arXiv subjects

Shin'ichirou Yoshida

Publications and source records attributed to Shin'ichirou Yoshida.

At least 19 recordsLinked to original sources

arXiv2026

Surface gravity wave on a neutron star ocean trapped around a magnetic pole

A warm neutron star is expected to have a fluid "ocean" of heavy elements at its outermost part of the outer crust. As is on the terrestrial ocean, the neutron star ocean also has surface gravity waves. Around a magnetic pole of a star, the ocean may have a dip due to the strong magnetic pressure coming into play in the hydrostatic balance of the ocean. The dip enables the surface gravity wave to be trapped around the magnetic pole to form eigenmodes. The frequency of the mode is much lower than the dynamical frequency at the stellar surface, owing to the weak Coriolis force and the gradient in the ocean's depth that makes the eigenmodes present. We solve the equation of surface gravity waves in the local $β$-plane approximation and obtain the spectrum of discrete eigenmodes. We see that there are no axisymmetric modes and that the mode frequency decreases and asymptotes to zero as the number of nodes of the corresponding eigenfunction increases. This is reminiscent of the g-modes in the context of asteroseismology. Observations of X-ray binaries containing neutron stars reveal that some of the systems exhibit low-frequency quasi-periodic oscillations (QPOs) whose frequency is $1-10^3$mHz. We investigate whether the eigenmodes considered here may explain the low-frequency QPO spectrum. It is suggested that some of the QPOs in the system whose neutron star spins at the period less than $10$s may be consistent with the model. As far as the spin period is larger than $10$s, the eigenmode frequencies are too low to explain the observed QPOs.

astro-ph.HE↗

arXiv2023

Bifurcation and stability of uniformly rotating homogeneous ellipsoids surrounded by a massive thin ring

We examine the effects of a massive concentric ring around a spheroid or an ellipsoid with uniform density and uniform rotation. Equilibrium sequences of axisymmetric Maclaurin-like spheroid and triaxial Jacobi-like ellipsoids are obtained. Due to the gravitational field of the ring, Maclaurin-like spheroid does not have a spherical limit when the object's angular frequency vanishes. At a critical value of the eccentricity of the spheroid's meridional section, a triaxial Jacobi-like ellipsoid bifurcates. When a parameter characterizing the gravitational field of the ring is smaller than a threshold, the bifurcation points of Maclaurin-like and Jacobi-like ellipsoids exist and the critical eccentricity is slightly larger than that of the classical Maclaurin-to-Jacobi bifurcation. When the parameter exceeds the threshold, the Maclaurin-like spheroid does not have the bifurcation point and the Jacobi-like ellipsoid appears at the lower eccentricity than the Maclaurin-like spheroid. By comparisons of the energy of the ellipsoids with the same angular momentum, it is shown that the critical point of bifurcation does not correspond to the onset of the secular instability of Maclaurin-like spheroid. It is concluded that the gravitational field of a massive ring surrounding a uniformly rotating spheroid stabilizes it against a bar-shaped deformation due to viscous dissipations.

physics.class-ph↗

arXiv2022

A numerical modeling of rotating substellar objects up to mass-shedding limits

Rotation may affect the occurrence of sustainable hydrogen burning in very low-mass stellar objects by the introduction of centrifugal force to the hydrostatic balance as well as by the appearance of rotational break-up of the objects (mass-shedding limit) for rapidly rotating cases. We numerically construct the models of rotating very low-mass stellar objects that may or may not experience sustained nuclear reaction (hydrogen-burning) as their energy source. The rotation is not limited to being slow so the effect of the rotational deformation of them is not infinitesimally small. Critical curves of sustainable hydrogen burning in the parameter space of mass versus central degeneracy, on which the nuclear energy generation balances the surface luminosity, are obtained for different values of angular momentum. It is shown that if the angular momentum exceeds the threshold $J_0=8.85\times 10^{48}{\rm erg}~{\rm s}$ the critical curve is broken up into two branches with lower and higher degeneracy because of the mass-shedding limit. Based on the results, we model mechano-thermal evolutions of substellar objects, in which cooling, as well as mass/angular momentum reductions, are followed for two simplified cases. The case with such external braking mechanisms as magnetized wind or magnetic braking is mainly controlled by the spin-down timescale. The other case with no external braking leads to the mass-shedding limit after gravitational contraction. Thereafter the object sheds its mass to form a ring or a disc surrounding it and shrinks.

astro-ph.SR↗

arXiv2021

Near-Chandrasekhar-Mass Type Ia Supernovae from the Double-Degenerate Channel

Recent observational evidence has demonstrated that white dwarf (WD) mergers are a highly efficient mechanism for mass accretion onto WDs in the galaxy. In this paper, we show that WD mergers naturally produce highly-magnetized, uniformly-rotating WDs, including a substantial population within a narrow mass range close to the Chandrasekhar mass ($M_{\rm Ch}$). These near-$M_{\rm Ch}$ WD mergers subsequently undergo rapid spin up and compression on a $\sim 10^2$ yr timescale, either leading to central ignition and a normal SN Ia via the DDT mechanism, or alternatively to a failed detonation and SN Iax through pure deflagration. The resulting SNe Ia and SNe Iax will have spectra, light curves, polarimetry, and nucleosynthetic yields similar to those predicted to arise through the canonical near-$M_{\rm Ch}$ single degenerate (SD) channel, but with a $t^{-1}$ delay time distribution characteristic of the double-degenerate (DD) channel. Furthermore, in contrast to the SD channel, WD merger near-$M_{\rm Ch}$ SNe Ia and SNe Iax will not produce observable companion signatures. We discuss a range of implications of these findings, from SNe Ia explosion mechanisms, to galactic nucleosynthesis of iron peak elements including manganese.

astro-ph.HE↗

arXiv2020

Fermionic dark stars spin slowly

We study r-mode instability of rotating compact objects composed of asymmetric and self-interacting Fermionic dark matter (dark stars). It is argued that the instability limit the angular frequency of the stars less than half of the Keplerian frequency. This may constrain the stars as alternatives to fast spinning pulsars or rapidly rotating Kerr black holes.

astro-ph.HE↗

arXiv2020

Some properties of doubly-degenerate stars

We investigate critical masses of and circular geodesics around doubly-degenerate stars (DDSs) which are composed of cold nuclear matter as well as cold Fermionic dark matter (DM). We here consider asymmetric dark Fermion with self-interaction as a DM candidate. These stars have core-envelope structures and are categorized into baryon-enveloped and DM-enveloped, according to the composition of their envelope. It is seen that the baryon-enveloped and DM-enveloped classes have their own critical masses determined mainly by the dominant component in their envelope. For a typical parameter sets, we see that a balanced mixture of two species may lead to smaller masses than if the either of the species is dominant. We also show that for a highly DM-enveloped case circular orbits in the vacuum region terminates at the innermost stable circular orbit (ISCO) in vacuum, but circular orbits of smaller radius are possible in the DM-envelope forming a gap between the ISCO and the inner orbits.

astro-ph.HE↗

arXiv2020

Decihertz gravitational waves from double white dwarf merger remnants

Close binaries of double white dwarfs (DWDs) inspiral and merge by emitting gravitational wave (GW). Orbital motion of some of these binaries are expected to be observed at low frequency band by space-borne laser interferometric detectors of GW. The merger remnant may suffer thermonuclear runaway and explode as type Ia supernova if they are massive enough. As GW sources the remnants have so far been scarcely studied. Here we propose a new mechanism of GW emission from DWD merger remnants which may be observed by planned GW detectors in decihertz frequency band. A remnant is temporarily expected to have a high degree of differential rotation as a consequence of merger process. It is then unstable to oscillation modes whose azimuthal pattern speed coincides with the stellar rotation. We solve eigenvalue problem of differentially rotating remnants and identify unstable eigenmodes which may be categorized to inertial modes. The estimate of characteristic strain of GW shows that they may be detectable within the distance of the Virgo cluster by planned gravitational wave observatories targeting the decihertz band.

astro-ph.HE↗

arXiv2019

On the central engine of the fastest-declining Type I supernova SN2019bkc

An analytic model is presented for the fastest-declining Type I supernova SN2019bkc. The model of the central engine consists of the magnetic dipole radiation from a non-explosive remnant of a double white dwarf merger. We consider the viscous evolution of the rotating remnant, which may lead to the diminishing energy supply in an appropriate time scale for the light curve.

astro-ph.HE↗

arXiv2019

Tidal disruptions of rotating stars by a supermassive black hole

We study tidal disruption events of rotating stars by a supermassive black hole in a galactic nucleus by using a smoothed-particle hydrodynamics (SPH) code. We compare mass infall rates of tidal-disruption debris of a non-rotating and of a rotating star when they come close to the supermassive black hole. Remarkably the mass distribution of debris bound to the black hole as a function of specific energy shows clear difference between rotating and non-rotating stars, even if the stellar rotation is far from the break-up limit. The debris of a star whose initial spin is parallel to the orbital angular momentum has a mass distribution which extends to lower energy than that of non-rotating star. The debris of a star with anti-parallel spin has a larger energy compared with a non-rotating counterpart. As a result, debris from a star with anti-parallel spin is bound more loosely to the black hole and the mass-infall rate rises later in time, while that of a star with a parallel spin is tightly bound and falls back to the black hole earlier. The different rising timescales of mass-infall rate may affect the early phase of flares due to the tidal disruptions. In the Appendix we study the disruptions by using a uniform-density ellipsoid model which approximately takes into account the effect of strong gravity of the black hole. We find the mass infall rate reaches its maximum earlier for strong gravity cases because the debris is trapped in a deeper potential well of the black hole.

astro-ph.HE↗

arXiv2019

Rotating white dwarf models with finite-temperature envelope

We present new numerical method to compute structures of differentially rotating white dwarfs with thermal stratification. Our models have cores composed of ions and completely degenerate electrons and have isentropic envelopes composed of ions, photons, partially degenerate electrons and positrons. The models are intended to mimic very early phases of remnants of white dwarf binary mergers, some of which may lead to type Ia supernovae. The effect of hot envelope to increase the mass depends on its chemical composition through the mean molecular weight of the envelope. For uniformly rotating models, we see only a small increase in mass even in the presence of hot envelope. Differential rotation changes it drastically and super-Chandrasekhar mass model whose mass doubles the Chandrasekhar mass of the degenerate star for some parameter choices. We also compute quasi-equilibrium evolutionary sequences of remnants by fixing either total angular momentum or entropy in the envelope. Existence of these sequences depends on various factors such as the remnant mass, the profile of differential rotation, the entropy and the chemical composition of the envelope.

astro-ph.HE↗

arXiv2017

Modeling differential rotations of compact stars in equilibriums

Outcomes of numerical relativity simulations of massive core collapses or binary neutron star mergers with moderate masses suggest formations of rapidly and differentially rotating neutron stars. Subsequent fall back accretion may also amplify the degree of differential rotations. We propose new formulations for modeling differential rotations of those compact stars, and present selected solutions of differentially rotating, stationary, and axisymmetric compact stars in equilibriums. For the cases when rotating stars reach break-up velocities, the maximum masses of such rotating models are obtained.

astro-ph.HE↗

arXiv2016

On a role of corotation radius in the low $T/W$ dynamical instability of differentially rotating stars

We investigate the nature of so-called low $T/W$ dynamical instability in a differentially rotating star by focusing on the role played by the corotation radius of the unstable oscillation modes. An one dimensional model of linear perturbation, which neglects dependence of variables on the coordinate along the rotational axis of the star, is solved to obtain stable and unstable eigenmodes. A linear eigenmode having a corotation radius, at which azimuthal pattern speed of the mode coincides with the stellar angular velocity, is categorized to either a complex (growing or damping) mode or a purely real mode belonging to a continuous spectrum of frequency. We compute canonical angular momentum and its flux to study eigenmodes with corotation radius. In a dynamically unstable mode, sound wave transports its angular momentum in such a way that the absolute value of the angular momentum is increased on both sides of the corotation radius. We further evaluate growth of amplitude of reflected sound wave incident to a corotation point and find that the over-reflection of the wave and the trapping of it between the corotation radius and the surface of the star may qualitatively explain dependences of eigenfrequencies on the stellar differential rotation. The results suggest that the low $T/W$ instability may be caused by over-reflection of sound waves trapped mainly between the surface of the star and a corotation radius.

astro-ph.HE↗

arXiv2016

Do triaxial supramassive compact stars exist?

We study quasiequilibrium solutions of triaxially deformed rotating compact stars -- a generalization of Jacobi ellipsoids under relativistic gravity and compressible equations of state (EOS). For relatively stiff (piecewise) polytropic EOSs, we find supramassive triaxial solutions whose masses exceed the maximum mass of the spherical solution, but are always lower than those of axisymmetric equilibriums. The difference in the maximum masses of triaxial and axisymmetric solutions depends sensitively on the EOS. If the difference turns out to be only about 10%, it will be strong evidence that the EOS of high density matter becomes substantially softer in the core of neutron stars. This finding opens a novel way to probe phase transitions of high density nuclear matter using detections of gravitational waves from new born neutron stars or magnetars under fallback accretion.

astro-ph.HE↗

arXiv2016

Unstable normal modes of low T/W dynamical instabilities in differentially rotating stars

We investigate the nature of low T/W dynamical instabilities in differentially rotating stars by means of linear perturbation. Here, T and W represent rotational kinetic energy and the gravitational binding energy of the star. This is the first attempt to investigate low T/W dynamical instabilities as a complete set of the eigenvalue problem. Our equilibrium configuration has "constant" specific angular momentum distribution, which potentially contains a singular solution in the perturbed enthalpy at corotation radius in linear perturbation. We find the unstable normal modes of differentially rotating stars by solving the eigenvalue problem along the equatorial plane of the star, imposing the regularity condition on the center and the vanished enthalpy at the oscillating equatorial surface. We find that the existing pulsation modes become unstable due to the existence of the corotation radius inside the star. The feature of the unstable mode eigenfrequency and its eigenfunction in the linear analysis roughly agrees with that in three-dimensional hydrodynamical simulations in Newtonian gravity. Therefore, our normal mode analysis in the equatorial motion proves valid to find the unstable equilibrium stars efficiently. Moreover, the nature of the eigenfunction that oscillates between corotation and the surface radius for unstable stars requires reinterpretation of the pulsation modes in differentially rotating stars.

astro-ph.SR↗

arXiv2015

Gravitational Wave Signatures of Ab Initio Two-Dimensional Core Collapse Supernova Explosion Models for 12-25 Solar Masses Stars

We present the gravitational waveforms computed in ab initio two-dimensional core collapse supernova models evolved with the Chimera code for progenitor masses between 12 and 25 solar masses. All models employ multi-frequency neutrino transport in the ray-by-ray approximation, state-of-the-art weak interaction physics, relativistic transport corrections such as the gravitational redshift of neutrinos, two-dimensional hydrodynamics with the commensurate relativistic corrections, Newtonian self-gravity with a general relativistic monopole correction, and the Lattimer-Swesty equation of state with 220 MeV compressibility, and begin with the most recent Woosley-Heger nonrotating progenitors in this mass range. All of our models exhibit robust explosions. Therefore, our waveforms capture all stages of supernova development: 1) a relatively short and weak prompt signal, 2) a quiescent stage, 3) a strong signal due to convection and SASI activity, 4) termination of active accretion onto the proto-neutron star, and 5) a slowly increasing tail that reaches a saturation value. Fourier decomposition shows that the gravitational wave signals we predict should be observable by AdvLIGO for Galactic events across the range of progenitors considered here. The fundamental limitation of these models is in their imposition of axisymmetry. Further progress will require counterpart three-dimensional models, which are underway.

astro-ph.HE↗

arXiv2013

Assessment of Cowling approximation in computing ellipticity of a magnetized non-barotropic star

A deformation of a neutron star due to its own magnetic field is an important issue in gravitational wave astronomy, since a misaligned rotator with small ellipticity may emit continuous gravitational wave that may be observed by ground-based detectors. Recently Mastrano et al. (2011,2013) evaluated deformations induced by both poloidal and toroidal magnetic field in non-barotropic model stars by neglecting the gravitational field perturbation (Cowling approximation). Following their treatment in non-barotropic fluid and magnetic configurations, we here assess the effect of gravitational perturbation that they neglected. We show that the ellipticity computed with gravitational perturbation is roughly twice as large as that obtained by Cowling approximation. We should allow this amount of error in using the neat analytic treatment proposed by them.

astro-ph.HE↗

arXiv2013

Axisymmetric and stationary structures of magnetized barotropic stars with extremely strong magnetic fields deep inside

We have succeeded in obtaining magnetized star models that have extremely strong magnetic fields in the interior of the star. In our formulation, arbitrary functions of the magnetic flux function appear in the expression for the current density. By appropriately choosing the functional form for one of the arbitrary functions that corresponds to the distribution of the toroidal current density, we have obtained configurations with magnetic field distributions that are highly localized within the central part and near the magnetic axis region. The absolute values of the central magnetic fields are stronger than those of the surface region by two orders of magnitude. By applying our results to magnetars, the internal magnetic poloidal fields could be 10^17 G, although the surface magnetic fields are about 10^15 G in the case of magnetars. For white dwarfs, the internal magnetic poloidal fields could be 10^12 G, when the surface magnetic fields are 10^9-10^10 G .

astro-ph.HE↗

arXiv2013

Slowly rotating neutron stars with small differential rotation: equilibrium models and oscillations in the Cowling approximation

Newly born neutron stars can present differential rotation, even if later it should be suppressed by viscosity or a sufficiently strong magnetic field. And in this early stage of its life, a neutron star is expected to have a strong emission of gravitational waves, which could be influenced by the differential rotation. We present here a new formalism for modelling differentially rotating neutron stars: working on the slow rotation approximation and assuming a small degree of differential rotation, we show that it is possible to separate variables in the Einstein field equations. The dragging of inertial frames is determined by solving three decoupled ODEs. After we establish our equilibrium model, we explore the influence of the differential rotation on the f and r-modes of oscillation of the neutron star in the Cowling approximation, and we also analyze an effect of the differential rotation on the emission of gravitational radiation from the f-modes. We see that the gravitational radiation from the f-modes is slightly suppressed by introducing differential rotation to the equilibrium stars.

gr-qc↗