A model of low-mass neutron stars with a quark core (original) (raw)

Neutron stars, supernova explosions and the transition to quark matter

Nuclear Physics A, 2000

The transition to quark matter can take place in neutron stars. The structure of a hybrid star, containing a core made of quark matter is discussed. The maximum mass of the non-rotating hybrid star turns out to be ∼ 1.6M ⊙. Possible signatures of the quark phase as pulsar's timing, cooling rate etc. are briefly analyzed. The deconfinement transition can also take place during the pre-supernova collapse. This possibility is studied by introducing a finite temperature EOS. The dependence of the latter on the proton fraction is shown to be crucial. The softening of the EOS at densities just above nuclear matter saturation density for Z/A ∼ 0.3 helps in obtaining an explosion. At the same time, at larger densities the EOS is stiff enough to support a neutron star compatible with observations.

SUPERDENSE STARSWITH A QUARK CORE

NATO Science Series II: Mathematics, Physics and Chemistry, 2006

Series of neutron star models with strange quark cores are constructed on the basis of an extensive set of calculated realistic equations of state of superdense matter with quark-hadron phase transition. For some models a new local maximum on the stable branch of the star mass -central pressure diagram is revealed. This maximum arises along with the appearance of a sharp fracture on the core, which is characteristic for models of layered stars with x ¡ £ ¢ d ¤ ¥

Neutron Stars with a Quark Core. I. Equations of State

Astrophysics, 2003

An extensive set of realistic equations of state for superdense matter with a quark phase transition is derived on the basis of the three equations of state for neutron matter and the eight variants of strange quark-gluon plasmas in the MIT quark bag model. The characteristics of the phase transitions are described and the calculated equations of state with a density jump are studied in detail.

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

Astrophysics, 2000

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 stars 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.

Low-Mass Quark Stars or Quark White Dwarfs

2001

An equation of state is considered that, in superdense nuclear matter, results in a phase transition of the first kind from the nucleon state to the quark state with a transition parameter

Effects of quark matter nucleation on the evolution of proto-neutron stars

Astronomy & Astrophysics, 2011

Context. A phase of strong interacting matter with deconfined quarks is expected in the core of a massive neutron star. If this deconfinement phase transition is of the first order, as suggested by many models inspired by quantum chromodynamics, then it will be triggered by the nucleation of a critical size drop of the (stable) quark phase in the metastable hadronic phase. Within these circumstances it has been shown that cold (T = 0) pure hadronic compact stars above a threshold value of their gravitational mass (central pressure) are metastable with respect to the "decay" (conversion) to quark stars (i.e., compact stars made at least in part of quark matter). This stellar conversion process liberates a huge amount of energy (a few 10 53 erg), and it could be the energy source of some of the long gamma ray bursts. Aims. The main goal of the present work is to establish whether a newborn hadronic star (proto-hadronic star) could survive the early stages of its evolution without "decaying" to a quark star. To this aim, we study the nucleation process of quark matter in hot (T 0) β-stable hadronic matter, with and without trapped neutrinos, using a finite temperature equation of state (EOS) for hadronic and quark matter. Methods. The finite-temperature EOS for the hadronic and for the quark phases were calculated using the nonlinear Walecka model and the MIT bag model, respectively. The quantum nucleation rate was calculated making use of the Lifshitz & Kagan nucleation theory. The thermal nucleation rate was calculated using the Langer nucleation theory. Results. We calculate and compare the nucleation rate and the nucleation time due to thermal and quantum nucleation mechanisms. We compute the crossover temperature above which thermal nucleation dominates the finite temperature quantum nucleation mechanism. We next discuss the consequences of quark matter nucleation for the physics and the evolution of proto-neutron stars. We introduce the new concept of limiting conversion temperature and critical mass M cr for proto-hadronic stars, and we show that protohadronic stars with a mass M < M cr could survive the early stages of their evolution without decaying to a quark star. We extend the concept of maximum mass of a "neutron star" with respect to the classical one introduced by Oppenheimer & Volkoff to account for the existence of two distinct families of compact stars (hadronic stars and quark stars) as predicted by the present scenario.

Quark matter nucleation in neutron stars and astrophysical implications

The European Physical Journal A, 2016

A phase of strong interacting matter with deconfined quarks is expected in the core of massive neutron stars. We investigate the quark deconfinement phase transition in cold (T = 0) and hot β-stable hadronic matter. Assuming a first order phase transition, we calculate and compare the nucleation rate and the nucleation time due to quantum and thermal nucleation mechanisms. We show that above a threshold value of the central pressure a pure hadronic star (HS) (i.e. a compact star with no fraction of deconfined quark matter) is metastable to the conversion to a quark star (QS) (i.e. a hybrid star or a strange star). This process liberates an enormous amount of energy, of the order of 10 53 erg, which causes a powerful neutrino burst, likely accompanied by intense gravitational waves emission, and possibly by a second delayed (with respect to the supernova explosion forming the HS) explosion which could be the energy source of a powerful gamma-ray burst (GRB). This stellar conversion process populates the QS branch of compact stars, thus one has in the Universe two coexisting families of compact stars: pure hadronic stars and quark stars. We introduce the concept of critical mass M cr for cold HSs and proto-hadronic stars (PHSs), and the concept of limiting conversion temperature for PHSs. We show that PHSs with a mass M < M cr could survive the early stages of their evolution without decaying to QSs. Finally, we discuss the possible evolutionary paths of proto-hadronic stars.

Quark Deconfinement and Implications for the Radius and the Limiting Mass of Compact Stars

The Astrophysical Journal, 2004

We study the consequences of the hadron-quark deconfinement phase transition in stellar compact objects when finite size effects between the deconfined quark phase and the hadronic phase are taken into account. We show that above a threshold value of the central pressure (gravitational mass) a neutron star is metastable to the decay (conversion) to a hybrid neutron star or to a strange star. The mean-life time of the metastable configuration dramatically depends on the value of the stellar central pressure. We explore the consequences of the metastability of "massive" neutron stars and of the existence of stable compact quark stars (hybrid neutron stars or strange stars) on the concept of limiting mass of compact stars. We discuss the implications of our scenario on the interpretation of the stellar mass and radius extracted from the spectra of several X-ray compact sources. Finally, we show that our scenario implies, as a natural consequence a two step-process which is able to explain the inferred "delayed" connection between supernova explosions and GRBs, giving also the correct energy to power GRBs.

Formation Of Quark Matter In Neutron Stars

At very large densities and/or temperatures a quark-hadron phase transition is expected to take place. Simulations of QCD on lattice at zero baryon density indicate that the transition occurs at Tc ∼ 150 − 170 MeV. The calculations indicate that transition is likely to be second order or a cross over phenomenon. Although the lattice simulations have not given any indication on when the transition occurs at nonzero baryon density, the transition is expected to occur around the densities of few times nuclear matter density. Also, there is a strong reason to believe that the quark matter formed after the phase transition is in colour superconducting phase. The matter densities in the interior of neutron stars are expected to be several times the nuclear matter density and therefore the neutron star cores may possibly consist of quark matter. One then expects that this quark matter is formed during the collapse of supernova. Starting with the assumption that the quark matter, when formed consists of predominantly u and d quarks, we consider the evolution of strange quarks by weak interactions in the present work. The reaction rates and time required to reach the chemical equilibrium are computed here. Our calculations show that the chemical equilibrium is reached in about 10 −7 seconds. Further more during and immediately after the equilibration process enormous amount of energy is released and copious numbers of neutrinos are produced. We show that for reasonable models of nuclear equations of state the amount of energy released could be as high as 10 53 ergs and as many as 10 58 neutrinos may be emitted during the quark matter formation.

Radial Modes of Neutron Stars with a Quark Core

The Astrophysical Journal, 2002

We make a first calculation of eigenfrequencies of radial pulsations of neutron stars with quark cores in a general relativistic formalism given by Chandrasekhar. The equations of state (EOS) used to estimate such eigenfrequencies have been derived by taking proper care of the hadron-quark phase transition. The hadronic EOS's have been obtained in the framework of the Brueckner-Hartree-Fock and relativistic mean field theories, whereas the quark EOS has been derived within the MIT bag model. We find that the periods of oscillations of neutron stars with a quark core show a kink, which is associated with the presence of a mixed phase region. Also, oscillation periods show significant differences between ordinary neutron stars and neutron stars with dynamical quark phases. Subject headings: dense matterequation of statestars: neutronstars: oscillations Nuclear matter at sufficiently high density and temperature is expected to undergo a phase transition to a quark-gluon plasma. The indirect evidences from heavy-ion experiments at CERN (Heinz 2000; Heinz & Jacobs 2000) and more recently at RHIC (Blaizot 2001) assume to confirm the formation of a quark-gluon plasma. Such a phase transition might occur inside neutron stars, because these are cold and very compact astrophysical objects. It is therefore very interesting to study the effects of possible phase transitions in neutron stars observable like maximum gravitational masses, radii, oscillation frequencies, etc. In the present letter, we analyze the consequences of a hadron-quark phase transition on the periods of radial oscillations in neutron stars. In fact, more than three decades ago, Cameron (1965) made a suggestion that vibration of neutron stars could excite motions that can have interesting astrophysical implications. There are several investigations of vibrating neutron stars and the simple dimensional analysis suggest that the period of fundamental mode would be the order of milliseconds. More than two decades later, Cutler et al. (1990) concluded that neutron stars of about one solar mass and radius about 10 km give periods (3-5) ms, and these turned out to be relatively insensitive to the exact value of central density.