Observational Constraints on Quark Matter in Neutron Stars

We study the observational constraints of mass and redshift on the properties of the equation of state （EOS） for quark matter in compact stars based on the quasi-particle description. We discuss two scenarios; strange stars and hybrid stars. We construct the equations of state utilizing an extended MIT bag model taking the medium effect into account for quark matter and the relativistic mean field theory for hadron matter. We show that quark matter may exist in strange stars and in the interior of neutron stars. The bag constant is a key parameter that affects strongly the mass of strange stars. The medium effect can lead to the stiffer hybrid-star EOS approaching the pure hadronic EOS, due to the reduction of quark matter, and hence the existence of heavy hybrid stars. We find that a middle range coupling constant may be the best choice for the hybrid stars being compatible with the observational constraints.

Strange Stars: Can Their Crust Reach the Neutron Drip Density?

The electrostatic potential of electrons near the surface of static strange stars at zero temperature is studied within the frame of the MIT bag model. We find that for QCD parameters within rather wide ranges, if the nuclear crust on the strange star is at a density leading to neutron drip, then the electrostatic potential will be insufficient to establish an outwardly directed electric field, which is crucial for the survival of such a crust. If a minimum gap width of 200 fm is brought in as a more stringent constraint, then our calculations will completely rule out the possibility of such crusts. Therefore, our results argue against the existence of neutron-drip crusts in nature.

Singlet pairing gaps of neutrons and protons in hyperonic neutron stars

The ^1 S0 nucleonic superfluids are investigated within the relativistic meanfield model and Bardeen-Cooper-Schrieffer theory in hyperonic neutron stars. The ^1 S0 pairing gaps of neutrons and protons are calculated based on the Reid soft-core interaction as the nucleon-nucleon interaction. In particular, we have studied the influence of degrees of freedom for hyperons on the ^1 S0 nucleonic pairing gap in neutron star matter. It is found that the appearance of hyperons has little impact on the baryonic density range and the size of the ^1S0 neutronic pairing gap; the ^1S0 protonic pairing gap also decreases slightly in this region where ρB = 0.0-0.393 fm^-3. However, if baryonic density becomes greater than 0.393 fm^-3, the ^1S0 protonic pairing gap obviously increases. In addition, the possible range for a protonic superfluid is obviously enlarged due to the presence of hyperons. In our results, the hyperons change the 1 So protonic pairing gap, which must change the cooling properties of neutron stars.

R-mode instability of strange stars and observations of neutron stars in LMXBs

Using a realistic equation of state （EOS） of strange quark matter, namely, the modified bag model, and considering the constraints on the parameters of EOS by the observational mass limit of neutron stars, we investigate the r-mode instability window of strange stars, and find the same result as in the brief study of Haskell, Degenaar and Ho in 2012 that these instability windows are not consistent with the spin frequency and temperature observations of neutron stars in low mass X-ray binaries.

Influence of a scalar-isovector δ-meson field on the quark phase structure in neutron stars

The deconfinement phase transition from hadronic matter to quark matter in the interior of compact stars is investigated. The hadronic phase is described in the framework of relativistic mean-field theory, where the scalar-isovector 6-meson effec- tive field is also taken into account. The MIT bag model for describing a quark phase is used. The changes of the parameters of phase transition caused by the presence of a δ-meson field are explored. Finally, alterations in the integral and structural parameters of hybrid stars due to both a deconfinement phase transition and inclusion of a δ-meson field are discussed.

Calculation of the structural properties of a strange quark star in the presence of a strong magnetic field using a density dependent bag constant

We have calculated the structural properties of a strange quark star with a static model in the presence of a strong magnetic field. To this end, we use the MIT bag model with a density dependent bag constant. To parameterize the density dependence of the bag constant, we have used our results for the lowest order constrained variational calculation of the asymmetric nuclear matter. By calculating the equation of state of strange quark matter, we have shown that the pressure of this system increases by increasing both density and magnetic field. Finally, we have investigated the effect of density dependence of the bag constant on the structural properties of a strange quark star.

Effects of non-Newtonian gravity on the properties of strange stars

The properties of strange star matter are studied in the equivparticle model with inclusion of non-Newtonian gravity. It is found that the inclusion of non-Newtonian gravity makes the equation of state stiffer if Witten＇s conjecture is true. Correspondingly, the maximum mass of strange stars becomes as large as two times the solar mass, and the maximum radius also becomes bigger. The coupling to boson mass ratio has been constrained within the stability range of strange quark matter.

Microscopic Magnetic Dipole Radiation in Neutron Stars

There is a ^3P2 neutron superfluid region in NS （neutron star） interior. For a rotating NS the ^3P2 superfluid region is like a system of rotating magnetic dipoles. It will give out electromagnetic radiation, which may provide a new heating mechanism of NSs. This mechanism plus some cooling agent may give a sound explanation to NS glitches.

Spectroscopic study of Be-shell stars:4 Her and 88 Her

We present an optical spectroscopic study based on 41 spectra of 4 Her and 32 spectra of 88 Her, obtained over a period of 6 months. We estimate the rotational velocity of these stars from HeI lines in the blue spectral region （4000-4500 A）. We find that these stars are likely to be rotating at a fractional critical rotation of -0.80. We measure the average Ip/lc ratio to quantify the strength of the Ha line and obtain 1.63 for 4 Her and 2.06 for 88 Her. The radius of the Ha emission region is estimated to be Ra/R. -5.0, assuming a Keplerian disk. These stars are thus found to be fast rotators with a relatively small Hoe emission region. We detect V/R variation of the Ha spectral line during the observed period. We re-estimate the periods for both stars and obtain a period of ,-46 d and its harmonic of 23.095 d for 4 Her, and a period of -86 d for 88 Her. As these two cases are shell stars with binaries and have low Ha EW with the emission region closer to the central star, the V/R variation and a change in period may be an effect of the binary on the circumstellar disk.

Cooling of hybrid stars with spin down compression

We study the cooling of hybrid stars coupled with spin-down. Due to the spindown of hybrid stars, the interior density continuously increases and different neutrino reactions may be triggered （from the modified Urca process to the quark and nucleon direct Urca process） at different stages of evolution. We calculate the rate of neutrino emissivity of different reactions and simulate the cooling curves of the rotational hybrid stars. The results show that the cooling curves of hybrid stars clearly depend on a magnetic field if the direct Urca reactions occur during the spin-down. Comparing the results of the rotational star model with the transitional static model, we find the cooling behavior of the rotational model is more complicated; the temperature of the star is higher, especially when direct Urca reactions appear in the process of rotation. Then, we find that the predicted temperatures of some rotating hybrid stars are compatible with the pulsar＇s data which are in contradiction with the results of the transitional method.

Optical/IR studies of Be stars in NGC 6834 with emphasis on two specific stars

We present optical and infrared photometric and spectroscopic studies of two Be stars in the 70-80-Myr-old open cluster NGC 6834. NGC 6834（1） has been reported as a binary from speckle interferometric studies whereas NGC 6834（2） may possibly be a γ Cas-like variable. Infrared photometry and spectroscopy from the United Kingdom Infrared Telescope （UKIRT）, and optical data from various facilities are combined with archival data to understand the nature of these candidates. High signal-to-noise near-IR spectra obtained from UKIRT have enabled us to study the optical depth effects in the hydrogen emission lines of these stars. We have explored the spectral classification scheme based on the intensity of emission lines in the H and K bands and contrasted it with the conventional classification based on the intensity of hydrogen and helium absorption lines. This work also presents hitherto unavailable UBV CCD photometry of NGC 6834, from which the evolutionary state of the Be stars is identified.

First-order phase transitions in rotating hybrid stars and pulsar glitches

The first order deconfinement phase transitions in rotating hybrid stars are studied and it is found that if the surface tension is sufficiently large, the transition from metastable hadron matter to stable mixed hadron-quark matter during the spindown history of a hybrid star can cause a glitch.

Phase structures of neutral dense quark matter and applicationto strange stars

In the contact interaction model,the quark propagator has only one solution,namely,the chiral symmetry breaking solution,at vanishing temperature and density in the case of physical quark mass.We generalize the condensate feedback onto the coupling strength from the 2 flavor case to the 2+1 flavor case,and find the Wigner solution appears in some regions,which enables us to tackle chiral phase transition as two-phase coexistences.At finite chemical potential,we analyze the chiral phase transition in the conditions of electric charge neutrality andβequilibrium.The four chemical potentials,μ_(u),μ_(d),μ_(s) and He,are constrained by three conditions,so that one inde-pendent variable remains:we choose the average quark chemical potential as the free variable.All quark masses and number densities suffer discontinuities at the phase transition point.The strange quarks appear after the phase trans-ition since the system needg more energy to produce a d.-quark than an s-quark.Taking the EOS as an input,the TOV equations are solved numerically,and we show that the mass--radius relation is sensitive to the EOS.The max-imum mass of strange quark stars is not susceptible to the parameter Aq we introduced.

Strange star candidates revised within a quark model with chiral mass scaling

We calculate the properties of static strange stars using a quark model with chiral mass scaling. The results are characterized by a large maximum mass （-1.6 M⊙） and radius （-10km）. Together with a broad collection of modern neutron star models, we discuss some recent astrophysical observational data that could shed new light on the possible presence of strange quark matter in compact stars. We conclude that none of the present astrophysical observations can prove or confute the existence of strange stars.

The effects of strong magnetic fields on neutron star structure:lowest order constrained variational calculations

We investigate the effects of strong magnetic fields upon the large-scale properties of neutron and protoneutron stars. In our calculations, the neutron star mat- ter was approximated by pure neutron matter. Using the lowest order constrained vari- ational approach at zero and finite temperatures, and employing AV18 potential, we present the effects of strong magnetic fields on the gravitational mass, radius, and gravitational redshift of neutron and protoneutron stars. It is found that the equation of state for a neutron star becomes stiffer with an increase of magnetic field and tem- perature. This leads to larger values of the maximum mass and radius for the neutron stars.

Maximum mass of a hot neutron star with a quark core

We have considered a hot neutron star with a quark core, a mixed phase of quark-hadron matter, and a hadronic matter crust and have determined the equation of state of the hadronic phase and the quark phase. We have then found the equation of state of the mixed phase under the Gibbs conditions. Finally, we have computed the structure of a hot neutron star with a quark core and compared our results with those of the neutron star without a quark core. For the quark matter calculations, we have used the MIT bag model in which the total energy of the system is considered as the kinetic energy of the particles plus a bag constant. For the hadronic matter calculations, we have used the lowest order constrained variational formalism. Our calculations show that the results for the maximum gravitational mass of a hot neutron star with a quark core are substantially different from those of a neutron star without the quark core.

FRB 171019:an event of binary neutron star merger?

The fast radio burst,FRB 171019,was relatively bright when discovered first by ASKAP but was identified as a repeater with three faint bursts detected later by GBT and CHIME.These observations lead to the discussion of whether the first bright burst shares the same mechanism with the following repeating bursts.A model of binary neutron star merger is proposed for FRB 171019,in which the first bright burst occurred during the merger event,while the subsequent repeating bursts are starquake-induced,and generally fainter,as the energy release rate for the starquakes can hardly exceed that of the catastrophic merger event.This scenario is consistent with the observation that no later burst detected is as bright as the first one.

The effect of dynamical quark mass on the calculation of a strange quark star's structure

We discuss the dynamical behavior of strange quark matter components, in particular the effects of density dependent quark mass on the equation of state of strange quark matter. The dynamical masses of quarks are computed within the Nambu-Jona-Lasinio model, then we perform strange quark matter calculations em- ploying the MIT bag model with these dynamical masses. For the sake of compar- ing dynamical mass interaction with QCD quark-quark interaction, we consider the one-gluon-exchange term as the effective interaction between quarks for the MIT bag model. Our dynamical approach illustrates an improvement in the obtained equation of state values. We also investigate the structure of the strange quark star using Tolman- Oppenheimer-Volkoff equations for all applied models. Our results show that dynamical mass interaction leads to lower values for gravitational mass.

Calculation of the structural properties of asymmetrical nuclear matter

The structural properties of asymmetrical nuclear matter have been calculated,employing the AV 18 potential for different values of proton to neutron ratio.These calculations have also been made for the case of symmetrical nuclear matter with the UV14,AV14 and AV 18 potentials.In our calculations,we used the lowest order constrained variational method to compute the correlation function of the system.

超新星核中的非奇异－奇异夸克物质相变

最近Ｇｅｎｔｉｌｅ等研究了超新星核区从核物质到夸克物质的一级相变．沿着他们的工作，本文研究从两味夸克物质到三味夸克物质的相变过程．我们发现相变时标小于１０－７秒．超新星的中心温度和核区的中微子总能量明显增大，这不仅会增加超新星爆发的成功机会，而且会提高复活激波的能量，同时会影响新生中子星的冷却．核区存在Ｓｃｈｗａｒｚｓｃｈｉｌｄ对流．