Fridolin Weber

Strange quark matter and compact stars

Abstract Astrophysicists distinguish between three different kinds of compact stars. These are white dwarfs, neutron stars, and black holes. The former contain matter in one of the densest forms found in the Universe which, together with the unprecedented progress in observational astronomy, makes such stars superb astrophysical laboratories for a broad range of most striking physical phenomena. These range from nuclear processes on the stellar surface to processes in electron degenerate matter at subnuclear densities to boson condensates and the existence of new states of baryonic matter—such as color superconducting quark matter—at supernuclear densities. More than that, according to the strange matter hypothesis strange quark matter could be more stable than nuclear matter, in which case neutron stars should be largely composed of pure quark matter possibly enveloped in thin nuclear crusts. Another remarkable implication of the hypothesis is the possible existence of a new class of white dwarfs. This article aims at giving an overview of all these striking physical possibilities, with an emphasis on the astrophysical phenomenology of strange quark matter. Possible observational signatures associated with the theoretically proposed states of matter inside compact stars are discussed as well. They will provide most valuable information about the phase diagram of superdense nuclear matter at high baryon number density but low temperature, which is not accessible to relativistic heavy ion collision experiments.

The Cooling of compact stars

Abstract The cooling of a compact star depends very sensitively on the state of matter at supranuclear densities, which essentially controls the neutrino emission, as well as on the structure of the stellar outer layers which control the photon emission. Open issues concern the hyperon population, the presence of meson condensates, superfluidity and superconductivity, and the transition of confined hadronic matter to quark matter. This paper describes these issues and presents cooling calculations based on a broad collection of equations of state for neutron star matter and strange matter. These results are tested against the body of observed cooling data.

Constraints on the high-density nuclear equation of state from the phenomenology of compact stars and heavy-ion collisions

A new scheme for testing nuclear matter equations of state (EoSs) at high densities using constraints from neutron star (NS) phenomenology and a flow data analysis of heavy-ion collisions is suggested. An acceptable EoS shall not allow the direct Urca process to occur in NSs with masses below 1.5M� , and also shall not contradict flow and kaon production data of heavy-ion collisions. Compact star constraints include the mass

Signal of Quark Deconfinement in the Timing Structure of Pulsar Spin-Down

(May, 12, 1997)The conversion of nuclear matter to quark matter in thecore of a rotating neutron star alters its moment of inertia.Hence the epoch over which conversion takes place will be sig-naled in the spin-down characteristics of pulsars. We find thatan observable called the braking index should be easily mea-surable during the transition epoch and can have a value farremoved (by orders of magnitude) from the canonical value ofthree expected for magnetic dipole radiation, and may haveeither sign. The duration of the transition epoch is governedby the slow loss of angular momentum to radiation and isfurther prolonged by the reduction in the moment of inertiacaused by the phase change which can even introduce an eraof spin-up. We estimate that about one in a hundred pulsarsmay be passing through this phase. The phenomenon is anal-ogous to “bachbending” observed in the moment of inertia ofrotating nuclei observed in the 1970’s, which also signaled achange in internal structure with changing spin.97.60Lf, 97.60.Gb, 97.10.Cv

Thermal Evolution of Compact Stars

Abstract A collection of modern, field-theoretical equations of state is applied to the investigation of cooling properties of compact stars. These comprise neutron stars as well as hypothetical strange-matter stars, made up of absolutely stable 3-flavor strange-quark matter. Various uncertainties in the behavior of matter at supernuclear densities, e.g., hyperonic degrees of freedom, behavior of coupling strengths in matter, pion and meson condensation, superfluidity, transition to quark matter, absolute stability of strange-quark matter, and last but not least the many-body technique itself are tested against the body of observed cooling data.

Electrically charged strange quark stars

The possible existence of compact stars made of absolutely stable strange quark matter--referred to as strange stars--was pointed out by Witten almost a quarter of a century ago. One of the most amazing features of such objects concerns the possible existence of ultrastrong electric fields on their surfaces, which, for ordinary strange matter, is around 10{sup 18} V/cm. If strange matter forms a color superconductor, as expected for such matter, the strength of the electric field may increase to values that exceed 10{sup 19} V/cm. The energy density associated with such huge electric fields is on the same order of magnitude as the energy density of strange matter itself, which, as shown in this paper, alters the masses and radii of strange quark stars at the 15% and 5% levels, respectively. Such mass increases facilitate the interpretation of massive compact stars, with masses of around 2M{sub {center_dot}}, as strange quark stars.

Baryon composition and macroscopic properties of neutron stars

Abstract Starting from a relativistic many-baryon/lepton lagrangian density — in which charged baryons (i.e. p, n, Σ ±,0 , Λ , Ξ 0,− , Δ ++,+0,− ) interact via the exchange of scalar-, vector-, and isovector mesons (i.e., σ, ω, π, ρ), respectively — both the baryon composition and gross structural parameters of neutron stars (like radius, gravitational mass, moment of inertia, red shift) are calculated. The constraint of charge neutrality of neutron star matter, which demands for the incorporation of leptons (i.e., e − , μ − ) into the theory, has been explicitly taken into account. The equation of state, which serves as an input for solving the Oppenheimer- Volkoff equation of stellar structure, is calculated for the Hartree as well as the Hartree-Fock approximation. Special emphasis is put towards comparing the outcome for both of these approximations. For example, we found for the maximum stable neutron star mass M / M ⊙ = 1.98 for the Hartree approximation and values of 2.18 and 2.31 for the Hartree-Fock treatments, respectively. Furthermore, the baryon compositions calculated for both of these approaches differ considerably from each other. Specifically, the Hartree-Fock approximation may not be suitable for the inclusion of the phenomenon of pion condensation in hadronic matter. The mathematical treatment of the many-baryon/lepton field theory is based on the Green function technique, as developed in some foregoing investigations for hot and dense matter.

Quark deconfinement in high-mass neutron stars

In this paper, we explore whether or not quark deconfinement may occur in high-mass neutron stars such as J1614-2230 (1.97 \pm 0.04 M_Sun) and J0348+0432 (2.01 \pm 0.04 M_Sun). Our study is based on a non-local extension of the SU(3) Nambu Jona-Lasinio (n3NJL) model with repulsive vector interactions among the quarks. This model goes beyond the frequently used local version of the Nambu Jona-Lasinio (NJL) model by accounting for several key features of QCD which are not part of the local model. Confined hadronic matter is treated in the framework of non-linear relativistic mean field theory. We find that both the local as well as the non-local NJL model predict the existence of extended regions of mixed quark-hadron (quark-hybrid) matter in high-mass neutron stars with masses of 2.1 to 2.4 M_Sun. Pure quark matter in the cores of neutron stars is obtained for certain parametrizations of the hadronic lagrangian and choices of the vector repulsion among quarks. The radii of high-mass neutron stars with quark-hybrid matter and/or pure quark matter cores in their centers are found to lie in the canonical range of 12 to 13 km.

EMMI rapid reaction task force meeting on quark matter in compact stars

The recent measurement of two solar mass pulsars has initiated an intense discussion on its impact on our understanding of the high-density matter in the cores of neutron stars. A task force meeting was held from October 7-10, 2013 at the Frankfurt Institute for Advanced Studies to address the presence of quark matter in these massive stars. During this meeting, the recent observational astrophysical data and heavy-ion data was reviewed. The possibility of pure quark stars, hybrid stars and the nature of the QCD phase transition were discussed and their observational signals delineated.

Phase Transition and Spin Clustering of Neutron Stars in X-Ray Binaries

We study the spin evolution of X-ray neutron stars in binary systems, which are being spun up by mass transfer from accretion disks. Our investigation reveals that a quark phase transition resulting from the changing central density induced by the changing spin, can lead to a pronounced peak in the frequency distribution of X-ray neutron stars. This finding provides one of several possible explanations available in the literature, or at least a contributor to part of the observed anomalous frequency distribution of neutron stars in low-mass X-ray binaries (LMXBs), which lie in a narrow band centered at about 300 Hz, as found by the Rossi Explorer (RXTE).The spins of X-ray neutron star accretors in low-mass binaries are found to cluster at about 300 Hz with the exception of a few higher frequency objects. We find that a postulated phase transition induced by the centrifugally driven dilution in the density profile of the star can produce a similar feature. It takes from 107 to 109 yr, depending on the mass accretion rate, to expel the high-density phase from the core. The corresponding growth in the moment of inertia retards spin-up during this epoch. Normal mass accretion-driven spin-up resumes at its completion. A phase change triggered by the changing spin and the accompanying evolution of the moment of inertia has its analog in rotating nuclei as was discovered in the 1970s.