arXiv subjects

Toshiki Maruyama

Publications and source records attributed to Toshiki Maruyama.

At least 37 records · Page 2Linked to original sources

arXiv2015

Cooling of Compact Stars with Color Superconducting Quark Matter

We show a scenario for the cooling of compact stars considering the central source of Cassiopeia A (Cas A). The Cas A observation shows that the central source is a compact star with high effective temperature, and it is consistent with the cooling without exotic phases. The Cas A observation also gives the mass range of $M \geq 1.5 M_\odot$. It may conflict with the current cooling scenarios of compact stars that heavy stars show rapid cooling. We include the effect of the color superconducting (CSC) quark matter phase on the thermal evolution of compact stars. We assume the gap energy of CSC quark phase is large ($Δ\gtrsim \mathrm{10 MeV}$), and we simulate the cooling of compact stars. We present cooling curves obtained from the evolutionary calculations of compact stars: while heavier stars cool slowly, and lighter ones indicate the opposite tendency.

astro-ph.SR↗

arXiv2015

Coexistence of Kaon Condensation and Hyperons in Hadronic Matter and Its Relevance to Quark Matter

Coexistence of kaon condensation and hyperons, which may be realized in neutron stars, is investigated on the basis of the relativistic mean-field theory combined with the effective chiral Lagrangian. It is shown that kaon-condensed phase in hyperon-mixed matter is plausible, but it leads to significant softening of the equation of state (EOS).We discuss indispensable effects which make the EOS stiffer so as to be consistent with recent neutron-star observations.

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arXiv2013

Finite size effects in hadron-quark phase transition by the Dyson-Schwinger method

We study the hadron-quark phase transition, taking into account the finite-size effects for neutron star matter. For the hadron phase, we adopt a realistic equation of state within the framework of the Brueckner-Hartree-Fock theory. For the quark phase, we apply the Dyson-Schwinger method. The properties of the mixed phase are clarified by considering the finite-size effects. We find that, if the surface tension is strong enough, the equation of state becomes to be close the one with the Maxwell condition, though we properly adopt the Gibbs conditions. This result is qualitatively the same with the one by the use of the simple bag model. We also find that the mass-radius relation by the EoS is consistent with the observations of massive neutron stars.

astro-ph.HE↗

arXiv2013

Nuclear "pasta" structures in low-density nuclear matter and neutron star crust

In neutron star crust, non-uniform structure of nuclear matter is expected, which is called the "pasta" structure. From the recent studies of giant flares in magnetars, these structures might be related to some observables and physical quantities of the neutron star crust. To investigate the above quantities, we numerically explore the pasta structures with a fully threedimensional geometry and study the properties of low-density nuclear matter, based on the relativistic mean-field model and the Thomas-Fermi approximation. We observe typical pasta structures for fixed proton number-fraction and two of them for cold catalyzed matter. We also discuss the crystalline configuration of "pasta".

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arXiv2013

Shear oscillations in the hadron-quark mixed phase

Inside neutron stars, the hadron-quark mixed phase is expected during the first order phase transition from the hadron phase to the quark phase. The geometrical structure of the mixed phase strongly depends on the surface tension at the hadron-quark interface. We evaluate the shear modulus which is one of the specific properties of the hadron-quark mixed phase. As an application, we study shear oscillations due to the hadron-quark mixed phase in neutron stars. We find that the frequencies of shear oscillations depend strongly on the surface tension; with a fixed stellar mass, the fundamental frequencies are almost proportional to the surface tension. Thus, one can estimate the value of surface tension via the observation of stellar oscillations with the help of the information on the stellar mass.

astro-ph.HE↗

arXiv2013

Cooling of Compact Stars with Color Superconducting Phase in Quark Hadron Mixed Phase

We present a new scenario for the cooling of compact stars considering the central source of Cassiopeia A (Cas A). The Cas A observation shows that the central source is a compact star that has high effective temperature, and it is consistent with the cooling without exotic phases. The observation also gives the mass range of $M \geqslant 1.5 M_\odot$, which may conflict with the current plausible cooling scenario of compact stars. There are some cooled compact stars such as Vela or 3C58, which can be barely explained by the minimal cooling scenario, which includes the neutrino emission by nucleon superfluidity (PBF). Therefore, we invoke the exotic cooling processes, where a heavier star cools faster than lighter one. However, the scenario seems to be inconsistent with the observation of Cas A. Therefore, we present a new cooling scenario to explain the observation of Cas A by constructing models that include a quark color superconducting (CSC) phase with a large energy gap; this phase appears at ultrahigh density region and reduces neutrino emissivity. In our model, a compact star has CSC quark core with a low neutrino emissivity surrounded by high emissivity region made by normal quarks. We present cooling curves obtained from the evolutionary calculations of compact stars: while heavier stars cool slowly, and lighter ones indicate the opposite tendency without considering nucleon superfluidity. Furthermore, we show that our scenario is consistent with the recent observations of the effective temperature of Cas A during the last 10 years, including nucleon superfluidity.

astro-ph.SR↗

arXiv2012

Amorphous state in the mixed phase of quark-hadron phase transition in protoneutron stars

We study the hadron-quark mixed phase in protoneutron stars, where neutrinos are trapped and lepton number becomes a conserved quantity besides the baryon number and electric charge. Considering protoneutron-star matter as a ternary system, the Gibbs conditions are applied together with the Coulomb interaction. We find that there no crystalline ("pasta") structure appears in the regime of high lepton-number fraction; the size of pasta becomes very large and the geometrical structure becomes mechanically unstable due to the charge screening effect. Consequently the whole system is separated into two bulk regions like an amorphous state, where the surface effect is safely neglected. There, the local charge neutrality is approximately attained, so that the equation of state is effectively reduced to the one for a binary system. Hence, we conclude that there is no possibility for the density discontinuity to appear in protoneutron-star matter, which is a specific feature in a pure system. These features are important when considering astrophysical phenomena such as supernova explosions or radiation of the gravitational wave from protoneutron stars.

astro-ph.HE↗

arXiv2012

Thermodynamical description of hadron-quark phase transition and its implications on compact-star phenomena

One of the most promising possibilities may be the appearance of quark matter in astrophysical phenomena in the light of recent progress in observations. The mechanism of deconfinement is not well understood, but the thermodynamical aspects of the hadron-quark (HQ) phase transition have been extensively studied in recent years. Then the mixed phase of hadron and quark matter becomes important; the proper treatment is needed to describe the HQ phase transition and derive the equation of state (EOS) for the HQ matter, based on the Gibbs conditions for phase equilibrium. We here adopt a EOS based on the baryon-baryon interactions including hyperons for the hadron phase, while we use rather simple EOS within the MIT bag model in the quark phase. For quark matter we further try to improve the previous EOS by considering other effective models of QCD. One of the interesting consequences may be the appearance of the inhomogeneous structures called "pasta", which are brought about by the surface and the Coulomb interaction effects. We present here a comprehensive review of our recent works about the HQ phase transition in various astrophysical situations: cold catalyzed matter, hot matter and neutrino-trapped matter. We show how the pasta structure becomes unstable by the charge screening of the Coulomb interaction, thermal effect or the neutrino trapping effect. Such inhomogeneous structure may affect astrophysical phenomena through its elasticity or thermal properties. Here we also discuss some implications on supernova explosion, gravitational wave and cooling of compact stars.

astro-ph.HE↗

arXiv2012

Molecular Dynamics for Dense Matter

We review a molecular dynamics method for nucleon many-body systems called the quantum molecular dynamics (QMD) and our studies using this method. These studies address the structure and the dynamics of nuclear matter relevant to the neutron star crusts, supernova cores, and heavy-ion collisions. A key advantage of QMD is that we can study dynamical processes of nucleon many-body systems without any assumptions on the nuclear structure. First we focus on the inhomogeneous structures of low-density nuclear matter consisting not only of spherical nuclei but also of nuclear "pasta", i.e., rod-like and slab-like nuclei. We show that the pasta phases can appear in the ground and equilibrium states of nuclear matter without assuming nuclear shape. Next we show our simulation of compression of nuclear matter which corresponds to the collapsing stage of supernovae. With increase of density, a crystalline solid of spherical nuclei change to a triangular lattice of rods by connecting neighboring nuclei. Finally, we discuss the fragment formation in expanding nuclear matter. Our results suggest that a generally accepted scenario based on the liquid-gas phase transition is not plausible at lower temperatures.

nucl-th↗

arXiv2012

Non-congruence of liquid-gas phase transition of asymmetric nuclear matter

We first explore the liquid-gas mixed phase in a bulk calculation, where two phases coexist without the geometrical structures. In the case of symmetric nuclear matter, the system behaves congruently, and the Maxwell construction becomes relevant. For asymmetric nuclear matter, on the other hand, the phase equilibrium is no more attained by the Maxwell construction since the liquid and gas phases are non-congruent; the particle fractions become completely different with each other. One of the origins of such non-congruence is attributed to the large symmetry energy. Subsequently we explore the charge-neutral nuclear matter with electrons by fully applying the Gibbs conditions to figure out the geometrical (pasta) structures in the liquid-gas mixed phase. We emphasize the effects of the surface tension and the Coulomb interaction on the pasta structures. We also discuss the thermal effects on the pasta structures.

nucl-th↗

arXiv2012

Nuclear pasta in supernovae and neutron stars

In supernova cores and neutron star crusts, nuclei with exotic shapes such as rod-like and slab-like nuclei are expected to exist. These nuclei are collectively called nuclear "pasta". For the past decades, existence of the pasta phases in the equilibrium state has been studied using various methods. Recently, the formation process of the pasta phases, which has been a long-standing problem, has been unveiled using molecular dynamics simulations. In this review, we first provide the astrophysical background of supernovae and neutron stars and overview the history of the study of the pasta phases. We then focus on the recent study on the formation process of the pasta phases. Finally, we discuss future important issues related to the pasta phases: their astrophysical evidence and consequences.

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arXiv2012

Three dimensional structure of low-density nuclear matter

We numerically explore the pasta structures and properties of low-density nuclear matter without any assumption on the geometry. We observe conventional pasta structures, while a mixture of the pasta structures appears as a metastable state at some transient densities. We also discuss the lattice structure of droplets.

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arXiv2011

Mixed phases during the phase transitions

Quest for a new form of matter inside compact stars compels us to examine the thermodynamical properties of the phase transitions. We closely consider the first-order phase transitions and the phase equilibrium on the basis of the Gibbs conditions, taking the liquid-gas phase transition in asymmetric nuclear matter as an example. Characteristic features of the mixed phase are figured out by solving the coupled equations for mean-fields and densities of constituent particles self-consistently within the Thomas-Fermi approximation. The mixed phase is inhomogeneous matter composed of two phases in equilibrium; it takes a crystalline structure with a unit of various geometrical shapes, inside of which one phase with a characteristic shape, called "pasta", is embedded in another phase by some volume fraction. This framework enables us to properly take into account the Coulomb interaction and the interface energy, and thereby sometimes we see the mechanical instability of the geometric structures of the mixed phase. Thermal effect on the liquid-gas phase transition is also elucidated. Similarly hadron-quark deconfinement transition is studied in hyperonic matter, where the neutrino-trapping effect as well as the thermal effect is discussed in relation to the properties of the mixed phase. Specific features of the mixed phase are elucidated and the equation of state is presented.

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arXiv2011

Three-dimensional calculation of inhomogeneous structure in low-density nuclear matter

In low-density nuclear matter which is relevant to the crust region of neutron stars and collapsing stage of supernovae, non-uniform structures called "nuclear pasta" are expected. So far, most works on nuclear pasta have used the Wigner-Seitz cell approximation with anzats about the geometrical structures like droplet, rod, slab and so on. We perform fully three-dimensional calculation of non-uniform nuclear matter for some cases with fixed proton ratios and in beta-equilibrium based on the relativistic mean-field model and the Thomas-Fermi approximation. In our calculation typical pasta structures are observed. However, there appears some difference in the density region of each pasta structure.

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arXiv2010

Signatures of hadron-quark mixed phase in gravitational waves

We calculate stellar oscillations including the hadron-quark mixed phase considering the finite size effects. We find that it is possible to distinguish whether the density discontinuity exists or not in the stars, even if one will observe the gravitational waves of the fundamental mode. Additionally, the normalized eigenfrequencies of pressure modes depend strongly on the stellar mass and on the adopted equation of state. Especially, in spite of the fact that the radius of the neutron star with $1.4M_\odot$, which is standard mass, is almost independent from the equation of state with quark matter, the frequencies of pressure modes depend on the adopted equation of state. Thus, via observing the many kinds of gravitational waves, it will be possible to make a restriction on the equation of state.

astro-ph.HE↗

arXiv2010

Pasta Structures of Quark-Hadron Phase Transition in Proto-Neutron Stars

We study the quark-hadron mixed phase in proto-neutron stars with the finite-size effects. In the calculations of pasta structures appeared in the mixed phase, the Gibbs conditions require the pressure balance and chemical equilibrium between two phases besides the thermal equilibrium. We find that the region of the mixed phase is limited due to thermal instability. Moreover, we study the effects of neutrinos to the pasta structures. As a result, we find that the existence of neutrinos make the pasta structures unstable, too. These characteristic features of the hadron-quark mixed phase should be important for the middle stage of the evolutions of proto-neutron stars.

astro-ph.HE↗

arXiv2010

Liquid-gas mixed phase in nuclear matter at finite temperature

We explore the geometrical structure of Liquid-gas (LG) mixed phase which is relevant to nuclear matter in the crust region of compact stars or supernovae. To get the equation of state (EOS) of the system, the Maxwell construction is found to be applicable to symmetric nuclear matter, where protons and neutrons behave simultaneously. For asymmetric nuclear matter, on the other hand, the phase equilibrium can be obtained by fully solving the Gibbs conditions since the components in the L and G phases are completely different. We also discuss the effects of surface and the Coulomb interaction on the mixed phase.

nucl-th↗

arXiv2010

Equation of State of Structured Matter at Finite Temperature

We investigate the properties of nuclear matter at the first-order phase transitions such as liquid-gas phase transition and hadron-quark phase transition. As a general feature of the first-order phase transitions of matter consisting of many species of charged particles, there appears a mixed phases with geometrical structures called ``pasta'' due to the balance of the Coulomb repulsion and the surface tension between two phases. The equation of state (EOS) of mixed phase is different from the one obtained by a bulk application of the Gibbs conditions or by the Maxwell construction due to the effects of the non-uniform structure. We show that the charge screening and strong surface tension make the EOS close to that of the Maxwell construction. The thermal effects are elucidated as well as the above finite-size effects.

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