arXiv subjects

Takuya Matsuda

Publications and source records attributed to Takuya Matsuda.

33 records · Page 2Linked to original sources

arXiv2004

Numerical Simulation of the Surface Flow on the Companion Star in a Close Binary System II. Construction of Doppler maps and application to Galactic supersoft X-ray sources

We perform three-dimensional numerical simulations of surface flows on the companion star in a semi-detached binary system and construct the corresponding Doppler maps. The mass ratio of the binary system, $q=M_2/M_1$, considered here is $q=0.05, 0.33, 0.5, 1, 2$, and 3. For all cases, we obtain the H-, L1-, and L2-eddies, as found in our previous work, and confirm that the flow pattern does not heavily depend on the mass ratio. We propose that this kind of problem be dubbed ``{\it stellar meteorology}.'' The Doppler maps at the position of the companion show a structure tilted towards clockwise direction and presenting deviations from the critical Roche surface due to the L1-eddy and the L2-eddy on the companion star. We apply our results to the Galactic supersoft X-ray source RX J0019.8+2156 and try to attribute the low radial velocity component of the emission lines of He II $λ4686$ observed recently to the irradiated spot on the surface of the companion rather than that of the white dwarf or the accretion disc. Based on the comparison between the observations and our constructed Doppler map, we estimate the mass of the companion star in RX J0019.8+2156 to be $\sim 2 M_{\odot}$ assuming the mass of the white dwarf star to be around $0.6 M_{\odot}$.

astro-ph↗

arXiv2002

Numerical simulation of the surface flow of a companion star in a close binary system

We simulate numerically the surface flow of a gas-supplying companion star in a semi-detached binary system. Calculations are carried out for a region including only the mass-losing star, thus not the mass accreting star. The equation of state is that of an ideal gas characterized by a specific heat ratio gamma, and the case with gamma=5/3 is mainly studied. A system of eddies appears on the surface of the companion star: an eddy in the low pressure region near the L1 point, one around the high pressure at the north pole, and one or two eddies around the low pressure at the opposite side of the L1 point. Gas elements starting near the pole region rotate clockwise around the north pole (here the binary system rotates counter-clockwise as seen from the north pole). Because of viscosity, the gas drifts to the equatorial region, switches to the counter-clockwise eddy near the L1 point and flows through the L1 point to finally form the L1 stream. The flow field in the L1 region and the structure of the L1 stream are also considered.

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arXiv2001

Direct Monte Carlo simulations of accretion discs

We apply the Direct Simulation Monte Carlo (DSMC) method, developed originally to calculate rarefied gas dynamical problems, to study the gas flow in an accretion disc in a close binary system. The method involves viscosity and thermal conduction automatically, and can thus simulate turbulent viscosity (alpha viscosity).

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arXiv2001

Application of the DSMC method to astrophysical flows

The Direct Simulation Monte Carlo (DSMC) method, developed originally to calculate rarefied gas dynamical problems, is applied to continuous flow including shocks assuming that the Knudsen number is sufficiently small. In particular, we study the formation of spiral shocks in the accretion disc of a close binary system. The method involves viscosity and thermal conduction automatically, and can thus simulate turbulent viscosity.

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arXiv2001

Numerical Simulation of Interaction between an L1 Stream and an Accretion Disk in a Close Binary System

The hydrodynamic behavior of an accretion disk in a close binary system is numerically simulated. Calculation is made for a region including the compact star and the gas-supplying companion. The equation of state is that of an ideal gas characterized by the specific heat ratio $γ$. Two cases with $γ$ of 1.01 and 1.2 are studied. Our calculations show that the gas, flowing from the companion via a Lagrangian L1 point towards the accretion disk, forms a fine gas beam (L1 stream), which penetrates into the disk. No hot spot therefore forms in these calculations. Another fact discovered is that the gas rotating with the disk forms, on collision with the L1 stream, a bow shock wave, which may be called an L1 shock. The disk becomes hot because the L1 shock heats the disk gas in the outer parts of the disk, so that the spiral shocks wind loosely even with $γ=1.01$. The L1 shock enhances the non-axisymmetry of the density distribution in the disk, and therefore the angular momentum transfer by the tidal torque works more effectively. The maximum of the effective $α$ becomes $\sim 0.3$. The 'hot spot' is not formed in our simulations, but our results suggest the formation of the 'hot line', which is the L1 shock elongated along the penetrating L1 stream.

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arXiv2001

Is Angular Momentum in an Accretion Disk Transported Inwards?

The derivation of the viscosity formula of accretion disks appearing in a number of textbooks is based on a mean free path theory of gas particles. However, this procedure, when followed precisely, leads to the incorrect conclusion that the angular momentum of an accretion disk flows from the outer part to the inner part.

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arXiv2000

Numerical simulation of accretion discs in close binary systems and discovery of spiral shocks

The history of hydrodynamic numerical simulations for accretion disks in close binary systems is reviewed, in which emphasis is placed, in particular, on the facts that spiral shock waves were numerically found in 1986 by researchers including one of the present authors and that spiral structure was discovered in IP Pegasi in 1997 by Steeghs et al. The results of our two and three-dimensional numerical simulations in recent years are then summarized, with comparison being made with observations.

astro-ph↗

arXiv1998

Three-dimensional SPH simulations of accretion discs

We discuss some 3D numerical simulations of accretion discs using the SPH method and a polytropic equation of state. We show that discs exist even for as large value of the polytropic index as 1.2, and that these discs are always in hydrostatic balance. We also show that even without any inflow, spiral shocks appear in the discs.

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arXiv1998

Effect of Radiation Drag on Hoyle-Lyttleton Accretion

Hoyle-Lyttleton type accretion is investigated, by taking account of not only the effect of radiation pressure but the effect of radiation drag. We calculate the trajectories of particles for three cases: only the effect of gravity is considered (case A); the effect of radiation pressure is taken into account (case B); the effect of radiation drag as well as radiation pressure is taken into account (case C). The accretion radii for former two cases are $2GM/v_{\infty}^2$ for case A and $2GM(1-Γ)/v_{\infty}^2$ for case B, where M is the mass of the accreted object, $v_{\infty}$ the relative velocity, and Gamma the normalized luminosity of the accreted object. We found that the accretion radius for case C is in between those of cases A and B under the present approximation; i.e., the accretion radius decreases due to radiation pressure while it increases due to radiation drag. In addition, the accretion radius for case C becomes larger as the incident velocity becomes fast. The effect of radiation drag becomes more and more important when the velocity of the incident particle is comparable to the light speed.

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arXiv1998

Two and Three Dimensional Numerical Simulations of Accretion Discs in a Close Binary System

We perform 2D and 3D numerical simulations of an accretion disc in a close binary system using the Simplified Flux vector Splitting (SFS) finite volume method. In our calculations, gas is assumed to be the ideal one, and we calculate the cases with gamma=1.01, 1.05, 1.1 and 1.2. The mass ratio of the mass-losing star to the mass-accreting star is one. Our results show that spiral shocks are formed on the accretion disc in all cases. In 2D calculations we find that the smaller gamma is, the more tightly the spiral winds. We observe this trend in 3D calculations as well in somewhat weaker sense. Recently, Steeghs, Harlaftis & Horne (1997) found the first convincing evidence for spiral structure in the accretion disc of the eclipsing dwarf nova binary IP Pegasi using the technique known as Doppler tomography. We may claim that spiral structure which we have discovered in numerical simulations before are now found observationally.

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arXiv1998

Spiral Structure in IP Peg: Confronting Theory and Observations

The first convincing piece of evidence of spiral structure in the accretion disc in IP Pegasi was found by Steeghs et al. (1997). We performed two kinds of 2D hydrodynamic simulations, a SFS finite volume scheme and a SPH scheme, with a mass ratio of 0.5. Both results agreed well with each other. We constructed Doppler maps and line flux-binary phase relations based on density distributions, the results agreeing well with those obtained by observation.

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arXiv1998

3D Finite Volume Simulation of Accretion Discs with Spiral Shocks

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arXiv1998

Application of numerical methods to modeling the stellar wind and interstellar medium interaction

We present a review of the application of numerical methods to solve the problem of the stellar/solar wind with the local interstellar medium. Most of approaches described are developed by the authors. They include both purely gas dynamic and magnetohydrodynamic methods. Some attention is given to the influence of the neutral component of the interstellar medium on the flow structure. Far-field boundary conditions are formulated which are usually required in astrophyscal application owing to the extremely large length scale of these problems. Results are presented of the stationary and nonstationary, axisymmetric and geniunly three-dimensional calculations. Newly developed high-resolution numerical methods are discussed which can be applied to a large variety of astrophysical problems. The bibliography includes 113 references. The review will be published in ``CFD Review 97'' by John Wiley.

physics.geo-ph↗

arXiv1998

Numerical Computation of Two Dimensional Wind Accretion of Isothermal Gas

A new numerical algorithm for calculating isothermal wind accretion flows has been developed and is applied here to the analysis of the hydrodynamics of two-dimensional plane symmetric accretion flows in wind-fed sources. Polar coordinates are used to ensure fine resolution near the object. It is found that a thin accretion column is formed which shows wave-like oscillations. Small accretion disks are formed temporarily around the object. Mass accretion rate and angular momentum accretion rate exhibit quasi-periodic oscillations. The amplitudes of the oscillations depend on the size of the inner boundary, the number of grid points and the method of calculation. For a smaller size of the inner boundary, finer grids and more accurate numerical schemes, the amplitudes of the oscillation become larger.

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arXiv1997

Spiral Shocks in 3D Accretion Discs

We have performed three-dimensional numerical simulations of accretion discs in a close binary system using the Smoothed Particle Hydrodynamics method. Our result show that, contrary to previous claims, 3D discs do exist even when the specific heat ratio of the gas is as large as gamma=1.2. Although the disc is clearly more spread in the z-direction in this case than it is for the quasi-isothermal one, the disc height is compatible with the hydrostatic balance equation. Our numerical simulations with gamma=1.2 also demonstrate that spiral shocks exist in 3D discs. These results therefore confirm previous 2D simulations.

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