# Shin'Ichirou YoshidaGraduate School of Arts and Sciences - The University of Tokyo · Earth Science and Astronomy

Shin'Ichirou Yoshida

Doctor of Science

## About

64

Publications

2,138

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1,273

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Citations since 2017

Introduction

[Current research interests]
Gravitational waves from rotating stars.
Instabilities of differentially rotating stars.
Evolution of double white dwarf merger remnants.
Modeling of MHD equilibrium of self-gravitating objects.
Quasi-periodic oscillations of compact X-ray binaries.
Compact stars composed of dark matter. Very low-mass stars and brown dwarfs.

**Skills and Expertise**

## Publications

Publications (64)

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...

We examine the effects of a massive concentric ring around a spheroid (i.e., axisymmetric ellipsoid) or an ellipsoid with uniform density and uniform rotation. Equilibrium sequences of axisymmetric Maclaurin-like spheroid and triaxial Jacobi-like ellipsoids are obtained which respectively correspond to the classical Maclaurin spheroids and the Jaco...

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 ste...

Comparison of the rotational frequency squared, $\Omega^2$, versus the ellipticity of rotating polytropes.
The ellipticity is defined as $\sqrt{1-(R_p/R_e)^2}$, where $R_p$ and $R_e$ are the polar and the equatorial radius, respectively.
$\Omega^2$ is normalized by $4\pi\rho_c$, where $\rho_c$ is the central density.

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 ste...

This preliminary version for RNAAS submission fits all the polytropic data (1 ≤ N ≤ 3) with a single Padé formula, as well as a single polynomial.) Giving a simple functional relation between the rotational frequency and the deformation for compress-ible stellar models is useful both in observational and theoretical astrophysics. Here I present sim...

Giving a simple functional relationship between the rotational frequency and
the centrifugal deformation for compressible stellar models is useful both in observational and theoretical astrophysics.
Here I present simple Pad\'{e} approximants for the rotational frequency of polytropic
stars (index $N=1, 1.5, 3$) as a function of the oblateness, whi...

Assessment of two of the frequently used approximations for rotating astrophysical objects, the Roche and the Darwin–Radau approximations, is performed by comparing the rotational frequencies computed by these approximations with those of self-consistent numerical hydrostatic solutions. Overall, the Darwin–Radau approximation performs better, thoug...

Slides from the local group colloquium.

In this study, we developed numerical models of brown dwarfs and extremely low-mass main-sequence stars that take into account the effect of rotation and investigated the conditions under which they can exist.

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...

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...

Close binaries of double white dwarfs (DWDs) inspiral and merge by emitting gravitational waves (GWs). Orbital motion of some of these binaries is expected to be observed at a low-frequency band by space-borne laser interferometric detectors of GWs. The merger remnant may suffer thermonuclear runaway and explode as SNe Ia if they are massive enough...

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 b...

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...

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...

Keywords: supernovae: individual (SN 2019bkc)
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
d...

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 phase...

We compare mass infall rates of tidal-disruption debris of a non-rotating and of a rotating star when they come close to a supermassive black hole at the center of a galaxy. 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...

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 pha...

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 t...

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...

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 e...

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...

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 formul...

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 correc...

Newly born neutron stars can present differential rotation, even if later it should be suppressed by viscosity or a sufficiently strong magnetic field. 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...

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. evaluated deformations induced by both poloidal and toroidal magnetic...

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 formal...

We present a new numerical code to compute non-axisymmetric eigenmodes of
rapidly rotating relativistic stars by adopting spatially conformally flat
approximation of general relativity. The approximation suppresses the radiative
degree of freedom of relativistic gravity and the field equations are cast into
a set of elliptic equations. The code is...

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...

We have obtained equilibrium states of magnetized disc-central compact object systems. Under the assumption of stationary and axisymmetry, we have derived generalized Grad-Shafranov equation (GS equation). We have succeed in solving GS equation and including not only poloidal but also toroidal magnetic fields under the ideal MHD approximation. We h...

The stability properties of relativistic stars against gravitational collapse to black holes is a classical problem in general
relativity. In 1988, a sufficient criterion for secular instability was established by Friedman, Ipser & Sorkin, who proved
that a sequence of uniformly rotating barotropic stars are secularly unstable on one side of a turn...

In this Letter we report brand new analytic stationary solutions of constant density stars with magnetic field and self-gravity.
These solutions include prolate configurations even for purely poloidal magnetic fields as well as oblate configurations.
These new analytic solutions are expressed in very simple forms and can be considered as generalize...

We analyze the impact of the choice rotation law on equilibrium sequences of
relativistic differentially-rotating neutron stars in axisymmetry. The maximum
allowed mass for each model is strongly affected by the distribution of angular
velocity along the radial direction and by the consequent degree of
differential rotation. In order to study the w...

We present a new formulation to compute numerically stationary and axisymmetric equilibria of magnetized and self-gravitating astrophysical fluids. Under the assumption of ideal MHD, the stream function for the flow can be chosen as a basic variable with which the Euler-Maxwell equations are cast into a set of basic equations, i.e. a generalized Be...

We present the gravitational wave signatures for a suite of axisymmetric core collapse supernova models with progenitors masses between 12 and 25 solar masses. These models are distinguished by the fact they explode and contain essential physics (in particular, multi-frequency neutrino transport and general relativity) needed for a more realistic d...

Unraveling the mechanism for core-collapse supernova explosions is an outstanding computational challenge and the problem remains essentially unsolved despite more than four decades of effort. However, much progress in realistic modeling has occurred recently through the availability of multi-teraflop machines and the increasing sophistication of s...

Much progress in realistic modeling of core-collapse supernovae has occurred
recently through the availability of multi-teraflop machines and the increasing
sophistication of supernova codes. These improvements are enabling simulations
with enough realism that the explosion mechanism, long a mystery, may soon be
delineated. We briefly describe the...

As one step towards a systematic modeling of the electromagnetic (EM) emission from an inspiralling black hole binary we consider a simple scenario in which the binary moves in a uniform magnetic field anchored to a distant circumbinary disc. We study this system by solving the Einstein-Maxwell equations in which the EM fields are chosen with astro...

We have investigated the "universality" of the twisted-torus equilibrium structures of the magnetic fields in deformed stars. The Tomimura-Eriguchi scheme for equilibrium structures of uniformly rotating magnetized stars is extended to treat equilibrium configurations of differentially rotating and meridional circulation-free magnetized polytropes...

We report our results of a study on non-axisymmetric oscillation modes of a torus with non-Keplerian rotational profile around Schwarzschild black hole. We study a simple toy equilibrium model of incompressible torus whose vertical structure is neglected. The master equation of the oscillation in this case becomes a second-order ordinary differenti...

Results from helically symmetric scalar field models and first results from a convergent helically symmetric binary neutron star code are reported here; these are models stationary in the rotating frame of a source with constant angular velocity omega. In the scalar field models and the neutron star code, helical symmetry leads to a system of mixed...

We study the nature of non-axisymmetric dynamical instabilities in differentially rotating stars with both linear eigenmode
analysis and hydrodynamic simulations in Newtonian gravity. We especially investigate the following three types of instability;
the one-armed spiral instability, the low T/|W| bar instability, and the high T/|W| bar instabilit...

We study the nature of non-axisymmetric dynamical instabilities in differentially rotating stars with both linear eigenmode analysis and hydrodynamic simulations in Newtonian gravity. We especially investigate the following three types of instability; the one-armed spiral instability, the low T/|W| bar instability, and the high T/|W| bar instabilit...

We investigate the dynamical one-armed spiral instability in differentially rotating stars with both eigenmode analysis and hydrodynamic simulations in Newtonian gravity. We find that the one-armed spiral mode is generated around the corotation radius of the star, and the distribution of angular momentum shifts inwards the corotation radius during...

We investigate dynamical instabilities appearing in differentially rotating stars with rather low value of T/|W| parameter (a ratio of rotational to gravitational energy), by using numerical non-linear hydrodynamics and linear eigenmode analysis. Particular attention are paid to the one-armed spiral instability and low T/|W| bar instability, both o...

This is the second in a series of papers investigating the oscillation properties of relativistic, non-self-gravitating tori orbiting around black holes. Extending the work done in a Schwarzschild background, here we consider the axisymmetric oscillations of vertically integrated tori in a Kerr space–time. The tori are modelled with a number of dif...

We develop a numerical scheme for obtaining the r-mode oscillations of rapidly rotating relativistic stars. In the present scheme, we neglect all metric perturbations and
only take account of the dynamics of the fluid in the background space-time of the unperturbed star (the relativistic Cowling
approximation). We also assume the star is barotropic...

This is the first of a series of papers investigating the oscillation properties of relativistic, non-selfgravitating tori orbiting around a black hole. In this initial paper we consider the axisymmetric oscillation modes of a torus constructed in a Schwarzschild spacetime. To simplify the treatment and make it as analytical as possible, we build o...

In this paper we have re-examined the damping effect of viscous boundary layer (VBL) on the r-mode instability with taking the superfluid motions in the inner crust into account...

We consider the spin evolution of highly magnetized neutron stars in a hypercritical inflow just after their birth in supernovae.
The presence of a strong magnetic field could deform the star and if the symmetry axis of the field is misaligned with that
of stellar rotation, the star will be an emitter of gravitational waves. Here we investigate the...

We show that, at first order in the angular velocity, the general relativistic description of Rossby-Haurwitz waves (the analogues of r-waves on a thin shell) can be obtained from the corresponding Newtonian one after a coordinate transformation. As an application, we show that the results recently obtained by Rezzolla and Yoshida (2001) in the ana...

Recent studies have raised doubts about the occurrence of r modes in Newtonian stars with a large degree of differential rotation. To assess the validity of this conjecture we have solved the eigenvalue problem for Rossby-Haurwitz waves (the analogues of r waves on a thin-shell) in the presence of differential rotation. The results obtained indicat...

Gravitational wave emission is considered to be the driving force for the evolution of short-period cataclysmic binary stars,
making them a potential test for the validity of general relativity. In spite of continuous refinements of the physical description,
a 10 per cent mismatch exists between the theoretical minimum period (Pturn ≃ 70 min) and t...

We have developed a new numerical scheme to solve r-mode oscillations of {\it rapidly rotating polytropic stars} in Newtonian gravity. In this scheme, Euler perturbations of the density, three components of the velocity are treated as four unknown quantities together with the oscillation frequency. For the basic equations of oscillations, the compa...

We have investigated the influence of the r-mode instability on hypercritically accreting neutron stars in close binary systems during their common envelope phases, based on the scenario proposed by Brown et al. On the one hand, neutron stars are heated by the accreted matter at the stellar surface, but on the other hand they are also cooled down b...

The first results of numerical analysis of classical r-modes of rapidly rotating compressible stellar models are reported. The full set of linear perturbation equations of rotating stars in Newtonian
gravity is solved numerically without the slow rotation approximation. A critical curve of gravitational wave emission induced
instability, which rest...

A numerical method of mode analysis of rapidly rotating relativistic stellar models by the Cowling approximation is applied to rotating neutron stars with realistic equations of state. For selected equations of state, eigenvalues and eigenfunctions of f-modes are numerically solved for stellar models from nonrotating to maximally rotating states.
N...

The relativistic Cowling approximation in which all metric perturbations are omitted is applied to nonaxisymmetric infinitesimal oscillations of uniformly rotating general relativistic polytropes.
Frequencies of lower order f-modes, which are important in analysis of secular instability driven by gravitational radiation, are investigated, and neutr...

Ergoregion instability of rapidly rotating relativistic stars found by
Friedman is investigated numerically by developing a totally different
formulation from the one presented by Comins & Schutz. Our new
scheme can provide solutions even for modes of small azimuthal number m,
which Comins & Schutz could not precisely investigate because of
their u...

We investigate secular instability of rapidly rotating polytropes by
taking account of the gravitational radiation reaction and the viscosity
of the neutron star matter. We use a new method to solve the linearized
equations describing the oscillation of rotating stars. The maximum
angular velocity of neutron stars is obtained when two dissipative
f...

Results of numerical analysis of classical r-modes in rapidly rotating compressible stellar models are presented. The full set of linear perturbation equations of rotating stars in New-tonian gravity are numerically solved without the slow rota-tion approximation. A critical curve of gravitational wave emission induced instability which restricts t...