arXiv subjects

Ch. C. Moustakidis

Publications and source records attributed to Ch. C. Moustakidis.

At least 19 recordsLinked to original sources

arXiv2026

Hadron-quark phase transitions along proto-neutron-star evolution

The new era of multi-messenger astronomy requires the accurate and self-consistent derivation of the nuclear equation of state at high temperature. In the present work, we focused on the calculation of hot hybrid equations of state, studying the different evolution stages of a proto-neutron star with a quark matter core (proto-hybrid star). For the hadronic matter we used two distinct Skyrme effective interactions, while for quark matter the well-known vector MIT bag model was employed. To model the era of trapped neutrinos in the system we considered the global conservation of lepton fraction which resulted in an equation of state with an extended mixed phase. For periods following the neutrino diffusion phase of a proto-neutron star, the equations of state were modelled using both the Maxwell and the Gibbs construction depending on the assumption for either local or global electric-charge conservation. With the use of the derived hybrid models, we solved the Tolman-Oppenheimer-Volkov equations to describe the corresponding hybrid star configurations. Finally, we investigated how the structure of proto-hybrid stars evolves, using constant rest mass sequences. We found that regardless of whether electric-charge is globally or locally conserved, the earlier stages of a hybrid star's life may play a crucial role on the determination of its maximum possible gravitational mass in later stages.

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arXiv2026

Impact of Kaon Condensation on the Thermal Evolution of the CCO in HESS J1731--347 Supernova Remnant

Recent analyses of the central compact object in the HESS J1731--347 supernova remnant suggest an unusual combination of a low mass and small radius, while its thermal emission indicates a relatively high surface temperature at its estimated age. Reconciling these structural and thermal properties within a unified theoretical framework may provide important constraints on the equation of state and composition of dense matter. In this work, we investigate the thermal consequences of negatively charged kaon condensation, an exotic phase that softens the equation of state and facilitate the reproduction of the inferred bulk properties of HESS J1731--347. We find that the onset of kaon condensation strongly accelerates the thermal evolution, leading to surface temperature substantially below the observationally inferred range. Within the adopted cooling framework, kaon condensation therefore cannot simultaneously account for the structural and thermal properties of HESS J1731--347.

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arXiv2026

Classification of Compact Stars via Machine Learning and Neural Network Models

Recent advances in multimessenger astronomy, particularly through gravitational-wave observations of compact-object mergers, have significantly improved our understanding of dense matter. Nevertheless, the internal composition of compact stars remains uncertain. Depending on the underlying equation of state (EoS), these objects may be neutron stars composed primarily of nucleons, quark stars made of deconfined quark matter, or hybrid stars containing both hadronic and quark phases. More exotic constituents, such as hyperons, meson condensates, or dark matter, have also been proposed. In this work, we investigate whether the internal composition of compact stars can be inferred from observable quantities, including mass, radius, and tidal deformability. To address this problem, we employ machine-learning and deep-learning techniques trained on a larg dataset of EoSs describing both neutron stars and quark stars. From these EoSs, we generate the corresponding mass radius relations spanning a wide range of stellar configurations. The resulting dataset is used to train and evaluate classification models aimed at identifying the nature of compact objects from their macroscopic properties. Our results indicate that suitable combinations of observables can distinguish neutron stars from quark stars with very high accuracy. These findings demonstrate the potential of machine-learning approaches as tools for probing the composition of dense matter. However, further studies incorporating additional scenarios, including hybrid stars and other exotic forms of matter, are required to establish the robustness and general applicability of this methodology.

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arXiv2026

Binding energy of compact stars and their non-radial oscillations

In the past years, a significant effort has been made with the scope of determining correlations, involving compact star properties, that are independent of the nuclear equation of state. Such universal relations are of utmost importance as they allow for the imposition of constraints on stellar properties without directly measuring them and they may also serve as a probe of General Relativity. In the present study, we investigated the possible existence of a universal relation between the binding energy of compact stars and the frequency of their non-radial oscillations. The main motivation was related to the fact that both of the aforementioned quantities might be measured in the occurrence of a supernova explosion. Interestingly, we found that there is a empirical relation between the oscillation frequency and the binding energy for both $f$ and $p_1$ modes, assuming hadronic stellar matter. The inclusion of hybrid equations of state, incorporating sharp phase transitions, was shown to result into deviations from the aforementioned quasi-universal relation.

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arXiv2026

Neutron Dark Decay and Exotic Compact Objects

Recent measurements of the compact star XTE J1814-338, with a mass of $M=1.2_{-0.05}^{+0.05}\ M_{\odot}$ and a radius of $R=7_{-0.4}^{+0.4} \ {\rm Km}$ alongside those of HESS J1731-347, which has a mass of $M=0.77_{-0.17}^{+0.20}\ M_{\odot}$ and a radius of $R=10.4_{-0.78}^{+0.86} \ {\rm Km}$, provide compelling evidence for the potential existence of exotic matter in neutron star cores. These observations offer important insights into the equation of state of dense nuclear matter. Recently, Fornal and Grinstein, in order to overcame the discrepancy between the neutron lifetime measured in beam and bottle experiments, proposed the existence of neutron dark decay. In the present work, an effort is made to connect the interpretation of the above exotic compact objects with the possible existence of dark particles, assumed to be products of neutron dark decay. Our hypothesis offers an advantage over comparable proposals, as the coexistence of dark matter and hadronic matter within neutron stars emerges from an intrinsic mechanism, thereby obviating the need to invoke external merger-related processes. It is still unclear to what extent the proposed dark decay of the neutron is affected by the extreme environment within neutron stars. Within this framework, we examined the case in which a mechanism suppressing the dark neutron decay becomes operative at densities few times above nuclear saturation density. We found that the proposed alternative explanation accommodates the simultaneous existence of neutron dark decay while consistently predicting both the two solar mass limit and the presence of compact objects with subsolar masses.

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arXiv2026

On the Cooling of Compact Stars in Light of the HESS J1731-347 Remnant

Recent analyses on the central compact object in the HESS J1731-347 supernova remnant reported not only surprising structural properties (mass $M$ and radius $R$), but also an interesting thermal evolution. More precisely, it has been estimated that $M=0.77^{+0.20}_{-0.17}M_\odot$ and $R=10.4^{+0.86}_{-0.78}$ km (at the $1σ$ level), while a redshited surface temperature of $153^{+4}_ {-2}$ keV at an age of 2-6 kyrs has been reported. In the present work, we conduct an in-depth investigation on the possible nature (hadronic, hybrid, quark) of this compact object by attempting to not only explain its mass and radius but also the corresponding estimations for its temperature and age. In the case of hybrid stars we also examine possible effects of the symmetry energy on the activation of different neutrino emitting process, and hence on the resulting cooling curves. We found that the reported temperature and age may be compatible to hadronic stellar configurations regardless of whether pairing effects are included. In the scenario of hybrid stars, we found that the strange quark matter core has to be in a superconducting state in order to reach an agreement with the observational constraints. In addition, the hadronic phase must be soft enough so that the direct Urca process is not activated. Furthermore, we have shown that the considered cooling constraints can be reconciled within the framework of strange stars. However, quark matter has to be in a superconducting state and the quark direct Urca process needs to be blocked.

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arXiv2025

Machine and Deep Learning Regression for Compact Object Equations of State

A central open problem in nuclear physics is the determination of a physically robust equation of state (EoS) for dense nuclear matter, which directly informs our understanding of the internal composition and macroscopic properties of compact objects such as neutron stars and quark stars. Traditional efforts have relied primarily on theoretical modeling grounded in nuclear and particle physics, with subsequent validation against empirical constraints from heavy ion collisions and, increasingly, multimessenger astrophysical observations. Recent developments, however, have introduced complementary analytical strategies that merge theoretical modeling with advanced data driven methodologies. In particular, Bayesian inference, machine learning, and deep learning have emerged as powerful tools for constraining the EoS and extracting physical insight from complex observational datasets. In this work, we employ state of the art machine learning and deep learning techniques to analyze mass radius relations of compact objects with the aim of reconstructing or inferring their underlying equations of state. The analysis is based on an extensive library of physically consistent, multimodal EoSs for neutron stars and a corresponding set for quark stars, each constructed to satisfy established theoretical and observational constraints. By leveraging the predictive capacity of these computational frameworks, we demonstrate the potential of data-driven approaches to provide refined insights into the behavior of matter at supranuclear densities and to contribute to a more unified understanding of the dense matter EoS.

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arXiv2025

Thermal Quarkyonic Matter and Its Implications for Neutron Star Structure

The structure and basic properties of dense nuclear matter still remain one of the open problems of Physics. In particular, the composition of the matter that composes neutron stars is under theoretical and experimental investigation. Among the theories that have been proposed, apart from the classical one where the composition is dominated by hadrons, the existence or coexistence of deconfined quark matter is a dominant guess. An approach towards this solution is the phenomenological view according to which the existence of quarkyonic matter plays a dominant role in the construction of the equation of state (EOS). According to it the structure of the EOS is based on the existence of the quarkyonic particle which is a hybrid state of a particle that combines properties of hadronic and quark matter with a corresponding representation in momentum space. In this paper we propose a phenomenological model for hot quarkyonic matter, borrowed from corresponding applications in hadronic models, where the interaction in the quarkyonic matter depends not only on the position but also on the momentum of the quarkyonic particles. This consideration, as we demonstrate, can have a remarkable consequence on the shape of the EOS and thus on the properties of neutron stars, especially in those for which the effect of temperature is significant, offering a sufficiently flexible model. Comparison with recent observational data can place constraints on the parameterization of the particular model and help improve its reliability.

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arXiv2025

Statistical complexity as a probe of mass and phase structure in compact objects

In this work, we present a comprehensive and systematic study of the statistical complexity, originally introduced by López-Ruiz, Mancini, and Calbet [Phys. Lett. A 209, 321-326 (1995)], across a broad range of compact star models. We explore how complexity correlates not only with macroscopic observables such as mass and radius, but also with the microscopic characteristics of the underlying equation of state. By incorporating both realistic equations of state and analytical solutions to Einstein's field equations, we demonstrate that gravitational mass plays a dominant role in determining the behavior of complexity. Furthermore, we show that strong phase transitions within the stellar interior, such as those hypothesized in hybrid stars, can manifest as distinct features in the complexity profile, offering a potential informational signature of such transitions. This work offers new insights into the link between information theory and compact object physics, highlighting complexity's potential as a diagnostic tool in astrophysics.

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arXiv2025

Simultaneous explanation of XTE J1814-338 and HESS J1731-347 objects using ${K^{-}}$ and ${\bar{K}^{0}}$ condensates

The recent observation of the compact star XTE J1814-338 with a mass of $M=1.2^{+0.05}_{-0.05}~{\rm M_{\odot}}$ and a radius of $R=7^{+0.4}_{-0.4}$ km, together with the HESS J1731-347, which has a mass of $M=0.77^{+0.20}_{-0.17}~{\rm M_{\odot}}$ and a radius of $R=10.4^{+0.86}_{-0.78}$ km, shows they provide evidence for the possible presence of exotic matter in the core of neutron stars and significantly enhance our understanding of the equation of state for the dense nuclear matter. In the present study, we investigate the possible existence of negative charged kaons and neutral anti-kaons in neutron stars by employing the relativistic mean field model with first order kaonic (${K^{-}}$ and ${\bar{K}^{0}}$) condensates. To the best of our knowledge, this represents a first alternative attempt aimed to explain the bulk properties of the XTE J1814-338 object and at the same time the HESS J1731-347 object, using a mixture of kaons condensation in dense nuclear matter. In addition, we compare our analysis approach with the recent observation of PSR J0437-4715 and PSR J1231-1411 pulsars, proposing that to simultaneously explain the current variety of astrophysical objects, it is essential to resurrect a scenario of two distinct branches, each corresponding to a different composition of nuclear matter.

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arXiv2025

Neutron Dark Decay in Neutron Stars: The Role of the Symmetry Energy

We conduct a systematic investigation of the influence of the nuclear symmetry energy on the proposed neutron decay into dark matter particles within the cores of neutron stars. Unlike the majority of previous studies that considered only pure neutron matter, the present analysis is extended to encompass $β$-stable nuclear matter. Furthermore, in relation to previous studies, the interactions between dark matter and baryons are incorporated and systematically studied regarding their effect on the structure of neutron stars. Our findings indicate that the nuclear symmetry energy plays a critical role in shaping the total equation of state (EoS) for dense neutron star matter containing dark sector components. The strength of interactions among dark matter particles, as well as between dark matter and baryons, is shown to be pivotal in determining both the composition and the macroscopic properties of neutron stars. The concurrent tuning of interaction strengths alongside the symmetry energy parameters may facilitate a more accurate reproduction of recent observational data relevant to neutron star properties. In any case, the extent to which the proposed dark decay of the neutron is affected by the extreme conditions prevailing in the interior of neutron stars remains an open problem.

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arXiv2025

Neutron Star with Dark Matter Admixture: A Candidate for Bridging the Mass Gap

Neutron stars, white dwarfs and black holes are the after death remnants of massive stars. However, according to the most recent observations, the neutron stars maximum mass is between $2.0-2.5 M_{\odot}$ while black holes of less than 5 $M_{\odot}$ has not yet been observed. The region between the most massive neutron star and the least massive black hole is called the mass-gap. If indeed its existence is confirmed by future observations, that indicates a gap in our understanding which seeks for explanation. In addition, the existence of compact objects within the mass-gap should also be supported with the help of possible new theoretical scenarios. In this study, we propose a possible explanation for the existence of compact objects within the mass-gap region. Specifically, we propose that the mass-gap region could be bridged by the existence of a hybrid compact object, composed of hadronic and self interacting and non-annihilating fermionic dark matter, considering that the interaction between these two fluids it is only gravitational. Fundamental questions about how these objects form and how they can be detected are also addressed.

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arXiv2025

Implications of the recent neutron decay measurements on the properties of compact objects -- a dark star with nucleonic shell ?

Recent experimental observation suggests that neutron decay is always accompanied by emission of electron while in 1% of cases proton is not emitted. We develop a scenario kinematically compatible with experimental observation, where neutron decay results in production of two dark matter particles of about half the mass of neutron and test properties of neutron stars with admixture of such particles. Constraints on mass and coupling to vector dark boson are obtained. The structure of the compact object is modified to a dark star with a shell of nucleonic matter around the nuclear saturation density.

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arXiv2025

Confronting recent light compact star observations with color-flavor locked quark matter

Recent analyses on the properties of the central compact object in the HESS J1731-347 remnant and the PSR J1231-1411 pulsar indicated that these two compact objects are characterized by similar (low) masses and possibly different radii. This paper aims at reconciling the aforementioned measurements by utilizing the widely employed color-flavor locked (CFL) MIT bag model. The main objective is related to the examination of the acceptable values for the color superconducting gap $Δ$ and the bag parameter $B$. Furthermore, our analysis involves two distinct hypotheses for the nature of compact stars. Firstly, we considered the case of absolute stability for strange quark matter and we found that it is possible to explain both measurements, while also respecting the latest astronomical constraints on the masses and radii of compact stars. Secondly, we studied the case of hybrid stellar matter (transition from hadrons to quarks), and concluded that, when early phase transitions are considered, the simultaneous reconciliation of both measurements leads to results that are inconsistent to the existence of massive compact stars. However, we showed that all current constraints may be satisfied under the consideration that the HESS J1731-347 remnant contains a slow stable hybrid star.

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arXiv2025

Constraints on color-flavor locked quark matter in view of the HESS J1731-347 measurement

Astrophysical observations play a crucial role in understanding the processes within compact stars. A recent study measured the central object in the HESS J1731-347 supernova remnant (SNR), estimating its mass at $M=0.77^{+0.20}_ {-0.17} \ M_\odot$ and radius at $R = 10.40^{+0.86}_{-0.78} \ \mathrm{km}$, identifying it as the lightest neutron star ever observed. Conventional models suggest neutron stars form with a minimum gravitational mass of approximately $1.17 \ M_\odot$, raising the question of whether this object is a typical neutron star or possibly an "exotic" star. To investigate, we utilize the Color-Flavor Locked (CFL) equation of state (EoS), integrating data from the HESS J1731-347 measurement with pulsar observations and gravitational wave detections. Additionally, we construct hybrid EoS by combining the MDI-APR1 (hadronic) and CFL (quark) EoS, introducing a phase transition through Maxwell construction. Our findings reveal that absolutely stable CFL quark matter effectively explains all observed measurements, including the central object of HESS J1731-347, whereas hybrid models incorporating the CFL MIT Bag model cannot account for the masses of the most massive observed pulsars.

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arXiv2025

XTE J1814-338: A potential hybrid star candidate

A recent analysis on the properties of the XTE J1814-338 pulsar yielded a small radius value around ~ 7 km. Notably, this estimation is significantly lower compared to the currently inferred values for the radius of neutron stars (as they are derived from both theoretical calculations and corresponding observations). In this paper, we focus on the construction of hybrid equations of state (EOSs) for the possible reconciliation of the exotic XTE J1814-338 properties. Our analysis indicates that an equation of state involving a sufficiently strong phase transition could potentially lead to the explanation of the size and mass of such a compact object. By examining the sign of the $dM/dP_c$ slope, we found that stellar configurations compatible to XTE J1814-338 are only stable for a "stiff" low density phase. For a "soft" hadronic EOS selection the stability of the resulting configurations may not be satisfied in terms of the turning point criterion. However, a complementary analysis, for the radial oscillations of the aforementioned configurations, indicated their stability when a slow phase conversion is considered.

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arXiv2025

Speed of sound bounds and first-order phase transitions in compact stars

In the present study, we employ three distinct, physically motivated speed of sound bounds to construct hybrid models, where the high-density phase is described by the maximally stiff equation of state. In particular, we consider the bounds related to special relativity, relativistic kinetic theory and conformality. The low-density hadronic phase is described by a state-of-the-art microscopic relativistic Brueckner-Hartree-Fock theory. This work aims to access the effect of the different speed of sound constraints on the relevant parameter space of the key parameters of first-order phase transitions by utilizing recent astronomical data. This involves a systematic analysis that also includes two distinct schemes for the construction of hybrid models (abrupt and smooth). Finally, a relevant discussion is conducted on the possible occurrence of a thermodynamic inconsistency that is related to the stability of the high-density phase over hadronic matter at large densities.

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arXiv2025

Hybrid stars in light of the HESS J1731-347 remnant and the PREX-II experiment

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 $^{208}$Pb, points towards 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 hybrid equations of state are strongly disfavored by the observation of $\sim2 M_\odot$ pulsars. However, the introduction of a Gaussian density dependence yields results that are compatible with the conservative $2 M_\odot$ constraint. The utilization of recent data based on the observation of PSR J0030+0451, PSR J0952-0607 and GW190814 allows for the imposition of additional constraints on the relevant parameters and the stiffness of the two phases. Interestingly, we find that the derived hybrid equations of state do not satisfy the PSR J0030+0451 constraints in $1σ$ and only marginally agree with the $2σ$ estimations. In addition, we estimate that the observation of massive pulsars, like PSR J0952-0607, in combination with the existence of HESS J1731-347, may require a strong phase transition below $\sim 1.7n_0$. Finally, we show that the supermassive compact object involved in GW190814 could potentially be explained as a rapidly rotating hybrid star.

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