The conversion from neutron stars to strange stars and its implications

The conversion from neutron stars with different equation of states (EOSs) for neutron matter into strange stars with different EOSs for strange quark matter has been studied in a general relativistic numerical calculation in this paper. For hot neutron stars, their conversion may lead to great variations in their rotation periods, of which the magnitude would be greatly dependent upon the EOS for neutron matter, and of which the timescale would be greatly determined by the EOS for strange matter. This phenomenon appears as giant glitches, which might provide a probe of EOSs for both neutron matter and strange matter. But for cold neutron stars, their conversion may result in a population of gamma-ray bursts.     

Thermal evolution of rotating hybrid stars

As a neutron star spins down, the nuclear matter is continuously converted into quark matter due to the core density increase, and then latent heat is released. We have investigated the thermal evolution of neutron stars undergoing such deconfinement phase transition. We have taken into account the conversion in the frame of the general theory of relativity. The released energy has been estimated as a function of changed rate of deconfinement baryon number. The numerical solutions to the cooling equation are seen to be very different from those without the heating effect. The results show that neutron stars may be heated to higher temperatures which is well matched with pulsar's data despite the onset of fast cooling in neutron stars with quark matter cores. It is also found that the heating effect has a magnetic field strength dependence. This feature could be particularly interesting for high temperatures of low-field millisecond pulsars at a later stage. The high temperature could fit the observed temperature for PSR J0437−4715.     

Signal of quark deconfinement in thermal evolution neutron stars with deconfinement heating

As neutron stars spin-down and contract, the deconfinement phase transition can continue to occur, resulting in energy release (so-called deconfinement heating) in case of the first-order phase transition. The thermal evolution of neutron stars is investigated to combine phase transition and the related energy release self-consistently. We find that the appearance of deconfinement heating during spin-down result in not only the cooling delay but also the increase of surface temperature of stars. For stars characterized by intermediate and weak magnetic field strength, a period of increasing surface temperature could exist. Especially, a sharp jump in surface temperature can be produced as soon as quark matter appears in the core of stars with a weak magnetic field. We think that this may serve as evidence for the existence of deconfinement quark matter. The results show that deconfinement heating facilitates the emergence of such characteristic signature during the thermal evolution process of neutron stars.     

Low-Mass Quark Stars

More and more observational hints of quark stars are proposed these years though pulsars are considered conventionally to be normal neutron stars. The existence of low-mass quark stars is a direct consequence of the possibility that pulsar-like stars are actually quark stars, because of the ability that quark matter can confine itself by color interaction. After a brief introduction to the study of quark stars, the various astrophysical implications of low-mass quark stars are investigated. It is addressed that some of the transient unidentified γ -ray sources are probably merging quark stars. The observability of low-mass quark stars is discussed.     

早期中子星和夸克物质

夸克禁闭的解除与夸克物质的存在一直是物理学家极感兴趣的问题。尽管理论上已指出在超高温或超高密的条件下可以有夸克物质存在,但是由于地面实验室的条件所限,目前还不能通过实验证实这一点.宇宙中被观测到的中子星(例如crab和Vela脉冲星)的中心密度大于4倍的核物质密度,其中心温度也可以达到10~8—10~9K,于是人们希     

Neutron Stars With a Quark Core

Models of neutron stars with a quark core are calculated on the basis of an extensive set of equations of state for superdense matter. The possible existence of a new branch of stable layered neutron stars is revealed for some realistic equations of state of neutron matter.     

The r-mode instability windows of hybrid stars

Taking into account the peculiar properties of hybrid stars, stars containing both a core of strange quark matter and the solid crust of a neutron star, and employing a fully self-consistent second-order correction technique, we study the time scale of bulk viscosity dissipation at the low temperature limit (T < 10⁹ K) and with this time scale we calculate the critical spin frequency of the hybrid star. It is found that its minimal value is 704.42 Hz (corresponding to a pulse period of 1.42 ms). When this is compared with the periods of neutron and strange stars, a better interpretation of the observational data is obtained.     

Phase transition to quarks in dense neutron matter: Effect of instanton interactions

Instanton effects are found to affect non-trivially the neutron matter to quark matter phase transition density. The relevance of the results for neutron stars is pointed out.     

Neutron Stars with a Quark Core. II. Basic Integral and Structural Parameters

A broad sample of computed realistic equations of state of superdense matter with a quark phase transition is used to construct a series of models of neutron stars with a strange quark core. The integral characteristics of the stellar configurations are obtained: gravitational mass, rest mass, radius, relativistic moment of inertia, and red shift from the star's surface, as well as the mass and radius of the quark core within the allowable range of values for the central pressure. The parameters of some of the characteristic configurations of the calculated series are also given and these are studied in detail. It is found that a new additional region of stability for neutron stars with strange quark cores may exist for some models of the equation of state.     

The formation of submillisecond pulsars and the possibility of detection

Pulsars have been recognized to be normal neutron stars, but sometimes have been argued to be quark stars. Submillisecond pulsars, if detected, would play an essential and important role in distinguishing quark stars from neutron stars. We focus on the formation of such submillisecond pulsars in this paper. A new approach to the formation of a submillisecond pulsar (quark star) by means of the accretion-induced collapse (AIC) of a white dwarf is investigated. Under this AIC process, we found that: (i) almost all newborn quark stars could have an initial spin period of ∼0.1 ms; (ii) nascent quark stars (even with a low mass) have a sufficiently high spin-down luminosity and satisfy the conditions for pair production and sparking process and appear as submillisecond radio pulsars; (iii) in most cases, the times of newborn quark stars in the phase with spin period <1 (or <0.5) ms are long enough for the stars to be detected.
As a comparison, an accretion spin-up process (for both neutron and quark stars) is also investigated. It is found that quark stars formed through the AIC process can have shorter periods (≤0.5 ms), whereas the periods of neutron stars formed in accretion spin-up processes must be longer than 0.5 ms. Thus, if a pulsar with a period shorter than 0.5 ms is identified in the future, it could be a quark star.     

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.     

Cooling of quark stars in the colour superconductive phase: effect of photons from glueball decay

The cooling history of a quark star in the colour superconductive phase is investigated. Here we specifically focus on the two-flavour colour (2SC) phase where the novel process of photon generation via glueball (GLB) decay has already been investigated. The picture we present here can, in principle, be generalized to quark stars entering a superconductive phase where similar photon generation mechanisms are at play. As much as 10⁴⁵–10⁴⁷ erg of energy is provided by the GLB decay in the 2SC phase. The generated photons slowly diffuse out of the quark star, keeping it hot and radiating as a blackbody (with possibly a Wien spectrum in gamma-rays) for millions of years. We discuss hot radio-quiet isolated neutron stars in our picture (such as RX J185635–3754 and RX J0720.4–3125) and argue that their nearly blackbody spectra (with a few broad features) and their remarkably tiny hydrogen atmosphere are indications that these might be quark stars in the colour superconductive phase where some sort of photon generation mechanism (reminiscent of the GLB decay) has taken place. Fits to observed data of cooling compact stars favour models with superconductive gaps of  Δ_2SC∼ 15–35 MeV  and densities  ρ_2SC= (2.5–3.0) ×ρ_N  (ρ_N being the nuclear matter saturation density) for quark matter in the 2SC phase. If correct, our model combined with more observations of isolated compact stars could provide vital information to studies of quark matter and its exotic phases.     

Vibrational damping by quark matter inside neutron stars

Free quark state may exist in the central region of massive neutron stars. We discuss the damping of the vibration of such neutron stars through the quark weak interaction. The damping time scale may be as short as tens of milliseconds. Damping with such short time constant may possibly be reflected in some γ-ray burst phenomena. An attempt is made to explain the γ burst of 1979 March 5 by this mechanism.     

On surface tension for compact stars

In an earlier analysis it was demonstrated that general relativity gives higher values of surface tension in strange stars with quark matter than neutron stars. We generate the modified Tolman-Oppenheimer-Volkoff equation to incorporate anisotropic matter and use this to show that pressure anisotropy provides for a wide range of behaviour in the surface tension than is the case with isotropic pressures. In particular, it is possible that anisotropy drastically decreases the value of the surface tension.     

The swallowing of a quark star by a black hole

In three-dimensional smooth particle hydrodynamic simulations of the coalescence of a quark star with a pseudo-Newtonian black hole all of the quark matter is quickly accreted by the black hole. The Madsen–Caldwell–Friedman argument against the existence of quark stars may need to be re-examined.     

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．     

Structure of pair winds from compact objects with application to emission from bare strange stars

We present the results of numerical simulations of stationary, spherically outflowing, e ^± pair winds, with total luminosities in the range 10³⁴–10⁴² ergs s^?1. In the concrete example described here, the wind injection source is a hot, bare, strange star, predicted to be a powerful source of e ^± pairs created by the Coulomb barrier at the quark surface. We find that photons dominate in the emerging emission, and the emerging photon spectrum is rather hard and differs substantially from the thermal spectrum expected from a neutron star with the same luminosity. This might help distinguish the putative bare strange stars from 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...     

用中子星限制暗物质粒子散射截面

中子星可以通过重子物质和暗物质的相互作用吸积暗物质,且在一定条件下, 中子星吸积的暗物质粒子可以引发自引力塌缩形成小型黑洞, 生成的黑洞可能会进一步吞噬中子星.依据文献已有模型, 基于以上物理过程给出了在暗物质粒子不同质量下对暗物质粒子--中子的散射截面的限制.使用弱相互作用大质量粒子(Weakly Interacting Massive Particle, WIMP)模型, 并考虑暗物质粒子是玻色子的情形, 讨论了暗物质粒子有无自相互作用以及有无湮灭等条件下对限制暗物质参数的影响.既考虑了已发现的两个中子星系统来给出对暗物质参数空间的限制,也考虑了两个可能存在的年老中子星来预测未来观测可能对暗物质参数空间的限制.对于考虑玻色--爱因斯坦凝聚(Bose-Einstein Condensate, BEC)的玻色子暗物质, 在无自相互作用或有弱自相互作用, 无湮灭或有很小湮灭截面的条件下,中子星给出的间接观测对暗物质粒子-中子散射截面的限制的强度比XENON1T直接探测实验来得更强.未来, 如果在银心附近能观测到年老中子星, 其观测结果可以提升模型给出的对暗物质粒子--中子散射截面的限制, 从而帮助人们进一步理解暗物质.