Charalampos Moustakidis

The recent analysis on the central compact object in the HESS J1731-347 remnant suggests interestingly small values for its mass and radius. Such an observation favors soft nuclear models that may be challenged by the observation of massive compact stars. In contrast, the recent PREX-II experiment, concerning the neutron skin thickness of Pb 208, points toward stiff equations of state that favor larger compact star radii. In the present study, we aim to explore the compatibility between stiff hadronic equations of state (favored by PREX-II) and the HESS J1731-347 remnant in the context of hybrid stars. For the construction of hybrid equations of state we use three widely employed Skyrme models combined with the well-known vector MIT bag model. Furthermore we consider two different scenarios concerning the energy density of the bag. In the first case, that of a constant bag parameter, we find that the resulting …

We investigate the hypothetical X17 boson on neutron stars and quark stars (QSs) using various hadronic equation of states (EoSs) with phenomenological or microscopic origin. Our aim is to set realistic constraints on its coupling constant and the mass scaling, with respect to causality and various possible upper mass limits and the dimensionless tidal deformability Λ 1.4. In particular, we pay special attention to two main phenomenological parameters of the X17, one is related to the coupling constant g that it has with hadrons or quarks and the other with the in-medium effects through regulator C. Both are very crucial concerning the contribution on the total energy density and pressure. In the case of considering the X17 as a carrier of nuclear force in relativistic mean field theory, an admixture into the vector boson segment was constrained by 20% and 30%. In our investigation, we came to the general conclusion …

The nuclear symmetry energy plays important role on the structure of finite nuclei as well as on the bulk properties of neutron stars. However, its values at high densities are completely uncertain and the corresponding experimental data have a large error. One possibility to determine or at least estimate the values at high densities is with the help of neutron star observations. Recently, observations of gravitational waves from merging processes of binary neutron star systems provide useful information on both their radius and tidal deformability, quantities directly related to the symmetry energy. In this work, an attempt is made in this direction, namely to see how recent observations can help to constrain the structure of finite nuclei. In particular, in the present study we parameterize the equation of state which describes the asymmetric and symmetric nuclear mater with the help of the parameter , where is the incompressibility and the slope parameter. The parameter is a regulator of the stiffness of the equation of state. We expect that the values of affect both the properties of finite nuclei as well as of the neutron star properties (where the role of the isovector interaction plays important role). It is natural to expect that constraints, via the parameter on finite nuclei will imply constraints on the neutron star properties and vice versa. In view of the above statements we propose a simple but self-consistent method to examine simultaneously the effects of the parameter on the properties of finite nuclei and neutron stars. We found constraints on the latter systems via combination by the recent experiments (PREX-2) and observational data …

The synthesis of hyper-heavy elements is investigated under conditions simulating neutron star environment. The constrained molecular dynamics approach is used to simulate low energy collisions of extremely n-rich nuclei. A new type of the fusion barrier due to a “neutron wind” is observed when the effect of neutron star environment (screening of Coulomb interaction) is introduced implicitly. When introducing also a background of surrounding nuclei, the nuclear fusion becomes possible down to temperatures of 10 8 K and synthesis of extremely heavy and n-rich nuclei appears feasible. A possible existence of hyper-heavy nuclei in a neutron star environment could provide a mechanism of extra coherent neutrino scattering or an additional mechanism, resulting in x-ray burst or a gravitational wave signal and, thus, becoming another crucial process adding new information to the suggested models on neutron …

We systematically extend the statement that the configurational entropy provides an alternative approach to studying gravitational stability of compact objects, carried out in the previous work of Gleiser and Jiang [Phys. Rev. D 92, 044046 (2015)]. Inspired by that paper, we try to answer the crucial question; Is there any one-to-one correspondence between the minimum of the configurational entropy and the stability point for each realistic equation of state? In view of the above question, we focus on neutron stars, quark stars, as well as on the third family of compact stars (hybrid stars), where a possible phase transition may lead to the existence of twin stars (stars with equal mass but different radius). In each case, we use a large set of equations of state investigating the possibility to find correlations between the stability region obtained from the traditional perturbation methods to the one obtained by the minimum of …

Neutron stars are like nuclear physics laboratories, providing a unique opportunity to apply and search for new physics. In that spirit, we explored novel concepts of nuclear physics studied in a neutron star environment. Firstly, we investigated the reported 17 MeV boson, which has been proposed as an explanation to the 8Be, 4He and 12C anomaly, in the context of its possible influence on the neutron star structure, defining a universal Equation of State. Next, we investigated the synthesis of hyper-heavy elements under conditions simulating the neutron star environment.

Nambu and Jona-Lasinio (NJL) inspired by BCS theory of superconductivity, have qualitatively explained that strong enough attraction of quarks can break SU (Nf) A chiral symmetry spontaneously and, among many other effects, create near-massless pion’s. Chiral effective Lagrangian’s and related theory have lead to one important input, a nonzero quark condensate< qq>≠ 0Another explanation of chiral symmetry breaking by the instanton ensemble is based on the collectivization of the instanton zero modes, into the so called “zero mode zone"(ZMZ). Due to nonzero matrix elements of the Dirac operator, near-zero Dirac eigenvalues are residing within a strip of small width:

Till today, the nature of Dark Matter (DM) remains elusive despite all our efforts. This missing matter of the universe has not been observed by the already operating DM direct-detection experiments, but we can infer its gravitational effects. Galaxies and clusters of galaxies are most likely to contain DM trapped to their gravitational field. This leads us to the natural assumption that compact objects might contain DM too. Among the compact objects exist in galaxies, neutron stars considered as natural laboratories, where theories can be tested, and observational data can be received. Thus, many models of DM they have proposed it's presence in those stars. By employing the two fluid model, we discovered a stable area in the M-R diagram of a celestial formation consisting of neutron and DM that is substantial in size and vast in dimensions. This formation spans hundreds of kilometers in diameter and possesses a mass equivalent to 100 or more times that of our sun. To elucidate, this entity resembles an enormous celestial body of DM, with a neutron star at its core. This implies that a supramassive stellar compact entity can exist without encountering any issues of stability and without undergoing a collapse into a black hole. In any case, the present theoretical prediction can, if combined with corresponding observations, shed light on the existence of DM and even more on its basic properties.

In agreement with the constantly increasing gravitational wave events, new aspects of the internal structure of compact stars can be considered. A scenario in which a first-order transition takes place inside these stars is of particular interest, as it can lead, under certain conditions, to a third gravitationally stable branch (besides white dwarfs and neutron stars), the twin stars. The new branch yields stars with the same mass as normal compact stars but quite different radii. We focus on hybrid stars undergoing a hadron-to-quark phase transition near their core and how this new stable configuration arises. Emphasis is given on the aspects of the phase transition and its parametrization in two different ways—namely, with the Maxwell and Gibbs constructions. We systematically study the gravitational mass, the radius, and the tidal deformability, and we compare them with the predictions of the recent observation by the …

The observation and distinction of two compact stars with an identical mass but a different radius would be a clear sign of hadron-quark phase transition in nuclear matter. Motivated by studies searching for significant deviations in the observables of twin stars, we investigate the differences that manifest in their r-mode instability windows and spin-down evolution. Firstly, we obtain a set of hybrid equations of state (which predict the existence of a third stable branch of compact objects) by employing the well-known Maxwell construction within the phenomenological framework of constant speed of sound parametrization. Then, we systematically study the influence of certain parameters, such as the energy density jump (in the resulting hybrid equation of state) and the crust elasticity, on the deviations appearing in the r-mode instability windows and spin-down evolution of twin stars. We conclude that two stars with an …

In the past few years, a lot of studies devoted to the discovery of universal relations (equation of state independent relations). The significance of such expressions can be understood if we consider that they offer the opportunity for testing general relativity in a way that is independent of the nuclear equation of state and they also allow us to impose constraints on the structure of neutron stars. The aim of this work is twofold. Firstly, we wish to clarify if hot equations of state are able to reproduce established universal relations. Secondly, we investigate a possible universal connection between the binding energy and the dimensionless tidal deformability of a neutron star. These two bulk properties are associated with two very important candidates for multimessenger signals, binary neutron star mergers and supernova explosions. We find that the predictions of hot equations of state do not agree with the predictions from accepted universal relations. Subsequently, the use of universal relations, when thermal effects are present, may be erroneous. Additionally, we find that, for moderate neutron star masses, the binding energy and the dimensionless tidal deformability of a neutron star satisfy a universal relation. The latter allows us to impose constraints on the binding energy of 1.4 M sun neutron star, using information from the analysis of the GW170817 event. Finally, we are able to present a universal relation between the compactness, the binding energy and the dimensionless tidal deformability, which is independent of the employed equation of state for zero and finite temperature.

Over the last few years, the detection of gravitational waves from binary neutron star systems has rekindled our hopes for a deeper understanding of the unknown nature of ultradense matter. In particular, gravitational wave constraints on the tidal deformability of a neutron star can be translated into constraints on several neutron star properties using a set of universal relations. Apart from binary neutron star mergers, supernova explosions are also important candidates for the detection of multimessenger signals. Such observations may allow us to impose significant constraints on the binding energy of neutron stars. The purpose of the present study is twofold. Firstly, we investigate the agreement of finite temperature equations of state with established universal relations. Secondly, we examine the possible existence of a universal relation between the binding energy and the dimensionless tidal deformability, which are the bulk properties connected to the most promising sources for multimessenger signals. We find that hot equations of state are not always compatible with accepted universal relations. Therefore, the use of such expressions for probing general relativity or imposing constraints on the structure of neutron stars would be inconclusive (when thermal effects are present). Additionally, we show that the binding energy and the dimensionless tidal deformability exhibit a universal trend at least for moderate neutron star masses. The latter allows us to set bounds on the binding energy of a 1.4 M⊙ neutron star using data from the GW170817 event. Finally, we provide a relation between the compactness, the binding energy and the …

Supplementing the experimental data of excited states of nuclei with those of the canonical assemble, we determine the temperature-dependent specific heat formula, with finite system corrections, in the mass range of 22< A< 206. Phase transition structures in shapes of peaks in the specific heat diagram have been identified as distinct pairing processes and are used in comparison to the pairing energy correction term (Gilbert and Cameron, 1965). Through mass dependent pairing gap equations the addition of the proton–neutron pairing procedure and finally the distinction of the pairing tendency, of each nuclei, was made possible.

The study of binary neutron stars mergers by the detection of the emitted gravitational waves is one of the most promised tools to study the properties of dense nuclear matter at high densities. It is worth claiming that strong evidence that the temperature of the stars during the last orbits before coalescing is very low, T≪ 1 MeV (hereafter cold case or cold star; k B= 1), does not exist. Nevertheless, theoretical studies suggest that the temperature concerning the inspiral phase, could reach even a few MeV. The heating process of the interior of the neutron stars is as follows; tides transfer mechanical energy and angular momentum to the star at the expense of the orbit, where friction within the star converts the mechanical energy into heat. During the inspiral, these effects are potentially detectable. Different treatments have been used to estimate the transfer of the mechanical energy and the size of the tidal friction …

On 14 August 2019, the LIGO/Virgo collaboration observed a compact object with mass ∼2.59−0.09+0.08M⊙, as a component of a system where the main companion was a black hole with mass ∼23M⊙. A scientific debate initiated concerning the identification of the low mass component, as it falls into the neutron star–black hole mass gap. The understanding of the nature of GW190814 event will offer rich information concerning open issues, the speed of sound and the possible phase transition into other degrees of freedom. In the present work, we made an effort to probe the nuclear equation of state along with the GW190814 event. Firstly, we examine possible constraints on the nuclear equation of state inferred from the consideration that the low mass companion is a slow or rapidly rotating neutron star. In this case, the role of the upper bounds on the speed of sound is revealed, in connection with the dense nuclear matter properties. Secondly, we systematically study the tidal deformability of a possible high mass candidate existing as an individual star or as a component one in a binary neutron star system. As the tidal deformability and radius are quantities very sensitive on the neutron star equation of state, they are excellent counters on dense matter properties. We conjecture that similar isolated neutron stars or systems may exist in the universe and their possible future observation will shed light on the maximum neutron star mass problem.

The equation of state along with the hot neutron star matter provides an important framework for studying essential astrophysical phenomena, such as the formation of protoneutron stars and the neutron star merger remnants. The equations of state of the fluid in the interior of the star for the above dynamical phenomena are based on the momentum-dependent interaction model and state-of-the-art microscopic data. In particular, we construct them at finite temperature with beta-stable matter, and finite entropy per baryon with varying proton fractions. Furthermore, we investigate in detail the thermal and rotation with the Kepler frequency effects on neutron star quantities, including the mass and radius, the frequency, the Kerr parameter, the moment of inertia, the central baryon density, etc. Thermal support and its effect on isolated neutron stars, as well as on the postmerger remnants, could provide useful insight and robust constraints on the equation of state of nuclear matter.

Nuclear mass is a key property of atomic nuclei. The accurate determination of nuclear masses provides information for the nuclear shell structure and nuclear deformations. The FRS Ion Catcher experimental setup at GSI, Darmstadt was used to perform mass measurements of 252 Cf fragments using a multiple-reflection time-of-flight mass spectrometer. The reported results are compared to previous measurements and their implication in the estimation of two-neutron separation energy is discussed.

Neutron stars constitute a very promising natural laboratory for studying the properties of dense nuclear matter and the equation of state. One microscopic parameter that is of great interest is the speed of sound, especially the upper bound on it. This work is based on the idea to examine possible constraints on the speed of sound by using neutron stars. For this purpose, in our study, we use the observed effective tidal deformability from binary neutron star systems as a tool to impose constraints on the equation of state through the upper bound on the speed of sound. In our approach, we parametrize the stiffness of the equation of state by using the speed of sound, for various transition density values. The two recent observations of binary neutron star mergers from LIGO/VIRGO have been used to impose robust constraints. Furthermore, we extended our study in the hypothetical scenario of a very massive neutron star by using the recent observation of the GW190814 event. The possibility of the existence of such a massive non-rotating neutron star cannot be excluded according to our study. Furthermore, we postulate what kind of future observations would be useful to impose more stringent constraints on the properties of the equation of state.

Neutron stars are a natural laboratory for studying dense nuclear matter in its extreme condition. The speed of sound is a microscopic parameter of great interest for studying the equation of state. In this work, we examine possible constraints on the upper bound of the speed of sound. In our study, we use the measured effective tidal deformability from the two recently detected binary neutron star mergers to impose constraints on the equation of state by using the upper bound on the speed of sound. In our approach, the stiffness of the equation of state is parametrized via two parameters: the speed of sound and the transition density. Moreover, our study is extended in the case of a very massive neutron star, using the recent detection of the GW190814 system. The tidal deformability and the upper bound on the speed of sound for such a massive neutron star are studied. According to our study, such a massive non-rotating neutron star may be existing. Finally, we postulate the kind of future detections that could be useful to impose further constraints and broaden our knowledge on these open problems.

Neutron stars are the densest known objects in the universe and an ideal laboratory for the strange physics of super-condensed matter. Theoretical studies in connection with recent observational data of isolated neutron stars, as well as binary neutron stars systems, offer an excellent opportunity to provide robust solutions on the dense nuclear problem. In the present work, we review recent studies concerning the applications of various theoretical nuclear models on a few recent observations of binary neutron stars or neutron-star–black-hole systems. In particular, using a simple and well-established model, we parametrize the stiffness of the equation of state with the help of the speed of sound. Moreover, in comparison to the recent observations of two events by LIGO/VIRGO collaboration, GW170817 and GW190425, we suggest possible robust constraints. We also concentrate our theoretical study on the resent observation of a compact object with mass ∼2.59−0.09+0.08M⊙ (GW190814 event), as a component of a system where the main companion was a black hole with mass ∼23M⊙. There is scientific debate concerning the identification of the low mass component, as it falls into the neutron-star–black-hole mass gap. This is an important issue since understanding the nature of GW190814 event will offer rich information concerning the upper limit of the speed of sound in dense matter and the possible phase transition into other degrees of freedom. We systematically study the tidal deformability of a possible high-mass candidate existing as an individual star or as a component in a binary neutron star system. Finally, we provide some …

The knowledge of the equation of state is a key ingredient for many dynamical phenomena that depend sensitively on the hot and dense nuclear matter, such as the formation of protoneutron stars and hot neutron stars. In order to accurately describe them, we construct equations of state at FInite temperature and entropy per baryon for matter with varying proton fractions. This procedure is based on the momentum dependent interaction model and state-of-the-art microscopic data. In addition, we investigate the role of thermal and rotation effects on microscopic and macroscopic properties of neutron stars, including the mass and radius, the frequency, the Kerr parameter, the central baryon density, etc. The latter is also connected to the hot and rapidly rotating remnant after neutron star merger. The interplay between these quantities and data from late observations of neutron stars, both isolated and in matter of …

The study of neutron star mergers by the detection of the emitted gravitational waves is one of the most promised tools to study the properties of dense nuclear matter at high densities. It is worth claiming that, at the moment, strong evidence that the temperature of the stars is zero during the last orbits before coalescing does not exist. Contrariwise, there are some theoretical predictions suggesting that the star's temperature might even be a few MeV. According to the main theory, the tides transfer mechanical energy and angular momentum to the star at the expense of the orbit, where friction within the star converts the mechanical energy into heat. During the inspiral these effects are potentially detectable. Different treatments have been used to estimate the transfer of the mechanical energy and the size of the tidal friction, leading to different conclusions about the importance of pre-merger tidal effects. The present work is dedicated to the study of the effect of temperature on the tidal deformability of neutron stars during the inspiral of a neutron star system just before the merger. We applied a class of hot equations of state originated from various nuclear models and found that even for low values of temperature ( MeV) the effects on the basic ingredients of tidal deformability are not negligible. However, according to the main finding, the effect of the temperature on the tidal deformability is indistinguishable. The consequences of this unexpected result are discussed and analyzed.

One of the greatest interest and open problems in nuclear physics is the upper limit of the speed of sound in dense nuclear matter. Neutron stars, both in isolated and binary system cases, constitute a very promising natural laboratory for studying this kind of problem. This present work is based on one of our recent study, regarding the speed of sound and possible constraints that we can obtain from neutron stars. To be more specific, in the core of our study lies the examination of the speed of sound through the measured tidal deformability of a binary neutron star system (during the inspiral phase). The relation between the maximum neutron star mass scenario and the possible upper bound on the speed of sound is investigated. The approach that we used follows the contradiction between the recent observations of binary neutron star systems, in which the effective tidal deformability favors softer equations of state …

The nuclear symmetry energy plays an important role in the description of the properties of finite nuclei as well as neutron stars. Especially, for low values of baryon density, the accurate description of the crust-core interface strongly depends on the symmetry energy. Usually, the well known parabolic approximation is employed for the definition of the symmetry energy without avoiding some drawbacks. In the present paper, a class of nuclear models, suitable for the description of the inner and outer core of neutron stars, is applied in studying the effect of higher orders of the expansion of the energy on the location of the crust-core transition. The thermodynamical and dynamical methods are used for the determination of the transition density n t and pressure P t. The corresponding energy density functional is applied for the study of some relevant properties of both nonrotating and slowly rotating neutron stars. We …

In agreement with the gravitational-wave events which are constantly increasing, new aspects of the internal structure of compact stars have come to light. A scenario in which a first order transition takes place inside these stars is of particular interest as it can lead, under conditions, to a third gravitationally stable branch (besides white dwarfs and neutron stars). This is known as the twin star scenario. The new branch yields stars with the same mass as normal compact stars but quite different radii. In the current work, we focus on hybrid stars undergone a hadron to quark phase transition near their core and how this new stable configuration arises. Emphasis is to be given especially in the aspects of the phase transition and its parametrization in two different ways, namely with Maxwell construction and with Gibbs construction. Qualitative findings of mass-radius relations of these stars will also be presented.

The prediction of the equation of state of hot, dense nuclear matter is one of the most complicated and interesting problems in nuclear astrophysics. At the same time, knowledge of it is the basic ingredient for some of the most interesting studies. In the present work, we concentrate our study on the construction of the equation of state of hot, dense nuclear matter, related mainly to the interior of the neutron star. We employ a theoretical nuclear model, which includes momentum-dependent interaction among the nucleons, along with state-of-the-art microscopic calculations. Thermal effects are introduced in a self-consistent way, and a set of isothermal and isentropic equations of state are predicted. The predicted equations of state are used in order to acquire and extend the knowledge of the thermal effect on both nonrotating and rapidly rotating with the Kepler frequency neutron stars. The simultaneous study of …

The upper bound of the speed of sound in dense nuclear matter is one of the most interesting but still unsolved problems in nuclear physics. Theoretical studies in connection with recent observational data of isolated neutron stars as well as binary neutron stars systems offer an excellent opportunity to shed light on this problem. In the present work, we suggest a method to directly relate the measured tidal deformability (polarizability) of binary neutron stars system (before merger) to the maximum neutron star mass scenario and possible upper bound on the speed of sound. This method is based on the simple but efficient idea that while the upper limit of the effective tidal deformability favors soft equations of state, the recent high measured values of neutron star mass favor stiff ones. In the present work, first, using a simple well established model we parametrize the stiffness of the equation of state with the help of the …

The observation of maximally rotating neutron stars (in comparison to nonrotating ones) may provide more information on the behavior of nuclear matter at high densities. We provide a theoretical treatment concerning the effects of the upper bound of the sound speed in dense matter on the bulk properties of maximally rotating (at the mass-shedding limit) neutron stars. In particular, we consider two upper bounds for the speed of sound, v s= c and v s= c/3, and the one provided by relativistic kinetic theory. We investigate to what extent the possible predicted upper bounds (from various theories and conjectures) on the speed of sound constrain the ones of various key quantities, including the maximum mass and the corresponding radius, Keplerian frequency, Kerr parameter, and moment of inertia. We mainly focus on the lower proposed limit, v s= c/3, and we explore in which mass region a rotating neutron star …

Neutron stars are among the densest known objects in the universe and an ideal laboratory for the strange physics of supercondensed matter. While the simultaneous measurements of mass and radius of nonrotating neutron stars may impose constraints on the properties of the dense nuclear matter, the observation and study of maximally rotating ones, close to the mass-shedding limit, may lead to significantly further constraints. Theoretical predictions allow neutron stars to rotate extremely fast (even more than 2000 Hz). However, until this moment, the fastest observed rotating pulsar has a frequency of 716 Hz, much lower compared to the theoretical predictions. There are many suggestions for the mechanism which lead to this situation. In any case, the theoretical study of uniformly rotating neutron stars, along with accurate measurements, may offer rich information concerning the high-density part of the equation …

This book is intended for undergraduate or beginning graduate students. The net outcome is material to cover one integrated course on Nuclear and Particle Physics as well as Astrophysics. There are many advantages in teaching all these subjects together as they have become increasingly inseparable. From a theoretical point of view, understanding the similarities between atoms, nuclei and other hadrons and applying analogs from one to the other have been very effective in research and they have led to the development of all these fields. From an experimental point of view, a high energy experimentalist must understand nuclear physics, if he or she wants to construct new devices, like detectors, etc., appropriate for observing new high energy phenomena. Furthermore, an understanding of certain areas of astrophysics and the physics of the cosmos, demands a good grasp of both nuclear and particle physics. This book is intended as a menu from which the reader can pick material according to his or her taste and interests. The authors inserted proper cross references to make a specific selection by the reader from this menu as easily digestible as possible. The authors supplied sets of problems with varying degree of complexity, accompanied by hints or a sketch of the solution, if needed, in most chapters.