The Experts below are selected from a list of 315 Experts worldwide ranked by ideXlab platform
Kōji Uryū - One of the best experts on this subject based on the ideXlab platform.
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new code for equilibriums and Quasiequilibrium initial data of compact objects iii axisymmetric and triaxial rotating stars
Physical Review D, 2016Co-Authors: Kōji Uryū, Keisuke Taniguchi, Antonios Tsokaros, Filippo Galeazzi, Hideya Hotta, Misa Sugimura, Shinichirou YoshidaAbstract:We introduce new code for stationary and axisymmetric equilibriums, as well as for triaxial Quasiequilibrium initial data, of single rotating relativistic stars. The new code is developed as a part of our versatile initial data code for compact objects, Compact Object CALculator (cocal). In computing strong gravitational fields, the waveless formulation is incorporated into the cocal code on top of the previously developed Isenberg-Wilson-Mathews formulation (conformally flat thin-sandwich formulation). Also introduced is a new differential rotation law that contains two parameters to control an angular velocity profile and a transition from uniform to differential rotation. We present convergence tests and solution sequences for both uniformly and differentially rotating equilibriums of stationary axisymmetric compact stars, as well as for Quasiequilibrium initial data of uniformly rotating triaxial (nonaxisymmetric) compact stars. We also show comparisons of uniformly rotating axisymmetric solutions computed with three different codes: cocal, lorene, and the RNS code.
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new code for Quasiequilibrium initial data of binary neutron stars corotating irrotational and slowly spinning systems
Physical Review D, 2015Co-Authors: Antonios Tsokaros, Kōji Uryū, Luciano RezzollaAbstract:We present the extension of our cocal---Compact Object CALculator---code to compute general-relativistic initial data for binary compact-star systems. In particular, we construct Quasiequilibrium initial data for equal-mass binaries with spins that are either aligned or antialigned with the orbital angular momentum. The Isenberg-Wilson-Mathews formalism is adopted and the constraint equations are solved using the representation formula with a suitable choice of a Green's function. We validate the new code with solutions for equal-mass binaries and explore its capabilities for a wide range of compactnesses, from a white dwarf binary with compactness $\ensuremath{\sim}1{0}^{\ensuremath{-}4}$, up to a highly relativistic neutron-star binary with compactness $\ensuremath{\sim}0.22$. We also present a comparison with corotating and irrotational Quasiequilibrium sequences from the spectral code lorene [Taniguchi and Gourgoulhon, Phys. Rev. D 66, 104019 (2002)] and with different compactness, showing that the results from the two codes agree to a precision of the order of 0.05%. Finally, we present equilibria for spinning configurations with a nuclear-physics equation of state in a piecewise polytropic representation.
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Computation of gravitational waves from inspiraling binary neutron stars in Quasiequilibrium circular orbits: Formulation and calibration
Physical Review D, 2001Co-Authors: Masaru Shibata, Kōji UryūAbstract:Gravitational waves from binary neutron stars in Quasiequilibrium circular orbits are computed using an approximate method which we propose in this paper. In the first step of this method, we prepare general relativistic irrotational binary neutron stars in a Quasiequilibrium circular orbit, neglecting gravitational waves. We adopt the so-called conformal flatness approximation for a three-metric to obtain the Quasiequilibrium states in this paper. In the second step, we compute gravitational waves, solving linear perturbation equations in the background spacetime of the Quasiequilibrium states. Comparing numerical results with post Newtonian waveforms and luminosity of gravitational waves from two point masses in circular orbits, we demonstrate that this method can produce accurate waveforms and luminosity of gravitational waves. It is shown that the effects of tidal deformation of neutron stars and strong general relativistic gravity modify the post Newtonian results for compact binary neutron stars in close orbits. We indicate that the magnitude of a systematic error in Quasiequilibrium states associated with the conformal flatness approximation is fairly large for close and compact binary neutron stars. Several formulations for improving the accuracy of Quasiequilibrium states are proposed.Comment: 26 pages, to be published in PR
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computation of gravitational waves from inspiraling binary neutron stars in Quasiequilibrium circular orbits formulation and calibration
Physical Review D, 2001Co-Authors: Masaru Shibata, Kōji UryūAbstract:Gravitational waves from binary neutron stars in Quasiequilibrium circular orbits are computed using an approximate method which we propose in this paper. In the first step of this method, we prepare general relativistic irrotational binary neutron stars in a Quasiequilibrium circular orbit, neglecting gravitational waves. We adopt the so-called conformal flatness approximation for a three-metric to obtain the Quasiequilibrium states in this paper. In the second step, we compute gravitational waves, solving linear perturbation equations in the background spacetime of the Quasiequilibrium states. Comparing numerical results with post Newtonian waveforms and luminosity of gravitational waves from two point masses in circular orbits, we demonstrate that this method can produce accurate waveforms and luminosity of gravitational waves. It is shown that the effects of tidal deformation of neutron stars and strong general relativistic gravity modify the post Newtonian results for compact binary neutron stars in close orbits. We indicate that the magnitude of a systematic error in Quasiequilibrium states associated with the conformal flatness approximation is fairly large for close and compact binary neutron stars. Several formulations for improving the accuracy of Quasiequilibrium states are proposed.
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New numerical scheme to compute three-dimensional configurations of Quasiequilibrium compact stars in general relativity: Application to synchronously rotating binary star systems
Physical Review D, 1999Co-Authors: Fumihiko Usui, Kōji Uryū, Yoshiharu EriguchiAbstract:We develop a new numerical scheme to obtain Quasiequilibrium structures of binary neutron star systems and nonaxisymmetric compact stars as well as the space time around those systems in general relativity. Although, strictly speaking, there are no equilibrium states for binary configurations in general relativity, the time scale of changes in orbital motion due to gravitational wave radiation is long compared with the orbital period. Thus, we can assume that binary neutron star systems, and nonaxisymmetric systems in general are in ``Quasiequilibrium'' states. Concerning the Quasiequilibrium states of binary systems in general relativity, several investigations have been already carried out by assuming conformal flatness of the spatial part of the metric. However, the validity of the conformally flat treatment has not been fully analyzed except for axisymmetric configurations. Therefore, it is desirable to solve for the Quasiequilibrium states by developing totally different methods from the conformally flat scheme. In this paper, we present a new numerical scheme to solve the Einstein equations for three-dimensional configurations directly, without assuming conformal flatness, although we use the simplified metric for the space time. This new formulation is an extension of the scheme which has been successfully applied for structures of axisymmetric rotating compact stars in general relativity. It is based on the integral representation of the Einstein equations, and takes into account the boundary conditions at infinity. We have checked our numerical scheme by computing equilibrium sequences of binary polytropic star systems in Newtonian gravity and those of axisymmetric polytropic stars in general relativity. We have applied this numerical code to binary star systems in general relativity and have succeeded in obtaining several equilibrium sequences of synchronously rotating binary polytropes with the polytropic indices $N=0.0,$ $0.5,$ and $1.0.$ It should be noted that our equilibrium sequences are not those of constant baryon mass star models because there is no unique choice of parameters to keep the baryon mass constant for our polytropic relation.
Yoshiharu Eriguchi - One of the best experts on this subject based on the ideXlab platform.
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Quasiequilibrium sequences of synchronously rotating binary neutron stars with constant rest masses in general relativity another approach without using the conformally flat condition
Physical Review D, 2002Co-Authors: Fumihiko Usui, Yoshiharu EriguchiAbstract:We have computed Quasiequilibrium sequences of synchronously rotating compact binary star systems with constant rest masses. This computation has been carried out by using the numerical scheme which is different from the scheme based on the conformally flat assumption about the space. Stars are assumed to be polytropes with polytropic indices of N=0.5, N=1.0, and N=1.5. Since we have computed binary star sequences with a constant rest mass, they provide approximate evolutionary tracks of binary star systems. For relatively stiff equations of state (N < 1.0), there appear turning points along the Quasiequilibrium sequences plotted in the angular momentum -- angular velocity plane. Consequently secular instability against exciting internal motion sets in at those points. Qualitatively, these results agree with those of Baumgarte et al. who employed the conformally flat condition. We further discuss the effect of different equations of state and different strength of gravity by introducing two kinds of dimensionless quantities which represent the angular momentum and the angular velocity. Strength of gravity is renormalized in these quantities so that the quantities are transformed to values around unity. Therefore we can clearly see relations among Quasiequilibrium sequences for a wide variety of strength of gravity and for different compressibility.
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Quasiequilibrium sequences of synchronously rotating binary neutron stars with constant rest masses in general relativity: Another approach without using the conformally flat condition
Physical Review D, 2002Co-Authors: Fumihiko Usui, Yoshiharu EriguchiAbstract:We have computed Quasiequilibrium sequences of synchronously rotating compact binary star systems with constant rest masses. This computation has been carried out by using the numerical scheme which is different from the scheme based on the conformally flat assumption about the space. Stars are assumed to be polytropes with polytropic indices of N=0.5, N=1.0, and N=1.5. Since we have computed binary star sequences with a constant rest mass, they provide approximate evolutionary tracks of binary star systems. For relatively stiff equations of state (N < 1.0), there appear turning points along the Quasiequilibrium sequences plotted in the angular momentum -- angular velocity plane. Consequently secular instability against exciting internal motion sets in at those points. Qualitatively, these results agree with those of Baumgarte et al. who employed the conformally flat condition. We further discuss the effect of different equations of state and different strength of gravity by introducing two kinds of dimensionless quantities which represent the angular momentum and the angular velocity. Strength of gravity is renormalized in these quantities so that the quantities are transformed to values around unity. Therefore we can clearly see relations among Quasiequilibrium sequences for a wide variety of strength of gravity and for different compressibility.Comment: 15 pages (including 18 figures). Accepted for publication in Physical Review D. Scheduled to appear in 15 March, 200
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New numerical scheme to compute three-dimensional configurations of Quasiequilibrium compact stars in general relativity: Application to synchronously rotating binary star systems
Physical Review D, 1999Co-Authors: Fumihiko Usui, Kōji Uryū, Yoshiharu EriguchiAbstract:We develop a new numerical scheme to obtain Quasiequilibrium structures of binary neutron star systems and nonaxisymmetric compact stars as well as the space time around those systems in general relativity. Although, strictly speaking, there are no equilibrium states for binary configurations in general relativity, the time scale of changes in orbital motion due to gravitational wave radiation is long compared with the orbital period. Thus, we can assume that binary neutron star systems, and nonaxisymmetric systems in general are in ``Quasiequilibrium'' states. Concerning the Quasiequilibrium states of binary systems in general relativity, several investigations have been already carried out by assuming conformal flatness of the spatial part of the metric. However, the validity of the conformally flat treatment has not been fully analyzed except for axisymmetric configurations. Therefore, it is desirable to solve for the Quasiequilibrium states by developing totally different methods from the conformally flat scheme. In this paper, we present a new numerical scheme to solve the Einstein equations for three-dimensional configurations directly, without assuming conformal flatness, although we use the simplified metric for the space time. This new formulation is an extension of the scheme which has been successfully applied for structures of axisymmetric rotating compact stars in general relativity. It is based on the integral representation of the Einstein equations, and takes into account the boundary conditions at infinity. We have checked our numerical scheme by computing equilibrium sequences of binary polytropic star systems in Newtonian gravity and those of axisymmetric polytropic stars in general relativity. We have applied this numerical code to binary star systems in general relativity and have succeeded in obtaining several equilibrium sequences of synchronously rotating binary polytropes with the polytropic indices $N=0.0,$ $0.5,$ and $1.0.$ It should be noted that our equilibrium sequences are not those of constant baryon mass star models because there is no unique choice of parameters to keep the baryon mass constant for our polytropic relation.
Kanokwan Sitthithakerngkiet - One of the best experts on this subject based on the ideXlab platform.
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on the existence result for system of generalized strong vector Quasiequilibrium problems
Fixed Point Theory and Applications, 2011Co-Authors: Somyot Plubtieng, Kanokwan SitthithakerngkietAbstract:We introduce a new type of the system of generalized strong vector Quasiequilibrium problems with set-valued mappings in real locally convex Hausdorff topological vector spaces. We establish an existence theorem by using Kakutani-Fan-Glicksberg fixed-point theorem and discuss the closedness of strong solution set for the system of generalized strong vector Quasiequilibrium problem. The results presented in the paper improve and extend the main results of Long et al. (2008).
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Existence Result of Generalized Vector Quasiequilibrium Problems in Locally -Convex Spaces
Fixed Point Theory and Applications, 2011Co-Authors: Somyot Plubtieng, Kanokwan SitthithakerngkietAbstract:This paper deals with the generalized strong vector Quasiequilibrium problems without convexity in locally Open image in new window-convex spaces. Using the Kakutani-Fan-Glicksberg fixed point theorem for upper semicontinuous set-valued mapping with nonempty closed acyclic values, the existence theorems for them are established. Moreover, we also discuss the closedness of strong solution set for the generalized strong vector Quasiequilibrium problems.
Nan-jing Huang - One of the best experts on this subject based on the ideXlab platform.
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Upper Semicontinuity of Solution Mappings to Parametric Generalized Vector Quasiequilibrium Problems
Journal of Function Spaces, 2015Co-Authors: Shu-qiang Shan, Yu Han, Nan-jing HuangAbstract:We establish the upper semicontinuity of solution mappings for a class of parametric generalized vector Quasiequilibrium problems. As applications, we obtain the upper semicontinuity of solution mappings to several problems, such as parametric optimization problem, parametric saddle point problem, parametric Nash equilibria problem, parametric variational inequality, and parametric equilibrium problem.
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On the stability of solution mapping for parametric generalized vector Quasiequilibrium problems
Computers & Mathematics with Applications, 2012Co-Authors: Ren-you Zhong, Nan-jing HuangAbstract:In this paper, we study the solution stability for a class of parametric generalized vector Quasiequilibrium problems. By virtue of the parametric gap function, we obtain a sufficient and necessary condition for the Hausdorff lower semicontinuity of the solution mapping to the parametric generalized vector Quasiequilibrium problem. The results presented in this paper generalize and improve some main results of Chen et al. (2010) [34], and Zhong and Huang (2011) [35].
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Systems of Generalized Quasivariational Inclusion Problems with Applications in -Spaces
Fixed Point Theory and Applications, 2011Co-Authors: Ming-ge Yang, Nan-jing HuangAbstract:We first prove that the product of a family of -spaces is also an -space. Then, by using a Himmelberg type fixed point theorem in -spaces, we establish existence theorems of solutions for systems of generalized quasivariational inclusion problems, systems of variational equations, and systems of generalized Quasiequilibrium problems in -spaces. Applications of the existence theorem of solutions for systems of generalized Quasiequilibrium problems to optimization problems are given in -spaces.
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Symmetric strong vector Quasiequilibrium problems in Hausdorff locally convex spaces
Journal of Inequalities and Applications, 2011Co-Authors: Bin Chen, Nan-jing Huang, Yeol Je ChoAbstract:In this article, a new symmetric strong vector Quasiequilibrium problem in real locally convex Hausdorff topological vector spaces is introduced and studied. An existence theorem of solutions for the symmetric strong vector Quasiequilibrium problem by using Kakutani-Fan-Glicksberg fixed point theorem is obtained. Moreover, the closedness of the solution set for this problem is derived. The results presented in this article improve and extend some known results according to Long et al. [Math. Comput. Model. 47, 445-451 (2008)], Somyot and Kanokwan [Fixed Point Theory Appl. doi:10.1155/2011/475121], and Wang et al. [Bull. Malays. Math. Sci. Soc. http://www.emis.de/journals/BMMSS/pdf/acceptedpapers/2009-11-022_R1.pdf].
Luciano Rezzolla - One of the best experts on this subject based on the ideXlab platform.
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new code for Quasiequilibrium initial data of binary neutron stars corotating irrotational and slowly spinning systems
Physical Review D, 2015Co-Authors: Antonios Tsokaros, Kōji Uryū, Luciano RezzollaAbstract:We present the extension of our cocal---Compact Object CALculator---code to compute general-relativistic initial data for binary compact-star systems. In particular, we construct Quasiequilibrium initial data for equal-mass binaries with spins that are either aligned or antialigned with the orbital angular momentum. The Isenberg-Wilson-Mathews formalism is adopted and the constraint equations are solved using the representation formula with a suitable choice of a Green's function. We validate the new code with solutions for equal-mass binaries and explore its capabilities for a wide range of compactnesses, from a white dwarf binary with compactness $\ensuremath{\sim}1{0}^{\ensuremath{-}4}$, up to a highly relativistic neutron-star binary with compactness $\ensuremath{\sim}0.22$. We also present a comparison with corotating and irrotational Quasiequilibrium sequences from the spectral code lorene [Taniguchi and Gourgoulhon, Phys. Rev. D 66, 104019 (2002)] and with different compactness, showing that the results from the two codes agree to a precision of the order of 0.05%. Finally, we present equilibria for spinning configurations with a nuclear-physics equation of state in a piecewise polytropic representation.
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New code for Quasiequilibrium initial data of binary neutron stars: Corotating, irrotational and slowly spinning systems
arXiv: General Relativity and Quantum Cosmology, 2015Co-Authors: Antonios Tsokaros, Koji Uryu, Luciano RezzollaAbstract:We present the extension of our \cocal~- Compact Object CALculator - code to compute general-relativistic initial data for binary compact-star systems. In particular, we construct Quasiequilibrium initial data for equal-mass binaries with spins that are either aligned or antialigned with the orbital angular momentum. The Isenberg-Wilson-Mathews formalism is adopted and the constraint equations are solved using the representation formula with a suitable choice of a Green's function. We validate the new code with solutions for equal-mass binaries and explore its capabilities for a wide range of compactnesses, from a white dwarf binary with compactness $\sim 10^{-4}$, up to a highly relativistic neutron-star binary with compactness $\sim 0.22$. We also present a comparison with corotating and irrotational Quasiequilibrium sequences from the spectral code \lorene [Taniguchi and Gourgoulhon, Phys. Rev. D {\bf 66}, 104019 (2002)] and with different compactness, showing that the results from the two codes agree to a precision of the order of $0.05\%$. Finally, we present equilibria for spinning configurations with a nuclear-physics equation of state in a piecewise polytropic representation.
