密度依赖口袋常数下奇异物质的热力学自洽处理及其对混合星性质的影响

在MTT口袋模型的基础上采用密度依赖口袋常数,给出了奇异夸克物质的热力学关系,并用于描述奇异夸克物质及混合星内的夸克相,研究了奇异星、混合星的性质.结果表明,密度依赖口袋常数下,奇异夸克物质的压强公式中有一个附加项,而能量密度中则没有,从而保证了系统的热力学自洽性.在新的热力学关系下,奇异夸克物质的状态方程变软,相应的奇异星的引力质量和对应的半径均变小;混合星的状态方程也变软,其质量变小,而对应的半径也变小.说明经热力学自洽处理后该模型对中子星的状态方程及相应的质量-半径关系等都有显著的影响.     

Strange star surface: a crust with nuggets

We reexamine the surface composition of strange stars. Strange quark stars are hypothetical compact stars which could exist if strange quark matter was absolutely stable. It is widely accepted that they are characterized by an enormous density gradient (10(26) g/cm4) and large electric fields at the surface. By investigating the possibility of realizing a heterogeneous crust, comprised of nuggets of strange quark matter embedded in an uniform electron background, we find that the strange star surface has a much reduced density gradient and negligible electric field. We comment on how our findings will impact various proposed observable signatures for strange stars.     

Dark matter, neutron stars, and strange quark matter

We show that self-annihilating weakly interacting massive particle (WIMP) dark matter accreted onto neutron stars may provide a mechanism to seed compact objects with long-lived lumps of strange quark matter, or strangelets, for WIMP masses above a few GeV. This effect may trigger a conversion of most of the star into a strange star. We use an energy estimate for the long-lived strangelet based on the Fermi-gas model combined with the MIT bag model to set a new limit on the possible values of the WIMP mass that can be especially relevant for subdominant species of massive neutralinos.     

Radial oscillations of magnetized proto strange stars in temperature- and density-dependent quark mass model

We report on the study of the mass-radius (M–R) relation and the radial oscillations of magnetized proto strange stars. For the quark matter we have employed the very recent modification, the temperature- and density-dependent quark mass model of the well-known density-dependent quark mass model. We find that the effect of magnetic field, both on the maximum mass and radial frequencies, is rather small. Also a proto strange star, whether magnetized or otherwise, is more likely to evolve into a strange star rather than transform into a black hole.     

Neutral color-spin-locking phase in neutron stars

We present results for the spin-1 color-spin-locking (CSL) phase using a NJL-type model in two-flavor quark matter for compact stars applications. The CSL condensate is flavor symmetric and therefore charge and color neutrality can easily be satisfied. We find small energy gaps ≃ 1MeV, which make the CSL matter composition and the EoS not very different from the normal quark matter phase. We keep finite quark masses in our calculations and obtain no gapless modes that could have strong consequences in the late cooling of neutron stars. Finally, we show that the region of the phase diagram relevant for neutron star cores, when asymmetric flavor pairing is suppressed, could be covered by the CSL phase.     

Influences of the bag constant on properties of hybrid stars

Influences of the bag constant on the properties of hybrid stars are investigated by using relativistic mean field theory and the MIT bag model to describe the hadron phase and quark phase in the interior of neutron stars, respectively. Our results indicate that the onset of hadron-quark phase transition is put off and the appearance of hyperon species is increased with the increase in bag constant. As a result, the hybrid star equation of state for a mixed phase range stiffens whereas that of the quark phase range softens, and the gravitational mass as well as the corresponding radius of hybrid stars are increased obviously. The gravitational mass of a hybrid star is increased from 1.42 Mo (M<,⊙> is solar mass) to 1.63M<,⊙> and the corresponding radius is changed from 9.1 km to 12.2 km when the bag constant (B<'1/4>) is increased from 170 MeV to 200 MeV. It is interesting to find that hybrid star equations of state become non-smooth when the TM2 parameter sets in the framework of relativistic mean field theory used to describe the hadronic matter, and consequently, the third family of compact stars appear in the mass-radius relations of hybrid stars in the narrow scope of the bag constant from 175 MeV to 180 MeV. These show that the choice of the bag constant in the MIT bag model has significant influence on the properties of hybrid stars.     

Dense stars with exotic configurations

In this paper, we consider dense stars with configurations expected from the SU(3)_C×SU(2)_W× U(1) standard model of strong and electroweak interactions. Following a recent suggestion that strange matter, a form of (uds) quark matter, may be the true ground state of hadronic matter, we investigate the prospect for the existence of dense stars consisting partially, or entirely, of strange matter by comparing the relative stability between neutron matter and strange matter. It is found that the restriction on the maximum star mass holds in all cases, including a pure strange star, a pure neutron star, and a neutron star with a quark core. It is also found that the choice of both the bag constantB and the strong coupling constant _s has a decisive effect on the relative stability between strange matter and neutron matter. For currently accepted values of (B, _s), anA= dense starcannot consist entirely,nor partially, of strange matter. Nevertheless, such conclusion may be subject to change if corrections ofO ( _s ² ) or other effects are taken into account. Finally, we use the framework of Tolman, Oppenheimer, and Volkoff to analyze two cases of boson stars: gluon stars and stars consisting of massive scalar particles (massive bosons). It is found that, in the case of gluon stars, the presence of the bag constant in the QCD vacuum yields results very similar to that found in quark stars. On the other hand, soliton stars consisting of massive bosons exist if there is some background pressure which plays the role similar to the bag constant for lowering the matter pressure. The stability problem for both gluon stars and soliton stars is briefly discussed.     

Strange quark star and the parameter space of the quasi-particle model

The properties of strange quark stars are studied within the quasi-particle model. Taking into account chemical equilibrium and charge neutrality, the equation of state(EOS) of(2+ 1)-flavor quark matter is obtained. We illustrate the parameter spaces with constraints from two aspects: one is based on the astronomical results of PSR J0740+ 6620 and GW 170 817,and the other is based on the constraints proposed from the theoretical study of a compact star that the EOS must ensure the tidal deformability Λ_(1.4)=190_(-120)~(+390) and support a maximum mass above 1.97M_⊙. It is found that neither type of constraints can restrict the parameter space of the quasi-particle model in a reliable region and thus we conclude that the low mass compact star cannot be a strange quark star.     

模型参数对奇异星性质的影响

用有效质量口袋模型描述奇异夸克物质,研究了耦合常数和口袋常数的选取对奇异夸克物质的状态方程及奇异星性质的影响.结果表明,随着耦合常数和口袋常数的增大,奇异夸克物质的状态方程变软,相应的奇异星的引力质量和对应的半径均变小.当耦合常数从0.5增大到2.0时,奇异星的质量从1.43M_⊙(M_⊙=1.99×10³⁰ kg)减小到1.25M_⊙,相应的半径由8.3 km减小到7.7 km;当口袋常数B^1/4由160 MeV增大到175 MeV时,奇异星的质量和半径分别由1.47M_⊙和8.6 km减小到1.22M_⊙和7.4 km.这说明奇异夸克物质及奇异星的性质明显依赖于模型参数的取值. 关键词： 模型参数 奇异星 状态方程 质量-半径关系     

Slowly rotating neutron stars and hadronic stars in the chiral SU(3) quark mean-field model

The equations of state for neutron matter, strange and non-strange hadronic matter in the chiral SU(3) quark mean-field model are applied in the study of slowly rotating neutron stars and hadronic stars. The radius, mass, moment of inertia, and other physical quantities are carefully examined. The effect of the nucleon crust for the strange hadronic star is exhibited. Our results show that the rotation can increase the maximum mass of compact stars significantly. For a big enough mass of pulsars which cannot be explained as strange hadronic stars, theoretical approaches to increase the maximum mass are addressed.     

Maximum elastic deformations of compact stars with exotic equations of state

I make the first estimates of maximum elastic quadrupole deformations sustainable by alternatives to conventional neutron stars. Solid strange quark stars might sustain maximum ellipticities (dimensionless quadrupoles) up to a few times rather than a few times for conventional neutron stars, and hybrid quark-baryon or meson-condensate stars might sustain up to . Most of the difference is due to the shear modulus, which can be up to rather than in the inner crust of a conventional neutron star. Maximum solid strange star ellipticities are comparable to upper limits obtained for several known pulsars in a recent gravitational-wave search by LIGO. Maximum ellipticities of the more robust hybrid model will be detectable by LIGO at initial design sensitivity. A large shear modulus also strengthens the case for starquakes as an explanation for frequent pulsar glitches.     

Phase transition to color-flavor-locked matter and condensation of Goldstone bosons in neutron star matter

Deconfinement phase transition and condensation of Goldstone bosons in neutron star matter are investigated in a chiral hadronic model (also referred as to the FST model) for the hadronic phase (HP) and in the color-flavor-locked (CFL) quark model for the deconfined quark phase. It is shown that the hadronic-CFL mixed phase (MP) exists in the center of neutron stars with a small bag constant, while the CFL quark matter cannot appear in neutron stars when a large bag constant is taken. Color superconductivity softens the equation of state (EOS) and decreases the maximum mass of neutron stars compared with the unpaired quark matter. The K⁰ condensation in the CFL phase has no remarkable contribution to the EOS and properties of neutron star matter. The EOS and the properties of neutron star matter are sensitive to the bag constant B, the strange quark mass m_s and the color superconducting gap Δ. Increasing B and m_s or decreasing Δ can stiffen the EOS which results in the larger maximum masses of neutron stars.     

36Cl and41Ca depth profiles in a Hiroshima granite stone and the Dosimetry System 1986

Hybrid stars composed of a strange matter core surrounded by neutron matter are investigated. We apply star models based on phenomenological equations of state (EOS) from nuclear reactions including a phase transition between the hadronic phase and the quark gluon plasma. For specific equations of state hybrid stars might exist. While the nuclear part of the EOS has only a minor influence on the properties of hybrid stars, the EOS for the quark gluon phase has a crucial impact on the existence of such objects.     

Equation of state in hybrid stars and the stability window of quark matter

Properties of hybrid stars with a mixed phase composed of asymmetric nuclear matter and strange quark matter are studied. The quark phase is investigated by the quark quasiparticle model with a self-consistent thermodynamic and statistical treatment. We present the stability windows of the strange quark matter with respect to the interaction coupling constant versus the bag constant. We find that the appearance of the quark–hadron mixed phases is associated with the meta-stable or unstable regions of the pure quark matter parameters. The mass–radius relation of the hybrid star is dominated by the equation of state of quark matter rather than nuclear matter. The contour plots of the maximum mass of the hybrid star are shown in the plane of the coupling constant and the bag constant.     

Quark core stars,quark stars and strange stars

A recent one flavor (zero temperature) quark matter equation of state is generalized to several flavors. It is shown that quarks undergo a first order phase transition. In addition, this equation of state depends on few parameters, one in the two flavor case, two in the three flavor case, and these parameters can be constrained by phenomenology. This equation of state is then applied to 1) the hadronquark transition in neutron stars and the determination of quark star stability, 2) the investigation of strange matter stability and possible strange star existence.     

QCD vacuum and hypothesis of the appearance of quark matter in the interior of stars

The problem of whether “cold” (T ≤ 10 MeV) subhadronic matter may appear in the central domain of massive neutron stars is discussed. It is shown that nontrivial properties of the QCD vacuum—namely, the existence of a quark-gluon condensate in the hadronic phase and its destruction in the transition to the quark phase—oppose so efficiently the stability of the two-phase structure of a cold star (quark center and hadronic periphery) that the probability of this configuration is very low. The loss of stability at the stage of the phase transition is expected to result in the heating of the central domain up to very high temperatures (T ≥ 100 MeV). Some seemingly enigmatic astrophysical observations and global processes that control the evolution of compact heavy stars may originate from this mechanism.     

Neutron stars, hybrid stars and the equation of state

Gross properties of hybrid stars consisting of a core of strange matter surrounded by ordinary neutron matter are investigated. We discuss star models based on phenomenological equations of state from nuclear reactions including a phase transition between the hadronic phase and the quark-gluon plasma. For certain parameters, such equations of state support the existence of hybrid stars. The identification of such objects could provide detailed information on the properties of strange quark matter.     

Phase Transitions in Neutron Stars Within Modified Quark-Meson Coupling Model

K^- condensation and quark deconfinement phase transitions in neutron stars are investigated. We use the modified quark-meson coupling model for hadronic phase and the MIT bag model for quark phase. With the equation of state (EOS) solved self-consistently, we discuss the properties of neutron stars. We find that the EOS of pure hadron matter with condensed K^- phase should be ruled out by the redshift for star EXO0748-676, while EOS containing unpaired quark matter phase with B^1/4 being about 180 MeV could be consistent with both this observation and the best measured mass of star PSR 1913+16. But if the recent inferred massive star among Terzan 5 with M>1.68M_๏ is confirmed, all the present EOSes with condensed phase and deconfined phase would be ruled out.     

Properties of hybrid stars in an extended MIT bag model

The properties of hybrid stars are investigated in the framework of the relativistic mean field theory (RMFT) and an MIT bag model with density-dependent bag constant to describe the hadron phase (HP) and quark phase (QP), respectively. We find that the density-dependent B(p) decreases with baryon density p; this decrement makes the strange quark matter become more energetically favorable than ever, which makes the threshold densities of the hadron-quark phase transition lower than those of the original bag constant case. In this case, the hyperon degrees of freedom can not be considered. As a result, the equations of state of a star in the mixed phase (MP) become softer whereas those in the QP become stiffer, and the radii of the star obviously decrease. This indicates that the extended MIT bag model is more suitable to describe hybrid stars with small radii.     

Prospects of detecting baryon and quark superfluidity from cooling neutron stars

Baryon and quark superfluidity in the cooling of neutron stars are investigated. Future observations will allow us to constrain combinations of the neutron or Lambda-hyperon pairing gaps and the star's mass. However, in a hybrid star with a mixed phase of hadrons and quarks, quark gaps larger than a few tenths of an MeV render quark matter virtually invisible for cooling. If the quark gap is smaller, quark superfluidity could be important, but its effects will be nearly impossible to distinguish from those of other baryonic constituents.