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Search Results: 1 - 10 of 191497 matches for " D. Kawata "
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Theory of Stellar Population Synthesis with an application to N-Body simulations
S. Pasetto,C. Chiosi,D. Kawata
Physics , 2012, DOI: 10.1051/0004-6361/201219698
Abstract: Aims. We present here a new theoretical approach to population synthesis. The aim is to predict colour magnitude diagrams (CMDs) for huge numbers of stars. With this method we generate synthetic CMDs for N-body simulations of galaxies. Sophisticated hydrodynamic N-body models of galaxies require equal quality simulations of the photometric properties of their stellar content. The only prerequisite for the method to work is very little information on the star formation and chemical enrichment histories, i.e. the age and metallicity of all star-particles as a function of time. The method takes into account the gap between the mass of real stars and that of the star-particles in N-body simulations, which best correspond to the mass of star clusters with different age and metallicity, i.e. a manifold of single stellar sopulations (SSP). Methods. The theory extends the concept of SSP to include the phase-space (position and velocity) of each star. Furthermore, it accelerates the building up of simulated CMD by using a database of theoretical SSPs that extends to all ages and metallicities of interest. Finally, it uses the concept of distribution functions to build up the CMD. The technique is independent of the mass resolution and the way the N-body simulation has been calculated. This allows us to generate CMDs for simulated stellar systems of any kind: from open clusters to globular clusters, dwarf galaxies, or spiral and elliptical galaxies. Results. The new theory is applied to an N-body simulation of a disc galaxy to test its performance and highlight its flexibility.
Stellar dynamics around transient co-rotating spiral arms
Kawata D.,Grand R.J.J.,Cropper M.
EPJ Web of Conferences , 2012, DOI: 10.1051/epjconf/20121907006
Abstract: Spiral density wave theory attempts to describe the spiral pattern in spiral galaxies in terms of a long-lived wave structure with a constant pattern speed in order to avoid the winding dilemma. The pattern is consequently a rigidly rotating, long-lived feature. We run an N-body/SPH simulation of a Milky Way-sized barred disk, and find that the spiral arms are transient features whose pattern speeds decrease with radius, in such a way that the pattern speed is almost equal to the rotation curve of the galaxy. We trace particle motion around the spiral arms. We show that particles from behind and in front of the spiral arm are drawn towards and join the arm. Particles move along the arm in the radial direction and we find a clear trend that they migrate toward the outer (inner) radii on the trailing (leading) side of the arm. Our simulations demonstrate that tat all radii where there is a co-rotating spiral arm the particles continue to be accelerated (decelerated) by the spiral arm for long periods, which leads to strong migration.
Mining the Galactic Halo for Very Metal-Poor Stars
S. Salvadori,A. Ferrara,R. Schneider,E. Scannapieco,D. Kawata
Physics , 2009, DOI: 10.1111/j.1745-3933.2009.00772.x
Abstract: We study the age and metallicity distribution function (MDF) of metal-poor stars in the Milky Way halo as a function of galactocentric radius by combining N-body simulations and semi-analytical methods. We find that the oldest stars populate the innermost region, while extremely metal-poor stars are more concentrated within r < 60 kpc. The MDF of [Fe/H] < -2 stars varies only very weakly within the central 50 kpc, while the relative contribution of [Fe/H] < -2 stars strongly increases with r, varying from 16% within 7 kpc < r < 20 kpc up to > 40% for r > 20 kpc. This is due to the faster descent of the spatial distribution (as seen from Earth) of the more enriched population. This implies that the outer halo < 40 kpc is the best region to search for very metal-poor stars. Beyond ~ 60 kpc the density of [Fe/H] < -2 stars is maximum within dwarf galaxies. All these features are imprinted by a combination of (i) the virialization epoch of the star-forming haloes, and (ii) the metal enrichment history of the Milky Way environment.
Galactic Cannibalism: the Origin of the Magellanic Stream
S. T. Maddison,D. Kawata,B. K. Gibson
Physics , 2002, DOI: 10.1023/A:1019599415181
Abstract: We are in a privileged location in the Universe which allows us to observe galactic interactions from close range -- the merger of our two nearest dwarf satellite galaxies, the LMC and SMC. It is important to understand the local merger process before we can have confidence in understanding mergers at high redshift. We present high resolution Nbody+SPH simulations of the disruption of the LMC and SMC and the formation of the Magellanic Stream, and discuss the implications for galaxy formation and evolution.
Chemodynamical analysis of bulge stars for simulated disc galaxies
A. Rahimi,D. Kawata,Chris B. Brook,Brad K. Gibson
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.15752.x
Abstract: We analyse the kinematics and chemistry of the bulge stars of two simulated disc galaxies using our chemodynamical galaxy evolution code GCD+. First we compare stars that are born inside the galaxy with those that are born outside the galaxy and are accreted into the centre of the galaxy. Stars that originate outside of the bulge are accreted into it early in its formation within 3 Gyrs so that these stars have high [alpha/Fe] as well as having a high total energy reflecting their accretion to the centre of the galaxy. Therefore, higher total energy is a good indicator for finding accreted stars. The bulges of the simulated galaxies formed through multiple mergers separated by about a Gyr. Since [alpha/Fe] is sensitive to the first few Gyrs of star formation history, stars that formed during mergers at different epochs show different [alpha/Fe]. We show that the [Mg/Fe] against star formation time relation can be very useful to identify a multiple merger bulge formation scenario, provided there is sufficiently good age information available. Our simulations also show that stars formed during one of the merger events retain a systematically prograde rotation at the final time. This demonstrates that the orbit of the ancient merger that helped to form the bulge could still remain in the kinematics of bulge stars.
On the origin of high eccentricity halo stars
C. B. Brook,D. Kawata,B. K. Gibson,C. Flynn
Physics , 2002, DOI: 10.1023/A:1024062431154
Abstract: The present-day chemical and dynamical properties of the Milky Way are signatures of the Galaxy's formation and evolution. Using a self consistent chemodynamical evolution code we examine these properties within the currently favoured paradigm for galaxy formation - hierarchical clustering within a CDM cosmology. Our Tree N-body/Smoothed Particle Hydrodynamics code includes a self-consistent treatment of gravity, hydrodynamics, radiative cooling, star formation, supernova feedback and chemical enrichment. Two models are described which explore the role of small-scale density perturbations in driving the evolution of structure within the Milky Way. The relationship between metallicity and kinematics of halo stars are quantified and the implications for galaxy formation discussed. While high-eccentricity halo stars have previously been considered a signature of ``rapid collapse'', we suggest that many such stars may have come from recently accreted satellites.
Evolution for the Next Generation
Masakado Kawata
PLOS Biology , 2012, DOI: 10.1371/journal.pbio.0020071
Abstract:
Economic growth and trend changes in wildlife hunting
Yukichika Kawata
Acta agriculturae Slovenica , 2011, DOI: 10.2478/v10014-011-0007-7
Abstract: Petty-Clark's law suggests that as the economy of a country develops, its proportion of primary industries declines while those of its secondary and tertiary industries increase. Traditionally, hunting has played a crucial role in a country's food supply; however, currently, it is increasingly viewed as a leisure activity. This paper empirically examines whether Petty-Clark's law holds in case of hunting in European countries. The results reveal that the proportion of hunters across countries increases when the per capita GDP is between 5,000 and 15,000 USD. Once the per capita GDP crosses the 15,000 USD mark, two major trends are detected: the number of hunters continues to increase in some countries but decreases in some other countries. Finally, the number of hunters in a country stabilizes when its per capita GDP reaches around 25,000 to 30,000 USD.
Evolution for the Next Generation
Masakado Kawata
PLOS Biology , 2004, DOI: 10.1371/journal.pbio.0020071
Abstract:
Galaxy Formation from a Low-Spin Density Perturbation in a CDM Universe
Daisuke Kawata
Physics , 1999, DOI: 10.1093/pasj/51.6.931
Abstract: In order to understand the formation process of elliptical galaxies which are not rotationally supported, we have carried out numerical simulations of the galaxy formation from the density perturbation with a rotation corresponding to a small spin parameter. The three-dimensional TREE N-Body/SPH simulation code used in this paper includes the dark matter and gas dynamics, radiative cooling, star formation, supernova feedback, and metal enrichment. The initial condition is a slowly rotating, top-hat over-dense sphere on which the perturbations expected in a cold dark matter (CDM) universe are superposed. By means of the stellar population synthesis, we calculated the surface brightness profile, the metallicity distribution, and the photometric properties of the end-product, and found that these properties quantitatively agree with the observed properties of bright elliptical galaxies. Thus, we conclude that, in a CDM universe, the proto-galaxy which has a spin-parameter as small as 0.02 evolves into an elliptical galaxy.
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