakamoto T.
Astrophysical Journal 564 ( 1 II ) 2002.1
Joint Work
The shock wave heating model is one of the possible mechanisms that could form chondrules. Recently, it is suggested that the shock wave heating model can explain some chondrule features including the maximum temperature and cooling rates. Here we show that another two chondrule features, characteristic size and rotation rates, can be explained naturally, in the framework of the shock wave heating model. The sizes of chondrules are bounded by the balance between the surface tension and the ram pressure on a molten droplet. The acceptable upper bound is acrit~=1 mm. Moreover, when a chondrule forms through the disruption of a larger droplet, it acquires a spin of the order of ω~=1×103 rad s-1. Both features are consistent with observations....
DOI： 10.1086/338789
Other Link： http://orcid.org/0000-0002-3380-5302
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Hydrodynamics in astrophysics with gravity on Heterogeneous Multi-Computer System
T., Boku, J., Makino, H., Susa, M., Umemura, T., Fukushige, and, A. Ukawa
IPSJ Transactions on High Performance Computing Systems 41 5 2002.1
Joint Work
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A Shock-Wave Heating Model for Chondrule Formation: Peak Temperature of Precursor Particles and High Ambient Gas Pressure
8th Asian-Pacific Regional Meeting, Volume II 2002
Single Work
We examine, in a framework of the shock-wave heating model, the effects of evaporation on the heating of chondrule precursor particles and the stability of their molten state in the post-shock flow. What we have found are as follows; (1) latent heat cooling dominates radiative cooling when the temperature of the precursor particles is larger than Tc (where 1900 K <≈ Tc <≈ 2400 K), and decreases the peak temperature of the precursor particles by more than a few 100 K, and (2) the total gas pressure (ram plus static) acting on the precursor particles in the post-shock region is large enough to keep the molten precursor particles stable. Therefore, we can see that the shock-wave heating model satisfies some observational constraints for chondrule formation models (Jones et al. 2000), i.e., the peak temperature of the precursor particles and elevated ambient gas pressure....
Other Link： http://orcid.org/0000-0002-3380-5302
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Heterogeneous multi-computer system: A new platform for multi-paradigm scientific simulation
Boku T, Makino J, Susa H, Umemura M, Fukushige T, Ukawa A.
Proceedings of the International Conference on Supercomputing 26 - 34 2002
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Heterogeneous multi-computer system: A new paradigm of parallel processing
Boku T, Makino J, Susa H, Umemura M, Fukushige T, Ukawa A.
Proceedings - International Conference on Parallel Computing in Electrical Engineering, PARELEC 2002 3 - 8 2002
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Formation of Host Clouds of First Stars in the Early Universe
Susa H.
Progress of Theoretical Physics Supplement ( 147 SUPPL. ) 11 - 42 2002
Single Work
It is widely accepted that various structures of the universe are formed through the gravitational instability of the density perturbations imprinted by the quantum fluctuations. First stars are also formed as a consequence of the evolved density perturbations. However, they are not formed from the stellar size perturbations such as M ̃ 1-102 M☉. Recent theoretical investigations suggest the first stars are formed in more massive ``parent'' clouds, with M ≳ 106 M☉. The key physics of the formation of such small hosts is the formation of hydrogen molecules in them. Since the virial temperature of such clouds are less than 104 K, they are not cooled by atomic cooling processes. Therefore, hydrogen molecules are required as the main coolant of primordial gas at 104 K, because their rotation/vibration levels are excited even at such low temperature. In this chapter, gravitational collapse of density perturbations in Cold Dark Matter (CDM) universe is reviewed. Second, the formation processes of hydrogen molecules and the cooling mechanisms in primordial gas are summarized. Finally, the condition of the formation of cold dense gas clouds that host the first stars is shown....
DOI： 10.1143/PTPS.147.11
Other Link： http://orcid.org/0000-0002-3380-5302
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A shock-wave heating model for chondrule formation: evaporation effects on molten particles
AAS/Division for Planetary Sciences Meeting Abstracts #33 2001.11
Single Work
Chondrules are mm-sized, once-molten, spherical-shaped grains composed of silicate material in chondritic meteorites. It is considered that dust particles, which are chondrule precursors, were heated and melted, and cooled again to solidify in a short period of time, and formed chondrules, although the heating mechanism responsible for the chondrule formation has not been clearly understood yet. Shock-wave heating model is one of the popular models for the heating process. In the model, dust grains are heated due to the drag heating in the post shock flow. Conditions of the shock flow in which dust particles can be heated up to the melting point have been revealed in previous work. However, the heating condition is not enough to form the chondrules. We examined, in a framework of the shock-wave heating model, if the shock flow can keep the heated dust particles molten stably. We numerically simulated the heating process in the flow taking into account the evaporation from the dust surface and the ram pressure from the flow onto the particle. We found that the complete vaporization condition does not change significantly, though the evaporation and the latent heat cooling take place during the heating process. We also found that the total gas pressure acting on the particle, which is a summation of the static pressure and the ram pressure, exceeds the vapor pressure of the molten dust particle. Therefore, it is possible to form chondrules, which are once-molten dust particles, in the shock-wave heating model if the dust temperature increases up to the melting point....
Other Link： http://orcid.org/0000-0002-3380-5302
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A Shock-Wave Heating Model for Chondrule Formation: Maximal Size of Chondrules
AAS/Division for Planetary Sciences Meeting Abstracts #33 2001.11
Single Work
Several models of heating events, which formed chondrules, have been proposed to date, though a consensus has not been reached yet. Shock-wave heating model is one of the possible models. In the model, dust particles are heated by the drag heating in the post shock flow. The maximum temperature and the cooling rates of chondrules that are estimated from laboratory experiments can be obtained in the model. Here we show that two other features of chondrules, characteristic size and rotation rates, may also be explained naturally in the framework of the shock-wave heating model. In the model, the sizes of chondrules are bounded by the balance between the surface tension and the ram pressure on a molten droplet. As the size of a molten particle increases, the surface tension descreses. So a large droplet whose radius exceeds a critical value is expected to fragment into small droplets. The acceptable upper bound is around 1 mm, which is consistent with characteristic sizes of chondrules. The upper bound does not depend on the shock flow conditions significantly, i.e., almost all the shocks which heat the dust particles up to the melting point provide similar maximal sizes of chondrules. Moreover, when a chondrule forms through the distribution of a larger droplet, it acquires a spin of the order of 1 x 103 rad s-1. It is also consistent with observations....
Other Link： http://orcid.org/0000-0002-3380-5302
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A shock heating model for chondrule formation in a protoplanetary disk
Iida A, Nakamoto T, Susa H, Nakagawa Y.
Icarus 153 ( 2 ) 430 - 450 2001.10
Joint Work
Chondrule formation due to the shock heating of dust particles with a wide variety of shock properties are examined. We numerically simulate the steady postshock region in a framework of one-dimensional hydrodynamics, taking into account many of the physical and chemical processes that determine the properties of the region, especially nonequilibrium chemical reactions of gas species. We mainly focus on the maximum temperature of dust particles and their net cooling rate in relation to the chondrule formation. We derive the condition of chondrule formation for the shock velocity vs and the preshock density n0. For n0>10 14.5 cm -3, the shock velocity should be in a range 6 km s -1≤ vs≤7 km s -1, while for n0<10 14.5 cm -3, vs should be 6×( n0/10 14.5 cm -3) -1/5 km s -1≤ vs≤7×( n0/10 14.5 cm -3) -1/5 km s -1 for an initial dust particle radius of 0.1 mm. The condition has a small dependence on particle size. We find that the Keplerian velocities and equatorial plane densities around the asteroidal and Jupiter orbital regions of the minimum mass solar nebula model are suitable for chondrule formation. We also find that the gas pressure in the postshock region is much higher than the one in the standard nebula environment. Furthermore, we find that the net cooling rates of 0.1-1-mm-sized dust particles are about 10 2-10 5 K h -1, which are not too far from experimental values, though the melting region is optically thin. Those slow net cooling rates are maintained by drag heating in the cooling phase. These results indicate that the shock heating model can be regarded as a strong candidate for the mechanism of chondrule formation....
Other Link： http://orcid.org/0000-0002-3380-5302
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Criteria for the formation of population III objects in the ultraviolet background radiation
Kitayama T, Susa H, Umemura M, Ikeuchi S.
Monthly Notices of the Royal Astronomical Society 326 ( 4 ) 1353 - 1366 2001.10
Joint Work
We explore possibilities of collapse and star formation in Population III objects exposed to the external ultraviolet background (UVB) radiation. Assuming spherical symmetry, we solve self-consistently radiative transfer of photons, non-equilibrium H2 chemistry and gas hydrodynamics. Although the UVB does suppress the formation of low-mass objects, the negative feedback turns out to be weaker than previously suggested. In particular, the cut-off scale of collapse drops significantly below the virial temperature Tvir~104K at weak UV intensities (J21<~10-2), owing to both self-shielding of the gas and H2 cooling. Clouds above this cut-off tend to contract highly dynamically, further promoting self-shielding and H2 formation. For plausible radiation intensities and spectra, the collapsing gas can cool efficiently to temperatures well below 104K before rotationally supported and the final H2 fraction reaches ~10-3. Our results imply that star formation can take place in low-mass objects collapsing in the UVB. The threshold baryon mass for star formation is ~109Msolar for clouds collapsing at redshifts z<~3, but drops significantly at higher redshifts. In a conventional cold dark matter universe, the latter coincides roughly with that of the 1σ density fluctuations. Objects near and above this threshold can thus constitute `building blocks' of luminous structures, and we discuss their links to dwarf spheroidal/elliptical galaxies and faint blue objects. These results suggest that the UVB can play a key role in regulating the star formation history of the Universe....
DOI： 10.1111/j.1365-2966.2001.04669.x
Other Link： http://orcid.org/0000-0002-3380-5302
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The effects of radiative transfer on the reionization of an inhomogeneous universe
Nakamoto T, Umemura M, Susa H.
Monthly Notices of the Royal Astronomical Society 321 ( 4 ) 593 - 604 2001.3
Joint Work
Assuming simple dynamics for the growth of density fluctuations, we implement six-dimensional (6D) radiative transfer calculations to elucidate the effects of photon propagation during the reionization of an inhomogeneous universe. The ionizing sources are postulated to be AGN-like in this paper. The present simulations reveal that radiative transfer effects are still prominent considerably after the percolation epoch, in which patchy ionized regions connect with each other. In other words, owing to the collective opacity, the Universe does not become perfectly transparent against ionizing radiation even though strongly self-shielded regions disappear. It turns out that the inhomogeneity of the medium enhances the opacity effects and delays the end of reionization. Owing to such radiative transfer effects, the reionization in an inhomogeneous universe proceeds fairly slowly, in contrast to the prompt reionization in a homogeneous universe, and as a result the surface of reionization is not so sharply edged, but highly uneven. As a signature of the uneven surface of reionization, the cosmic IR background (CIB) radiation, which is produced by Lyα photons resulting from radiative recombination, could exhibit strong anisotropies, reflecting the amplitude of density fluctuations at the reionization era. The predicted CIB intensity lies on a level of possible detection by forthcoming IR space telescope facilities....
DOI： 10.1046/j.1365-8711.2001.04008.x
Other Link： http://orcid.org/0000-0002-3380-5302
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3D Radiative Transfer Effects on the Cosmic Reionization
The Physics of Galaxy Formation 2001
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Cosmological Formation of Subgalactic Objects and Radiative Feedback
The Physics of Galaxy Formation 2001
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Cosmic Reionization and Galaxy Formation
The Physics of Galaxy Formation 2001
Single Work
Using 3D radiative transfer calculations on the reionization of an inhomogeneous universe, QSO absorption line systems are simulated and they are compared with observations of Ly$\alpha$ continuum depression at high redshifts. By this comparison, it is found that the metagalactic UV intensity decreases rapidly with $z$ at $z>4$ as $I_{21}=0.5\exp [3(4-z)]$, and the reionization must have taken place between $z=6$ and 10. Based on this time-dependence of UV background intensity, we explore the collapse of pregalactic clouds in the UV background, and find that the self-shielding is prominent above a mass scale as $ M_{BIF}=3.0\times 10^{11} M_\odot [(1+z_c)/5]^{-4.2}(I_{21}/0.5)^{0.6}. $ This mass scale predicts the bifurcation of galactic morphology, and by confrontation with observations it turns out that the bifurcation mass successfully discriminates between elliptical and spiral galaxies....
Other Link： http://orcid.org/0000-0002-3380-5302
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Impacts of Radiative Feedback on the Formation of Primordial Galaxies
The Physics of Galaxy Formation 2001
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UV Background-Induced Bifurcation into Elliptical and Spiral Galaxies
Birth and Evolution of the Universe 2001
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The Physics of Galaxy Formation
The Physics of Galaxy Formation 2001
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Reionization of an Inhomogeneous Universe
Birth and Evolution of the Universe 2001
Single Work
Full radiative transfer in 3D space has been solved to pursue the reionization history in an inhomogeneous universe. It has been shown that the reionization of an inhomogeneous universe is not a prompt event, but a fairly slow process. Also, QSO absorption line systems are simulated with using the results of radiative transfer calculations. Ly$\alpha$ continuum depression implies that the metagalactic UV intensity decreases rapidly with $z$ at $z>5$, and the reionization must have taken place between $z=6$ and 10. Finally, it is stressed that H$\alpha$ forest is a more powerful tool to probe the reionization history and the density fluctuations in the universe at $z>5$....
Other Link： http://orcid.org/0000-0002-3380-5302
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Photoionization of a Clumpy Universe
Astrophysics and Space Science Library 2001