On the mass defect of strange stars

In the context of the MIT bag model we compute the mass defect and the gravitational packing factor for three models of strange stars and study the contribution of gravitational and internal energy to the total energy of the system. For strange stars it is possible to realize a model with negative internal energy, leading to a greater binding energy of these stars compared to neutron stars. This is the reason for the absence of configurations with negative mass defect for the models in question. We analyze the question of identifying the remnants of supernovae with neutron or strange stars.Translated fromAstrofizika, Vol. 38, No. 2, 1995.     

Fine structure on the surface of bare strange stars

The surface fine structure of bare strange stars is determined. The distribution of electrons and quarks in the surface layer is determined using a phenomenological Thomas-Fermi model. For the MIT bag model, the quark density at the free surface is found to fall off continuously to zero in a layer of finite thickness. Unlike the results of other authors, here the electric field in the transition layer changes direction. The coefficient of surface tension of the quark matter is determined in terms of this model. Depending on the model parameters, it is 60–150 MeV/Fm².Translated from Astrofizika, Vol. 48, No. 1, pp. 139–150 (February 2005).     

Superdense stars containing strange baryons

This article is a review dedicated to the centenary of V. A. Ambartsumyan. The author, who is one of the first students of Ambartsumyan and G. S. Sahakyan in the area of superdense cosmology, presents the results of his first studies carried out jointly with Sahakyan, which were published immediately after the pioneering work of his teachers and supplemented their efforts. The results of studies by the author and various coauthors on superdense stars containing strange quarks are also discussed.     

Rotational parameters of strange stars in comparison with neutron stars

I study stellar structures, i.e. the mass, the radius, the moment of inertia and the oblateness parameter at different spin frequencies for strange stars and neutron stars in a comparative manner. I also calculate the values of the radii of the marginally stable orbits and Keplerian orbital frequencies. By equating kHz QPO frequencies to Keplerian orbital frequencies, I find corresponding orbital radii. Knowledge about these parameters might be useful in further modeling of the observed features from LMXBs with advanced and improved future techniques for observations and data analysis.     

Remarks on the relative populations of neutron and strange stars

We consider a simple qualitative model to estimate the time-scale forneutronstrange matter decay in dense stellar environments. It is argued that a large mismatch between the former and the microscopic weak interaction time-scale suggests that a dual population of both types of compact objects is unlikely. Assuming the correctness of the strange matter hypothesis all of them should be strange stars. If one instead postulates accretion as the decisive feature for the conversion, a consideration of neutron stars structure indicates a fairly narrow range for the onset of the critical density before the corresponding Chandrasekhar mass is achieved.     

Seismic signatures of strange stars with crust

We study acoustic oscillations (eigenfrequencies, velocity distributions, damping times) of normal crusts of strange stars. These oscillations are very specific because of huge density jump at the interface between the normal crust and the strange matter core. The oscillation problem is shown to be self-similar. For a low (but non-zero) multipolarity l , the fundamental mode (without radial nodes) has a frequency of ∼300 Hz and mostly horizontal oscillation velocity; other pressure modes have frequencies ≳20 kHz and almost radial oscillation velocities. The latter modes are similar to radial oscillations (having approximately the same frequencies and radial velocity profiles). The oscillation spectrum of strange stars with crust differs from the spectrum of neutron stars. If detected, acoustic oscillations would allow one to discriminate between strange stars with crust and neutron stars and constrain the mass and radius of the star.     

Models of strange stars having a crust

Models of strange quark stars with a crust consisting of atomic nuclei and degenerate electrons, maintained by an electrostatic barrier at the surface of the strange quark matter, are investigated for a realistic range of parameters of the MIT bag model. The density at which neutrons escape from nuclei, ρ = ρ_drip, is taken as the maximum possible boundary density of the crust. Series of strange stars are calculated as a function of central density. Configurations with masses of 1.44 and 1.77 M{ie330-1} and a gravitational redshift Z_s = 0.23, corresponding to the best-known observational data, are investigated. The presence of a crust results in the existence of a minimum mass for strange stars, and also helps to explain the glitch phenomenon of pulsars within the framework of the existence of strange quark matter. Translated from Astrofizika, Vol. 42, No. 3, pp. 439–448, July–September, 1999.     

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.     

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.     

Pair emission from bare magnetized strange stars

The dominant emission from bare strange stars is thought to be electron–positron pairs, produced through spontaneous pair creation (SPC) in a surface layer of electrons tied to the star by a superstrong electric field. The positrons escape freely, but the electrons are directed towards the star and quickly fill all available states, such that their degeneracy suppresses further SPC. An electron must be reflected and gain energy in order to escape, along with the positron. Each escaping electron leaves a hole that is immediately filled by another electron through SPC. We discuss the collisional processes that produce escaping electrons. When the Landau quantization of the motion perpendicular to the magnetic field is taken into account, electron–electron collisions can lead to an escaping electron only through a multistage process involving higher Landau levels. Although the available estimates of the collision rate are deficient in several ways, it appears that the rate is too low for electron–electron collisions to be effective. A simple kinetic model for electron–quark collisions leads to an estimate of the rate of pair production that is analogous to thermionic emission, but the work function is poorly determined.     

The effect of r-mode instability on the evolution of isolated strange stars

We studied the evolution of isolated strange stars (SSs) synthetically, considering the influence of r -mode instability. Our results show that the cooling of SSs with non-ultrastrong magnetic fields is delayed by heating due to r -mode damping for millions of years, while the spin-down of the stars is dominated by gravitational radiation (GR). Especially for the SSs in a possible existing colour–flavour locked (CFL) phase, the effect of r -mode instability on the evolution of stars becomes extremely important because the viscosity, neutrino emissivity and specific heat involving pairing quarks are blocked. It leads to the cooling of these colour superconducting stars being very slow and the stars can remain at high temperature for millions of years, which differs completely from previous understanding. In this case, an SS in CFL phase can be located at the bottom of its r -mode instability window for a long time, but does not spin-down to a very low frequency for hours.     

Strange quark matter and models of strange stars

In the framework of the MIT bag model we consider absolutely stable strange quark matter consisting of u, d, and s quarks and electrons. For a realistic range of parameters of the quark bag we compute the threshold density for the appearance of strange quark matter that is realized on the surface of self-sustaining strange stars. On the basis of twelve calculated equations of state we give a detailed study of the series of configurations of strange stars consistent with the best known observational data. We show that the binding energy of the models depends essentially on the quark-gluon interaction constant _c.Translated fromAstrofizika, Vol. 37, No. 3, 1994.The authors are grateful to E. V. Chubaryan and A. M. Atoyan for assistance in overcoming the information blockade. The present paper was written in the framework of area 46/101 93-353, supported by the Ministry of Higher Education and Science of the Republic of Armenia.     

Anisotropic stellar structure equations for magnetized strange stars

The fact that a fermion system in an external magnetic field breaks the spherical symmetry suggests that its intrinsic geometry is axisymmetric rather than spherical. In this work we analyze the impact of anisotropic pressures, due to the presence of a magnetic field, in the structure equations of a magnetized quark star.We assume a cylindrical metric and an anisotropic energy momentum tensor for the source. We found that there is a maximum magnetic field that a strange star can sustain, closely related to the violation of the virial relations.     

The effects of strong interaction on the observational discrimination between neutron and strange stars

Strange stars are compact objects similar to neutron stars composed of strange matter. This paper investigates the observational effects of the strong interaction between quarks. We believe: 1) that the conversion of a neutron star to a strange star is a large “period glitch” which is determined by the strong interaction; 2) that the strong interaction results in effective damping of oscillation of hot strange stars, which could be a new mechanism of driving supernova explosions; 3) that the strong interaction increases the difference in rotation between strange and neutron stars under high temperatures, making the minimum period for strange stars lower than that for neutron stars.     

On the properties of strange modes

Properties of the so-called strange modes occurring in linear stability calculations of stellar models are discussed. The behaviour of these modes is compared for two different sets of stellar models, for very massive zero-age main-sequence stars and for luminous hydrogen-deficient stars, both with high luminosity-to-mass ratios. We have found that the peculiar behaviour of the frequencies of the strange modes with the change of a control parameter is caused by the pulsation amplitude of a particular eigenmode being strongly confined to the outer part of the envelope, around the density inversion zone. The frequency of a strange mode changes because the depth of the confinement zone changes with the control parameter. Weakly non-adiabatic strange modes tend to be overstable because the amplitude confinement quenches the effect of radiative damping. On the other hand, extremely non-adiabatic strange modes become overstable because the perturbation of radiation force (gradient of radiation pressure) provides a restoring force that can be out of phase with the density perturbation. We discuss this mechanism by using a plane-parallel two-zone model.     

Spin down of rotating compact magnetized strange stars in general relativity

We find that in general relativity slow down of the pulsar rotation due to the magnetodipolar radiation is more faster for the strange star with comparison to that for the ordinary neutron star of the same mass. Comparison with astrophysical observations on pulsars spindown data may provide an evidence for the strange star existence and, thus, serve as a test for distinguishing it from the neutron star.