arXiv subjects

Fridolin Weber

Publications and source records attributed to Fridolin Weber.

At least 55 records · Page 3Linked to original sources

arXiv2018

Competition between delta isobars and hyperons and properties of compact stars

The $Δ$-isobar degrees of freedom are included in the covariant density functional (CDF) theory to study the equation of state (EoS) and composition of dense matter in compact stars. In addition to $Δ$'s we include the full octet of baryons, which allows us to study the interplay between the onset of delta isobars and hyperonic degrees of freedom. Using both the Hartree and Hartree-Fock approximation we find that $Δ$'s appear already at densities slightly above the saturation density of nuclear matter for a wide range of the meson-$Δ$ coupling constants. This delays the appearance of hyperons and significantly affects the gross properties of compact stars. Specifically, $Δ$'s soften the EoS at low densities but stiffen it at high densities. This softening reduces the radius of a canonical $1.4 M_\odot$ star by up to 2~km for a reasonably attractive $Δ$ potential in matter, while the stiffening results in larger maximum masses of compact stars. We conclude that the hypernuclear CDF parametrizations that satisfy the 2$M_\odot$ maximum mass constraint remain valid when $Δ$ isobars are included, with the important consequence that the resulting stellar radii are shifted toward lower values, which is in agreement with the analysis of neutron star radii.

nucl-th↗

arXiv2018

Neutrino Emissivity in the Quark-Hadron Mixed Phase

In this work we investigate the effect a crystalline quark-hadron mixed phase can have on the neutrino emissivity from the cores of neutron stars. To this end we use relativistic mean-field equations of state to model hadronic matter and a nonlocal extension of the three-flavor Nambu-Jona-Lasinio model for quark matter. Next we determine the extent of the quark-hadron mixed phase and its crystalline structure using the Glendenning construction, allowing for the formation of spherical blob, rod, and slab rare phase geometries. Finally we calculate the neutrino emissivity due to electron-lattice interactions utilizing the formalism developed for the analogous process in neutron star crusts. We find that the contribution to the neutrino emissivity due to the presence of a crystalline quark-hadron mixed phase is substantial compared to other mechanisms at fairly low temperatures ($\lesssim 10^9$ K) and quark fractions ($\lesssim 30\%$), and that contributions due to lattice vibrations are insignificant compared to static-lattice contributions. There are a number of open issues that need to be addressed in a future study on the neutrino emission rates caused by electron-quark blob bremsstrahlung. Chiefly among them are the role of collective oscillations of matter, electron band structures, and of gaps at the boundaries of the Brillouin zones on bremsstrahlung, as discussed in the summary section of this paper. We hope this paper will stimulate studies addressing these issues.

nucl-th↗

arXiv2018

Cooling of hypernuclear compact stars

We study the thermal evolution of hypernuclear compact stars constructed from covariant density functional theory of hypernuclear matter and parameterizations which produce sequences of stars containing two-solar-mass objects. For the input in the simulations, we solve the Bardeen-Cooper-Schrieffer gap equations in the hyperonic sector and obtain the gaps in the spectra of $Λ$, $Ξ^0$ and $Ξ^-$ hyperons. For the models with masses $M/M_{\odot} \ge 1.5$ the neutrino cooling is dominated by hyperonic direct Urca processes in general. In the low-mass stars the $(Λp)$ plus leptons channel is the dominant direct Urca process, whereas for more massive stars the purely hyperonic channels $(Σ^-Λ)$ and $(Ξ^-Λ)$ are dominant. Hyperonic pairing strongly suppresses the processes on $Ξ^-$s and to a lesser degree on $Λ$s. We find that intermediate-mass $1.5 \le M/M_{\odot} \le 1.8$ models have surface temperatures which lie within the range inferred from thermally emitting neutron stars, if the hyperonic pairing is taken into account. Most massive models with $M/M_{\odot} \simeq 2$ may cool very fast via the direct Urca process through the $(Λp)$ channel because they develop inner cores where the $S$-wave pairing of $Λ$s and proton is absent.

astro-ph.HE↗

arXiv2017

Very Magnetized White Dwarfs with Axisymmetric Magnetic Field and the Importance of the Electron Capture and Pycnonuclear Fusion Reactions for their Stability

In this work, we study the properties of magnetized white dwarfs taking into account possible instabilities due to electron capture and pycnonuclear fusion reactions in the cores of such objects. The structure of white dwarfs is obtained by solving the Einstein-Maxwell equations with a poloidal magnetic field in a fully general relativistic approach. The stellar interior is composed of a regular crystal lattice made of carbon ions immersed in a degenerate relativistic electron gas. The onsets of electron capture reactions and pycnonuclear reactions are determined with and without magnetic fields. We find that magnetized white dwarfs violate the standard Chandrasekhar mass limit significantly, even when electron capture and pycnonuclear fusion reactions are present in the stellar interior. We obtain a maximum white dwarf mass of around $2.14\,M_{\odot}$ for a central magnetic field of $\sim 3.85\times 10^{14}$~G, which indicates that magnetized white dwarfs may play a role for the interpretation of superluminous type Ia supernovae. Furthermore, we show that the critical density for pycnonuclear fusion reactions limits the central white dwarf density to $9.35\times 10^9$ g/cm$^3$. As a result, equatorial radii of white dwarfs cannot be smaller than $\sim 1100$~km. Another interesting feature concerns the relationship between the central stellar density and the strength of the magnetic field at the core of a magnetized white dwarf. For high magnetic fields, we find that the central density increases (stellar radius decrease) with magnetic field strength, which makes ultramagnetized white dwarfs more compact. The opposite is the case, however, if the central magnetic field is less than $\sim 10^{13}$~G. In the latter case, the central density decreases (stellar radius increases) with central magnetic field strengths.

astro-ph.SR↗

arXiv2016

The Constant-Sound-Speed parameterization for NJL models of quark matter in hybrid stars

The discovery of pulsars as heavy as 2 solar masses has led astrophysicists to rethink the core compositions of neutron stars, ruling out many models for the nuclear equations of state (EoS). We explore the hybrid stars that occur when hadronic matter is treated in a relativistic mean-field approximation and quark matter is modeled by three-flavor local and non-local Nambu Jona-Lasinio (NJL) models with repulsive vector interactions. The NJL models typically yield equations of state that feature a first order transition to quark matter. Assuming that the quark-hadron surface tension is high enough to disfavour mixed phases, and restricting to EoSes that allow stars to reach 2 solar masses, we find that the appearance of the quark matter core either destabilizes the star immediately (this is typical for non-local NJL models) or leads to a very short hybrid star branch in the mass-radius relation (this is typical for local NJL models). Using the Constant-Sound-Speed parametrization we can see that the reason for the near-absence of hybrid stars is that the transition pressure is fairly high and the transition is strongly first order.

nucl-th↗

arXiv2016

Neutrino emissivity in the quark-hadron mixed phase of neutron stars

Numerous theoretical studies using various equation of state models have shown that quark matter may exist at the extreme densities in the cores of high-mass neutron stars. It has also been shown that a phase transition from hadronic matter to quark matter would result in an extended mixed phase region that would segregate phases by net charge to minimize the total energy of the phase, leading to the formation of a crystalline lattice. The existence of quark matter in the core of a neutron star may have significant consequences for its thermal evolution, which for thousands of years is facilitated primarily by neutrino emission. In this work we investigate the effect a crystalline quark-hadron mixed phase can have on the neutrino emissivity from the core. To this end we calculate the equation of state using the relativistic mean-field approximation to model hadronic matter and a nonlocal extension of the three-flavor Nambu-Jona-Lasinio model for quark matter. Next we determine the extent of the quark-hadron mixed phase and its crystalline structure using the Glendenning construction, allowing for the formation of spherical blob, rod, and slab rare phase geometries. Finally we calculate the neutrino emissivity due to electron-lattice interactions utilizing the formalism developed for the analogous process in neutron star crusts. We find that the contribution to the neutrino emissivity due to the presence of a crystalline quark-hadron mixed phase is substantial compared to other mechanisms at fairly low temperatures ($\lesssim 10^9$ K) and quark fractions ($\lesssim 30\%$), and that contributions due to lattice vibrations are insignificant compared to static-lattice contributions.

nucl-th↗

arXiv2014

Properties of High-Density Matter in Neutron Stars

This short review aims at giving a brief overview of the various states of matter that have been suggested to exist in the ultra-dense centers of neutron stars. Particular emphasis is put on the role of quark deconfinement in neutron stars and on the possible existence of compact stars made of absolutely stable strange quark matter (strange stars). Astrophysical phenomena, which distinguish neutron stars from quark stars, are discussed and the question of whether or not quark deconfinement may occur in neutron stars is investigated. Combined with observed astrophysical data, such studies are invaluable to delineate the complex structure of compressed baryonic matter and to put firm constraints on the largely unknown equation of state of such matter.

astro-ph.SR↗

arXiv2014

Hadron-Quark Phase Transition in Quark-Hybrid Stars

The recent discovery of the two-solar mass neutron stars $J1614-2230~(1.97 \pm 0.04 M_{\odot})$ and $J0348+0432~(2.01 \pm 0.04 M_{\odot})$ allows to consider the possible existence of deconfined quarks in the cores of neutron stars. Based on a non-local extension of the $SU(3)$ Nambu Jona-Lasinio model with vector interactions to describe the quark matter phase, and a non-linear Walecka model to represent the hadronic phase, a phase transition between these two phases can be constructed via the Gibbs conditions and imposing global electric charge neutrality condition. Depending on the strength of quark vector repulsion, we find that an extended region made of a mixed phase of quarks and hadrons may exist in high-mass neutron stars with masses up to $2.0-2.4 M_{\odot}$. The radii of these objects are between $12$ and $13$ km, as expected for neutron stars.

hep-ph↗

arXiv2014

EMMI Rapid Reaction Task Force Meeting on 'Quark Matter in Compact Star'

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 oservational 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.

astro-ph.HE↗

arXiv2013

Thermal Evolution of Rotating Neutron Stars

In this work we consider the thermal evolution of rigidly rotating neutron stars. In order to perform such study we first calculate the structure of rotating objects, which is considerably more complicated than that of spherical objects. The structure of rotating neutron stars is obtained by solving Einstein's equation for a rotationally deformed fluid distributions. The numerical method used is based on the the KEH. The equation of state used for computing the neutron star structure and composition is a simple relativistic mean field model, with parameter set G300. With the structure of rotating neutron stars computed we calculate the thermal evolution of these objects. In order to do so, we re-derive the thermal evolution equations to account for the metric of a rotating object. The cooling of neutron stars with different frequencies is then calculated. We show that the cooling of the star strongly depends on the frequency of the object, with higher frequencies stars showing a substantial temperature difference between the equator and poles.

astro-ph.HE↗

arXiv2013

Impact of Rotation on the Structure and Composition of Neutron Stars

Depending on mass and rotational frequency, gravity compresses the matter in the core regions of neutron stars to densities that are several times higher than the density of ordinary atomic nuclei. At such huge densities atoms themselves collapse, and atomic nuclei are squeezed so tightly together that new particle states may appear and novel states of matter, foremost quark matter, may be created. This feature makes neutron stars superb astrophysical laboratories for a wide range of physical studies. And with observational data accumulating rapidly from both orbiting and ground based observatories spanning the spectrum from X-rays to radio wavelengths, there has never been a more exiting time than today to study neutron stars. The Hubble Space Telescope and X-ray satellites such as Chandra and XMM-Newton in particular have proven especially valuable. New astrophysical instruments such as the Five hundred meter Aperture Spherical Telescope (FAST), the square kilometer Array (skA), Fermi Gamma-ray Space Telescope (formerly GLAST), and possibly the International X-ray Observatory (now Advanced Telescope for High ENergy Astrophysics, ATHENA) promise the discovery of tens of thousands of new non-rotating and rotating neutron stars. The latter are referred to as pulsars. This paper provides a short overview of the impact of rotation on the structure and composition of neutron stars. Observational properties, which may help to shed light on the core composition of neutron stars--and, hence, the properties of ultra-dense matter--are discussed.

astro-ph.SR↗

arXiv2013

Proton superconductivity and the masses of neutron stars

The unexpected temperature evolution of the neutron star in the Cassiopeia A supernova remnant (Cas A, for short) has renewed tremendous interest in the cooling mechanisms of neutron stars. In particular, the formation of superconducting protons and superfluid neutrons deep inside the cores of neutron stars have become focal points of the discussion. The purpose of this letter is to add a new aspect to this discussion, which focuses on the connection between proton superconductivity and the masses of neutron stars. Assuming (as is currently the case) that the temperature evolution of Cas A is largely controlled by superconducting protons, we study a series of phenomenological proton-pairing models to determine how deep into the stellar core superconducting protons actually penetrate. This allows us to establish a heretofore unknown relationship between the mass of the neutron star in Cas A and the penetration depth of the superconducting proton phase. This relationship can be used to either predict the depth of the superconducting proton phase, or, conversely, determine the mass of Cas A from a reliable calculation of the size of the proton superconducting phase in superdense neutron star matter. We emphasize that the strategy outlined in this paper can be applied to any other neutron star of similar age, whose temperature might be reliably monitored over a several years period. High-mass neutron stars, such as the recently discovered neutron stars J1614-2230 ($1.97 \pm 0.04\, \msun$) and J0348+0432 ($2.01 \pm 0.04 \, \msun$), appear particularly appealing as a significant fraction of the protons in their cores may be superconducting.

astro-ph.HE↗

arXiv2012

Impact of Rotation-Driven Particle Repopulation on the Thermal Evolution of Pulsars

Driven by the loss of energy, isolated rotating neutron stars (pulsars) are gradually slowing down to lower frequencies, which increases the tremendous compression of the matter inside of them. This increase in compression changes both the global properties of rotating neutron stars as well as their hadronic core compositions. Both effects may register themselves observationally in the thermal evolution of such stars, as demonstrated in this Letter. The rotation-driven particle process which we consider here is the direct Urca (DU) process, which is known to become operative in neutron stars if the number of protons in the stellar core exceeds a critical limit of around 11% to 15%. We find that neutron stars spinning down from moderately high rotation rates of a few hundred Hertz may be creating just the right conditions where the DU process becomes operative, leading to an observable effect (enhanced cooling) in the temperature evolution of such neutron stars. As it turns out, the rotation-driven DU process could explain the unusual temperature evolution observed for the neutron star in Cas A, provided the mass of this neutron star lies in the range of 1.5 to 1.9 \msun and its rotational frequency at birth was between 40 (400 Hz) and 70% (800 Hz) of the Kepler (mass shedding) frequency, respectively.

astro-ph.HE↗

arXiv2012

On the Transport Properties of a Quark-Hadron Coulomb Lattice in the Cores of Neutron Stars

Already more that 40 years ago, it has been suggested that because of the enormous mass densities in the cores of neutron stars, the hadrons in the centers of neutron stars may undergo a phase transition to deconfined quark matter. In this picture, neutron stars could contain cores made of pure (up, down, strange) quark matter which are surrounded by a mixed phase of quarks and hadrons. More than that, because of the competition between the Coulomb and the surface energies associated with the positively charged regions of nuclear matter and negatively charged regions of quark matter, the mixed phase may develop geometrical structures, similarly to what is expected of the sub-nuclear liquid-gas phase transition. In this paper we restrict ourselves to considering the formation of rare phase blobs in the mixed quark-hadron phase. The influence of rare phase blobs on the thermal and transport properties of neutron star matter is investigated. The total specific heat, $c_V$, thermal conductivity, $κ$, and electron-blob Bremsstrahlung neutrino emissivities, $ε_{ν,\text{BR}}$, of quark-hybrid matter are computed and the results are compared with the associated thermal and transport properties of standard neutron star matter. Our results show that the contribution of rare phase blobs to the specific heat is negligibly small. This is different for the neutrino emissivity from electron-blob Bremsstrahlung scattering, which turns out to be of the same order of magnitude as the total contributions from other Bremsstrahlung processes for temperatures below about $10^8$ K.

astro-ph.SR↗

arXiv2012

Structure of Quark Stars

This paper gives an brief overview of the structure of hypothetical strange quarks stars (quark stars, for short), which are made of absolutely stable 3-flavor strange quark matter. Such objects can be either bare or enveloped in thin nuclear crusts, which consist of heavy ions immersed in an electron gas. In contrast to neutron stars, the structure of quark stars is determined by two (rather than one) parameters, the central star density and the density at the base of the crust. If bare, quark stars possess ultra-high electric fields on the order of 10^{18} to 10^{19} V/cm. These features render the properties of quark stars more multifaceted than those of neutron stars and may allow one to observationally distinguish quark stars from neutron stars.

astro-ph.SR↗

arXiv2012

Thermal Evolution of Neutron Stars in 2 Dimensions

There are many factors that contribute to the breaking of the spherical symmetry of a neutron star. Most notably is rotation, magnetic fields, and/or accretion of matter from companion stars. All these phenomena influence the macroscopic structures of neutron stars, but also impact their microscopic compositions. The purpose of this paper is to investigate the cooling of rotationally deformed, two-dimensional (2D) neutron stars in the framework of general relativity theory, with the ultimate goal of better understand the impact of 2D effects on the thermal evolution of such objects. The equations that govern the thermal evolution of rotating neutron stars are presented in this paper. The cooling of neutron stars with different frequencies is computed self-consistently by combining a fully general relativistic 2D rotation code with a general relativistic 2D cooling code. We show that rotation can significantly influence the thermal evolution of rotating neutron stars. Among the major new aspects are the appearances of hot spots on the poles, and an increase of the thermal coupling times between the core and the crust of rotating neutron stars. We show that this increase is independent of the microscopic properties of the stellar core, but depends only on the frequency of the star.

astro-ph.HE↗

arXiv2010

Properties of Bare Strange Stars Associated with Surface Electric Fields

In this paper we investigate the electrodynamic surface properties of bare strange quark stars. The surfaces of such objects are characterized by the formation of ultra-high electric surface fields which might be as high as $\sim 10^{19}$ V/cm. These fields result from the formation of electric dipole layers at the stellar surfaces. We calculate the increase in gravitational mass associated with the energy stored in the electric dipole field, which turns out to be only significant if the star possesses a sufficiently strong {\em net} electric charge distribution. In part two of the paper, we explore the intriguing possibility of what happens when the electron layer (sphere) rotates with respect to the stellar strange matter body. We find that in this event magnetic fields can be generated which, for moderate effective rotational frequencies between the electron layer and the stellar body, agree with the magnetic fields inferred for several Central Compact Objects (CCOs). These objects could thus be comfortably interpreted as strange stars whose electron atmospheres rotate at frequencies that are moderately different ($\sim 10$ Hz) from the rotational frequencies of the strange star itself.

astro-ph.HE↗

arXiv2010

Vortex Dynamics in Color-Superconducting Quark stars: The Re-heating of Magnetars

Compact stars made of quark matter rather than confined hadronic matter, are expected to form a color superconductor. This superconductor ought to be threaded with rotational vortex lines within which the star's interior magnetic field is confined. The vortices (and thus magnetic flux) would be expelled from the star during stellar spin-down, leading to magnetic reconnection at the surface of the star and the prolific production of thermal energy. In this Letter, we show that this energy release can re-heat quark stars to exceptionally high temperatures, such as observed for Soft Gamma Repeaters (SGRs), Anomalous X-Ray pulsars (AXPs), and X-ray dim isolated neutron stars (XDINs). Moreover, our numerical investigations of the temperature evolution, spin-down rate, and magnetic field behavior of such superconducting quark stars suggest that SGRs, AXPs, and XDINs may be linked ancestrally. Finally, we discuss the possibility of a time delay before the star enters the color superconducting phase, which can be used to estimate the density at which quarks deconfine. We find this density to be five times that of nuclear saturation.

astro-ph.HE↗