Models Of Strange Stars Research Articles

Epicyclic frequencies for rotating strange quark stars: Importance of stellar oblateness

Kilohertz quasi periodic oscillations (QPOs) can be used as a probe of the inner regions of accretion disks in compact stars and hence also of the properties of the central object. Most models of kHz QPOs involve epicyclic frequencies to explain their origin. We compute the epicyclic frequencies of nearly circular orbits around rotating strange quark stars. The MIT bag model is used to model the equation of state of quark matter, and the uniformly rotating stellar configurations are computed in full general relativity. The vertical epicyclic frequency and the related nodal precession rate of inclined orbits are very sensitive to the oblateness of the rotating star. For rotating stellar models of moderate and high-mass strange stars, the sense of the nodal precession changes at a certain rotation rate. At lower stellar rotation rates, the orbital nodal precession is prograde, as it is in the Kerr metric, while at higher rotation rates, the precession is retrograde, as it is for Maclaurin spheroids. Thus, qualitatively, the orbits around rapidly rotating strange quark stars are affected more strongly by the effects of stellar oblateness than by the effects of general relativity. We show that epicyclic and orbital frequencies calculated numerically for small mass strange stars are in very good agreement with analytical formulas for Maclaurin spheroids.

Torsional oscillations of strange stars

Strange stars are one of the hypothetical compact stellar objects that can be formed after a supernova explosion. The existence of these objects relies on the absolute stability of strange collapsed quark matter with respect to standard nuclear matter. We discuss simple models of strange stars with a bare quark matter surface, thus standard nuclear matter is completely absent. In these models an electric dipole layer a few hundreds Fermi thick should exist close to the star surface. Studying the torsional oscillations of the electrically charged layer we estimate the emitted power, finding that it is of the order of 1045 erg/s, meaning that these objects would be among the brightest compact sources in the heavens. The associated relaxation times are very uncertain, with values ranging between microseconds and minutes, depending on the crust thickness. Although part of the radiated power should be absorbed by the electrosphere surrounding the strange star, a sizable fraction of photons should escape and be detectable.

R-mode instability in quark stars with a crystalline crust

The r-mode instability, believed to limit the rotation speed of compact stars, can provide empirical confirmation for the existence of stable deconfined phases of quark matter that are predicted by weak coupling calculations in Quantum Chromodynamics. We construct a model for strange quark stars as heavy as 2 solar masses that are made of superconducting quark matter in the bulk and a thin crystalline quark matter crust. This crystalline quark crust is sufficiently robust to withstand r-mode heating and viscous rubbing for realistic mode amplitudes O(10^{-2}), unlike a crust made of neutron-rich nuclei. The dissipation provided by viscous rubbing at the core-crust boundary is both necessary and sufficient to obtain stable rotation speeds that are consistent with the majority of rapidly spinning pulsars in low mass X-ray binaries. Our analysis implies that while bare strange stars are ruled out by the existence of rapidly spinning pulsars, a strange star with a quark matter crust is a distinct possibility.

Quadrupole moments of rotating neutron stars and strange stars

We present results for models of neutron stars and strange stars constructed using the Hartle-Thorne slow-rotation method with a wide range of equations of state, focusing on the values obtained for the angular momentum $J$ and the quadrupole moment $Q$, when the gravitational mass $M$ and the rotational frequency $\Omega$ are specified. Building on previous work, which showed surprising uniformity in the behaviour of the moment of inertia for neutron-star models constructed with widely-different equations of state, we find similar uniformity for the quadrupole moment. These two quantities, together with the mass, are fundamental for determining the vacuum space-time outside neutron stars. We study particularly the dimensionless combination of parameters $QM/J^2$ (using units for which $c=G=1$). This quantity goes to 1 in the case of a Kerr-metric black hole and deviations away from 1 then characterize the difference between neutron-star and black-hole space-times. It is found that $QM/J^2$ for both neutron stars and strange stars decreases with increasing mass, for a given equation of state, reaching a value of around 2 (or even less) for maximum-mass models, meaning that their external space-time is then rather well approximated by the Kerr metric. If $QM/J^2$ is plotter against compactness $R/2M$ (where $R$ is the radius), it is found that the relationship is nearly unique for neutron-star models, independent of the equation of state, while it is significantly different for strange stars. This gives a new way of possibly distinguishing between them.

Strange Matter and Neutron Stars

Strange matter may be the true ground state of hadronic matter. Phenomenological model of strange matter is presented and possibilities of formation of strange matter in neutron stars are considered. Possible scenario of conversion of a neutron star into a strange star is discussed. Models of strange stars with a bare quark surface and with a solid crust are presented. Properties of strange stars are reviewed.

A Relativistic Model for Strange Quark Star

We propose a spherically symmetric and anisotropic model for strange quark stars within the framework of MIT Bag model. Though the model is found to comply with all the physical requirements of a realistic star satisfying a strange matter equation of state (EOS), the estimated values the Bag constant for different strange star candidates like Her X-1, SAX J 1808.4-3658 and 4U 1820-30, clearly indicate that the Bag constant need not necessarily lie within the range of 60-80 MeV fm$^{-3}$ as claimed in the literature.

Strange stars in Krori–Barua space-time

The singularity space-time metric obtained by Krori and Barua\cite{Krori1975} satisfies the physical requirements of a realistic star. Consequently, we explore the possibility of applying the Krori and Barua model to describe ultra-compact objects like strange stars. For it to become a viable model for strange stars, bounds on the model parameters have been obtained. Consequences of a mathematical description to model strange stars have been analyzed.

Self-bound models of compact stars and recent mass-radius measurements

The exact composition of a specific class of compact stars, historically referred to as "neutron stars", is still quite unknown. Possibilities ranging from hadronic to quark degrees of freedom, including self-bound versions of the latter have been proposed. We specifically address the suitability of strange star models (including pairing interactions) in this work, in the light of new measurements available for four compact stars. The analysis shows that these data might be explained by such an exotic equation of state, actually selecting a small window in parameter space, but still new precise measurements and also further theoretical developments are needed to settle the subject.

Magnetospheric activity of bare strange quark stars

In Ruderman & Sutherland (RS75) model, the normal neutron stars as pulsars bear a severe problem, namely the binding energy problem that both ions (e.g., ${}_{26}^{56}$Fe) and electrons on normal neutron star surface can be pulled out freely by the unipolar generator induced electric field so that sparking on polar cap can hardly occur. {\bf This problem could be solved within the Partially Screened Gap (PSG) model in the regime of neutron stars}. However, in this paper we extensively study this problem in a bare strange quark star (BSS) model. We find that the huge potential barrier built by the electric field in the vacuum gap above polar cap could usually prevent electrons from streaming into the magnetosphere unless the electric potential of a pulsar is sufficiently lower than that at infinite interstellar medium. Other processes, such as the diffusion and thermionic emission of electrons have also been included here. Our conclusions are as follows: both positive and negative particles on a BSS's surface would be bound strongly enough to form a vacuum gap above its polar cap as long as the BSS is not charged (or not highly negative charged), and multi-accelerators could occur in a BSS's magnetosphere. Our results would be helpful to distinguish normal neutron stars and bare quark stars through pulsar's magnetospheric activities.

Magnetar oscillations pose challenges for strange stars

Abstract Compact relativistic stars allow us to study the nature of matter under extreme conditions, probing regions of parameter space that are otherwise inaccessible. Nuclear theory in this regime is not well constrained: one key issue is whether neutron stars are in fact composed primarily of strange quark matter. Distinguishing the two possibilities, however, has been difficult. The recent detection of seismic vibrations in the aftermath of giant flares from two magnetars (highly magnetized compact stars) is a major breakthrough. The oscillations excited seem likely to involve the stellar crust, the properties of which differ dramatically for strange stars. We show that the resulting mode frequencies cannot be reconciled with the observations for reasonable magnetar parameters. Ruling out strange star models would place a strong constraint on models of dense quark matter.

On relativistic models of strange stars

The superdense stars with mass-to-size ratio exceeding 0.3 are expected to be made of strange matter. Assuming that the 3-space of the interior space-time of a strange star is that of a three-paraboloid immersed in a four-dimensional Euclidean space, we obtain a two-parameter family of their physically viable relativistic models. This ansatz determines density distribution of the interior self-gravitating matter up to one unknown parameter. The Einstein’s field equations determine the fluid pressure and the remaining geometrical variables. The information about mass-to-size ratio together with the conventional boundary conditions lead to the determination of total mass, radius and other parameters of the stellar configuration.

The Energy-loss Rate of Neutrinos in the Urca Process of Strange Quark Matter in Weak and Ultra-strong Magnetic Fields

The influence of magnetic field on the energy-loss rate of neutrinos in the direct Urca process of quarks in the model of strange stars is investigated in this article. First, an improvement is made on the method of approximate calculation in the case of weak field, which can be extended to other processes of weak interactions. Then, a rigorous calculation is made for ultra-strong magnetic fields, of the energy-loss rate of neutrinos in the Urca process. The result shows that the emissivity strongly depends on the magnetic field, being directly proportional to the square of the magnetic field strength. More importantly, the dependence on temperature is different in the cases of strong and weak or null magnetic fields.

Rotating star initial data for a constrained scheme in numerical relativity

A new numerical code for computing stationary axisymmetric rapidly rotating stars in general relativity is presented. The formulation is based on a fully constrained-evolution scheme for 3 + 1 numerical relativity using the Dirac gauge and maximal slicing. We use both the polytropic and MIT bag model equations of state to demonstrate that the code can construct rapidly rotating neutron star and strange star models. We compare numerical models obtained by our code and a well-established code, which uses a different gauge condition, and show that the two codes agree to a high degree of accuracy.

Strange Star Equations of State Revisited

AbstractMotivated by recent suggestions that strange stars can be responsible for glitches and other observational features of pulsars, we review some possible equations of state and their implications for models of neutron, hybrid, and strange stars. We consider the MIT bag model and also strange matter in the colour–flavour locked phase. The central energy densities for strange stars are higher than the central densities of ordinary neutron stars. Strange stars are bound by the strong force and so can also rotate much faster than neutron stars. These results are only weakly dependent on the model used for the quark matter. If just one of the existing mass-to-radius ratio constraint is valid, most neutron stars equations of state are ruled out, but all the strange stars equations of state presented in this work remain consistent with the constraint.

The Inner Annular Gap for Pulsar Radiation: -Ray and Radio Emission

The inner annular gap (IAG), a new type of inner gap whose magnetic field lines intersect the null charge surface (NCS), is proposed to explain γ-ray and radio emission from pulsars. The IAG can be an important source for high-energy particles. The particles can radiate between the NCS and the IAG. Some observational characteristics in both γ-ray and radio bands, such as the Crab-like, Vela-like, and Geminga-like γ-ray emission beams, can be reproduced by the numerical method. It is predicted that the view angle ζ should be larger than the inclination angle (ζ > α), otherwise the γ-ray radiation will have little possibility to be observed. Whether the IAG (or cap) is sparking (or free flow) depends on the surface binding energy of the pulsar. Instead of neutron star models, the scenario of the IAG is favorable for bare strange star models, because bare strange stars can easily satisfy the requisite condition to form an IAG for both pulsars (Ω 0).

Models of Strange Stars with a Crust and Strange Dwarfs

Strange quark stars with a crust and strange dwarfs consisting of a compact strange quark core and an extended crust are investigated in terms of a bag model. The crust, which consists of atomic nuclei and degenerate electrons, has a limiting density of ρcr=ρdrip=4.3·1011 g/cm3. A series of configurations are calculated for two sets of bag model parameters and three different values of ρcr (109 g/cm3 ≤ ρcr ≤ ρdrip) to find the dependence of a star's mass M and radius R on the central density. Sequences of stars ranging from compact strange stars to extended strange dwarfs are constructed out of strange quark matter with a crust. The effect of the bag model parameters and limiting crust density ρcr on the parameters of the strange stars and strange dwarfs is examined. The strange dwarfs are compared with ordinary white dwarfs and observational differences between the two are pointed out.

Highly accurate calculation of rotating neutron stars

We give a detailed description of the recently developed multi-domain spectral method for constructing highly accurate general-relativistic models of rapidly rotating stars. For both "ordinary" and "critical" configurations, it is exhibited by means of representative examples, how the accuracy improves as the order of the approximation increases. Apart from homogeneous fluid bodies, we also discuss models of polytropic and strange stars.

What Can the Redshift Observed in EXO 0748–676 Tell Us?

The mass-radius relations for bare and crusted strange stars are calculated with the bag model. Comparing these relations with the observed one derived from redshift, we address that the conclusion, that EXO 0748-676 can not be a strange star, is incorrect. Various strange star models can show that EXO 0748-676 could have a mass of (1.3 ~ 1.7) M_sun and a radius of (8.4 ~ 11.4) km. It is proposed that some nascent strange stars could be bare and have mass \~ 0.1 M_sun, whereas their masses increase during a long accretion history.

A strongly magnetized strange star model for soft gamma-ray repeaters

Soft γ-ray repeaters (SGRs) have been currently argued as super-strongly magnetized neutron stars (magnetars) with magnetic fields with a few 10 14 G. Here we propose an alternate model, in which SGRs are young strange stars with superconducting cores and with poloidal magnetic fields of normal radio pulsars. For SGR 1806−20, its magnetic field is taken to be ∼3 × 10 13 G in order to match the stellar characteristic timescale and the age of the surrounding supernova remnant (SNR) associated with this burster. The movement of the magnetic flux tubes in these strange stars not only leads to crustal cracking of the stars, giving rise to deconfinement of crustal matter to strange matter, but also to movement of internal magnetic toruses together with the flux tubes. The former process will result in bursting and quiescent emission and the latter process will produce quasi-steady relativistic winds which will power the surrounding SNR. The observed persistent X-ray luminosity, bursting behavior and inferred wind luminosity of SGRs can be explained in this model. The possible relation between the sources of SGRs, anomalous X-ray pulsars and gamma-ray bursts is discussed.

Is SAX J1808.4-3658 a Strange Star?

One of the most important questions in the study of compact objects is the nature of pulsars, including whether they are composed of $\beta$-stable nuclear matter or strange quark matter. Observations of the newly discovered millisecond X-ray pulsar \sax with the Rossi X-Ray Timing Explorer place firm constraint on the radius of the compact star. Comparing the mass - radius relation of \sax with the theoretical mass - radius relation for neutron stars and for strange stars, we find that a strange star model is more consistent with SAX J1808.4-3658, and suggest that it is a likely strange star candidate.