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Universe
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Studies in Neutron Stars
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Studies in Neutron Stars

A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Compact Objects".

Deadline for manuscript submissions: closed (31 March 2024) | Viewed by 5495

Special Issue Editors

Dr. Ana Gabriela Grunfeld

E-Mail Website
Guest Editor
1. Department of Theoretical Physics, CNEA, Buenos Aires 1429, Argentina
2. CONICET, Godoy Cruz 2290, Ciudad Autónoma de Buenos Aires C1425FQB, Argentina
Interests: chiral effective models; finite temperature and density; QCD phase diagram; compact objects; magnetic fields
Prof. Dr. David Blaschke

E-Mail Website
Guest Editor
1. Institute of Theoretical Physics, University of Wroclaw, 50-204 Wroclaw, Poland
2. Center for Advanced Systems Understanding (CASUS), D-02826 Görlitz, Germany
3. Helmholtz-Zentrum Dresden-Rossendorf (HZDR), D-01328 Dresden, Germany
Interests: quantum field theory at finite temperature; dense hadronic matter and QCD phase transitions; quark matter in heavy-ion collisions, in compact stars, their mergers and in supernova explosions; pair production in strong fields
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

With the detection of the gravitational wave signal from the inspiral phase of the binary neutron star merger GW170817 by the LIGO-Virgo Collaboration in 2017, the new era of multi-messenger astronomy has begun. In this new era, neutron stars (NSs) play a crucial role, be it as pulsars in isolation or in binaries, as magnetars, as proto-neutron stars after supernova explosion or as companions in a merger.

NSs are the only objects whose emission encompasses all the available multi-messenger tracers: electromagnetic waves, cosmic rays, neutrinos, and gravitational waves (GW). They are the only laboratories where we can study the most extreme phases of matter: not only can we probe extremes of gravity and electromagnetism, but also, strong and weak interaction can be studied in regimes that we cannot hope to explore on Earth. The study of these objects transcends the traditional astrophysical approach and requires a multidisciplinary effort that spans from particle and nuclear physics to astrophysics, from experiment to theory, from gravitational waves to the electromagnetic spectrum.

The purpose of this Special Issue is to collect new original contributions as well as reviews in the broad field of NS studies. We welcome contributions exploring all aspects of NSs, from theories to observations. Submit your paper and select the Journal “Universe” and the Special Issue “Studies on Neutron Stars” via: MDPI submission system. Papers will be published on a rolling basis, and we will be pleased to receive your contributions when they are ready to be submitted.

Dr. Ana Gabriela Grunfeld
Prof. Dr. David Blaschke
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Universe is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • neutron star cooling
  • quark matter in neutron stars
  • bayesian analysis
  • binary neutron star mergers
  • hypernuclear matter
  • gravitational waves
  • NICER
  • strange dwarfs
  • mass twin stars
  • perturbative QCD

Published Papers (6 papers)

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Research

Jump to: Review

10 pages, 267 KiB  
Article
Estimate for the Neutrino Magnetic Moment from Pulsar Kick Velocities Induced at the Birth of Strange Quark Matter Neutron Stars
by Alejandro Ayala, Santiago Bernal-Langarica and Daryel Manreza-Paret
Universe 2024, 10(7), 301; https://doi.org/10.3390/universe10070301 - 20 Jul 2024
Viewed by 451
Abstract
We estimate the magnetic moment of electron neutrinos by computing the neutrino chirality flip rate that can occur in the core of a strange quark matter neutron star at birth. We show that this process allows neutrinos to anisotropically escape, thus inducing the [...] Read more.
We estimate the magnetic moment of electron neutrinos by computing the neutrino chirality flip rate that can occur in the core of a strange quark matter neutron star at birth. We show that this process allows neutrinos to anisotropically escape, thus inducing the star kick velocity. Although the flip from left- to right-handed neutrinos is assumed to happen in equilibrium, the no-go theorem does not apply because right-handed neutrinos do not interact with matter and the reverse process does not happen, producing the loss of detailed balance. For simplicity, we model the star core as consisting of strange quark matter. We find that even when the energy released in right-handed neutrinos is a small fraction of the total energy released in left-handed neutrinos, the process describes kick velocities for natal conditions, which are consistent with the observed ones and span the correct range of radii, temperatures and chemical potentials for typical magnetic field intensities. The neutrino magnetic moment is estimated to be μν∼3.6×10−18μB, where μB is the Bohr magneton. This value is more stringent than the bound found for massive neutrinos in a minimal extension of the standard model. Full article
(This article belongs to the Special Issue Studies in Neutron Stars)
30 pages, 2160 KiB  
Article
Isospin QCD as a Laboratory for Dense QCD
by Toru Kojo, Daiki Suenaga and Ryuji Chiba
Universe 2024, 10(7), 293; https://doi.org/10.3390/universe10070293 - 12 Jul 2024
Cited by 1 | Viewed by 396
Abstract
QCD with the isospin chemical potential μI is a useful laboratory to delineate the microphysics in dense QCD. To study the quark–hadron continuity, we use a quark–meson model that interpolates hadronic and quark matter physics at microscopic level. The equation of state [...] Read more.
QCD with the isospin chemical potential μI is a useful laboratory to delineate the microphysics in dense QCD. To study the quark–hadron continuity, we use a quark–meson model that interpolates hadronic and quark matter physics at microscopic level. The equation of state is dominated by mesons at low density but taken over by quarks at high density. We extend our previous studies with two flavors to the three-flavor case to study the impact of the strangeness, which may be brought by kaons (K+,K0)=(us¯,sd¯) and the UA(1) anomaly. In the normal phase, the excitation energies of kaons are reduced by μI in the same way as hyperons in nuclear matter at the finite baryon chemical potential. Once pions condense, kaon excitation energies increase as μI does. Moreover, strange quarks become more massive through the UA(1) coupling to the condensed pions. Hence, at zero and low temperature, the strange hadrons and quarks are highly suppressed. The previous findings in two-flavor models, sound speed peak, negative trace anomaly, gaps insensitive to μI, persist in our three-flavor model and remain consistent with the lattice results to μI∼ 1 GeV. We discuss the non-perturbative power corrections and quark saturation effects as important ingredients to understand the crossover equations of state measured on the lattice. Full article
(This article belongs to the Special Issue Studies in Neutron Stars)
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17 pages, 3765 KiB  
Article
Strange Quark Stars: The Role of Excluded Volume Effects
by G. Lugones and Ana G. Grunfeld
Universe 2024, 10(6), 233; https://doi.org/10.3390/universe10060233 - 24 May 2024
Cited by 2 | Viewed by 517
Abstract
We study cold strange quark stars employing an enhanced version of the quark-mass density-dependent model, which incorporates excluded volume effects to address non-perturbative QCD repulsive interactions. We provide a comparative analysis of our mass formula parametrization with previous models from the literature. We [...] Read more.
We study cold strange quark stars employing an enhanced version of the quark-mass density-dependent model, which incorporates excluded volume effects to address non-perturbative QCD repulsive interactions. We provide a comparative analysis of our mass formula parametrization with previous models from the literature. We identify the regions within the parameter space where three-flavor quark matter is more stable than the most tightly bound atomic nucleus (stability window). Specifically, we show that excluded volume effects do not change the Gibbs free energy per baryon at zero pressure, rendering the stability window unaffected. The curves of pressure versus energy density exhibit various shapes—convex upward, concave downward, or nearly linear—depending on the mass parametrization. This behavior results in different patterns of increase, decrease, or constancy in the speed of sound as a function of baryon number density. We analyze the mass–radius relationship of strange quark stars, revealing a significant increase in maximum gravitational mass and a shift in the curves toward larger radii as the excluded volume effect intensifies. Excluded volume effects render our models compatible with all modern astrophysical constraints, including the properties of the recently observed low-mass compact object HESSJ1731. Full article
(This article belongs to the Special Issue Studies in Neutron Stars)
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20 pages, 591 KiB  
Article
Nuclear Matter Equation of State in the Brueckner–Hartree–Fock Approach and Standard Skyrme Energy Density Functionals
by Isaac Vidaña, Jérôme Margueron and Hans-Josef Schulze
Universe 2024, 10(5), 226; https://doi.org/10.3390/universe10050226 - 17 May 2024
Viewed by 650
Abstract
The equation of state of asymmetric nuclear matter as well as the neutron and proton effective masses and their partial-wave and spin–isospin decomposition are analyzed within the Brueckner–Hartree–Fock approach. Theoretical uncertainties for all these quantities are estimated by using several phase-shift-equivalent nucleon–nucleon forces [...] Read more.
The equation of state of asymmetric nuclear matter as well as the neutron and proton effective masses and their partial-wave and spin–isospin decomposition are analyzed within the Brueckner–Hartree–Fock approach. Theoretical uncertainties for all these quantities are estimated by using several phase-shift-equivalent nucleon–nucleon forces together with two types of three-nucleon forces, phenomenological and microscopic. It is shown that the choice of the three-nucleon force plays an important role above saturation density, leading to different density dependencies of the energy per particle. These results are compared to the standard form of the Skyrme energy density functional, and we find that it is not possible to reproduce the BHF predictions in the (S,T) channels in symmetric and neutron matter above saturation density, already at the level of the two-body interaction, and even more including the three-body interaction. Full article
(This article belongs to the Special Issue Studies in Neutron Stars)
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14 pages, 1170 KiB  
Article
Compact Stars in the vBag Model and Its f-Mode Oscillations
by Heng-Yi Zhou, Wei Wei and Xia Zhou
Universe 2023, 9(6), 285; https://doi.org/10.3390/universe9060285 - 10 Jun 2023
Viewed by 1325
Abstract
Electromagnetic (EM) observations and gravitational wave (GW) measurements enable us to determine the mass and radius of neutron stars (NSs) and their tidal deformability, respectively. These parameters offer valuable insights into the properties of dense matter in NSs. In this study, the vector-interaction-enhanced [...] Read more.
Electromagnetic (EM) observations and gravitational wave (GW) measurements enable us to determine the mass and radius of neutron stars (NSs) and their tidal deformability, respectively. These parameters offer valuable insights into the properties of dense matter in NSs. In this study, the vector-interaction-enhanced bag model (vBag model) is employed to investigate strange and hybrid stars’ properties. The parameters of the vBag model are constrained using multi-messenger observations, revealing that strange stars are incompatible with current observations. In contrast, hybrid stars can exhibit a substantial mixed phase region and a thin hadronic shell. Furthermore, we present the frequencies and damping time of fundamental mode (f-mode) oscillations of hybrid stars and test their universal relations with compactness and tidal deformability. The findings indicate that the presence of mixed phase components leads to larger frequencies and shorter damping time of the f-mode oscillation of hybrid stars, and the softer equation of state (EoS) affects this behavior more significantly. The universal relations of hybrid stars in the vBag model can be described by fourth-order/seventh-order polynomials, which do not break the previous results. Full article
(This article belongs to the Special Issue Studies in Neutron Stars)
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Review

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25 pages, 2208 KiB  
Review
Measuring the Lense–Thirring Orbital Precession and the Neutron Star Moment of Inertia with Pulsars
by Huanchen Hu and Paulo C. C. Freire
Universe 2024, 10(4), 160; https://doi.org/10.3390/universe10040160 - 28 Mar 2024
Cited by 2 | Viewed by 1195
Abstract
Neutron stars (NSs) are compact objects that host the densest forms of matter in the observable universe, providing unique opportunities to study the behaviour of matter at extreme densities. While precision measurements of NS masses through pulsar timing have imposed effective constraints on [...] Read more.
Neutron stars (NSs) are compact objects that host the densest forms of matter in the observable universe, providing unique opportunities to study the behaviour of matter at extreme densities. While precision measurements of NS masses through pulsar timing have imposed effective constraints on the equation of state (EoS) of dense matter, accurately determining the radius or moment of inertia (MoI) of an NS remains a major challenge. This article presents a detailed review on measuring the Lense–Thirring (LT) precession effect in the orbit of binary pulsars, which would give access to the MoI of NSs and offer further constraints on the EoS. We discuss the suitability of certain classes of binary pulsars for measuring the LT precession from the perspective of binary star evolution and highlight five pulsars that exhibit properties promising to realise these goals in the near future. Finally, discoveries of compact binaries with shorter orbital periods hold the potential to greatly enhance measurements of the MoI of NSs. The MoI measurements of binary pulsars are pivotal to advancing our understanding of matter at supranuclear densities, as well as improving the precision of gravity tests, such as the orbital decay due to gravitational wave emission, and of tests of alternative gravity theories. Full article
(This article belongs to the Special Issue Studies in Neutron Stars)
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