arXiv subjects

Ignazio Bombaci

Publications and source records attributed to Ignazio Bombaci.

At least 37 records · Page 2Linked to original sources

arXiv2018

Neutron star properties from optimized chiral nuclear interactions

We adopt two- and three-body nuclear forces derived at the next-to-next-to-leading-order (N2LO) in the framework of effective chiral perturbation theory (ChPT) to calculate the equation of state (EOS) of $β$-stable neutron star matter using the Brueckner--Hartree--Fock many-body approach. We use the recent optimized chiral two-body nuclear interaction at N2LO derived by \cite{ekstrom1} and two different parametrizations of the three-body N2LO interaction: the first one is fixed to reproduce the saturation point of symmetric nuclear matter while the second one is fixed to reproduce the binding energies of light atomic nuclei. We show that in the second case the properties of nuclear matter are not well determined whereas in the first case various empirical nuclear matter properties around the saturation density are well reproduced. We also calculate the nuclear symmetry energy $E_{sym}$ as a function of the nucleonic density and compare our results with the empirical constraints obtained using the excitation energies of isobaric analog states in nuclei and the experimental data on the neutron skin thickness of heavy nuclei. We next calculate various neutron star properties and in particular the mass-radius and mass-central density relations. We find that the adopted interactions based on a fully microscopic framework, are able to provide an EOS which is consistent with the present data of measured neutron star masses and in particular with the mass $M=2.01\pm0.04 M_\odot$ of the neutron star in PSR J0348+0432. We finally consider the possible presence of hyperons in the stellar core and we find a softening of the EOS and a substantial reduction of the stellar maximum mass in agreement with similar calculations present in the literature.

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arXiv2018

Equation of state of dense nuclear matter and neutron star structure from nuclear chiral interactions

We report a new microscopic equation of state (EOS) of dense symmetric nuclear matter, pure neutron matter, and asymmetric and $β$-stable nuclear matter at zero temperature using recent realistic two-body and three-body nuclear interactions derived in the framework of chiral perturbation theory (ChPT) and including the $Δ(1232)$ isobar intermediate state. This EOS is provided in tabular form and in parametrized form ready for use in numerical general relativity simulations of binary neutron star merging. Here we use our new EOS for $β$-stable nuclear matter to compute various structural properties of non-rotating neutron stars.The EOS is derived using the Brueckner--Bethe--Goldstone quantum many-body theory in the Brueckner--Hartree--Fock approximation. Neutron star properties are next computed solving numerically the Tolman--Oppenheimer--Volkov structure equations. Our EOS models are able to reproduce the empirical saturation point of symmetric nuclear matter, the symmetry energy $E_{sym}$, and its slope parameter $L$ at the empirical saturation density $n_{0}$. In addition, our EOS models are compatible with experimental data from collisions between heavy nuclei at energies ranging from a few tens of MeV up to several hundreds of MeV per nucleon. These experiments provide a selective test for constraining the nuclear EOS up to $\sim 4 n_0$. Our EOS models are consistent with present measured neutron star masses and particularly with the mass $M = 2.01 \pm 0.04 \, M_{\odot}$ of the neutron stars in PSR~J0348+0432.

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arXiv2018

No axion-like particles from core-collapse supernovae?

A strong bound on the properties of axion-like particles (ALPs) has been set by assuming that ALPs are emitted by the protoneutron star just before the core-bounce in Galactic core-collapse supernovae, and that these ALPs subsequently convert to $γ$-ray photons which ought to be detected by a $γ$-ray mission. This argument has been applied to supernova 1987A to derive the bound on the ALP-photon coupling $g_{a γγ} \lesssim 5.3 \cdot 10^{- 12} \, {\rm GeV}^{- 1}$ for an ALP mass $m_a \lesssim 4.4 \cdot 10^{- 10} \, {\rm eV}$, and can be applied to the next Galactic supernova to derive the even stronger bound $g_{a γγ} \lesssim 2 \cdot 10^{- 13} \, {\rm GeV}^{- 1}$ for an ALP mass $m_a \lesssim 10^{- 9} \, {\rm eV}$. We carefully analyze the considered ALP production mechanism and find that it is oversimplified to an unacceptable extent. By taking into account the minimal ingredients required by a realistic analysis, we conclude that the previous results are doomed to failure. As a consequence, all papers quoting the above bound should be properly revised. Yet, since we are unable to rule out the possibility that protoneutron stars emit ALPs, in case a core-collapse supernova explodes in the Galaxy the $γ$-ray satellite missions active at that time should look for photons possibly coming from the supernova.

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arXiv2017

Two coexisting families of compact stars: observational implications for millisecond pulsars

It is usually thought that a single equation of state (EoS) model "correctly" represents cores of all compact stars. Here we emphasize that two families of compact stars, viz., neutron stars and strange stars, can coexist in nature, and that neutron stars can get converted to strange stars through the nucleation process of quark matter in the stellar center. From our fully general relativistic numerical computations of the structures of fast-spinning compact stars, known as millisecond pulsars, we find that such a stellar conversion causes a simultaneous spin-up and decrease in gravitational mass of these stars. This is a new type of millisecond pulsar evolution through a new mechanism, which gives rise to relatively lower mass compact stars with higher spin rates. This could have implication for the observed mass and spin distributions of millisecond pulsars. Such a stellar conversion can also rescue some massive, spin-supported millisecond pulsars from collapsing into black holes. Besides, we extend the concept of critical mass $M_{\rm cr}$ for the neutron star sequence (Berezhiani et al. 2003; Bombaci et al. 2004) to the case of fast-spinning neutron stars, and point out that neutron star EoS models cannot be ruled out by the stellar mass measurement alone. Finally, we emphasize the additional complexity for constraining EoS models, for example, by stellar radius measurements using X-ray observations, if two families of compact stars coexist.

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arXiv2017

Millisecond radio pulsars with known masses: parameter values and equation of state models

The recent fast growth of a population of millisecond pulsars with precisely measured mass provides an excellent opportunity to characterize these compact stars at an unprecedented level. This is because the stellar parameter values can be accurately computed for known mass and spin rate and an assumed equation of state (EoS) model. For each of the 16 such pulsars and for a set of EoS models from nucleonic, hyperonic, strange quark matter and hybrid classes, we numerically compute fast spinning stable stellar parameter values considering the full effect of general relativity. This first detailed catalogue of the computed parameter values of observed millisecond pulsars provides a testbed to probe the physics of compact stars, including their formation, evolution and EoS. We estimate uncertainties on these computed values from the uncertainty of the measured mass, which could be useful to quantitatively constrain EoS models. We note that the largest value of the central density $ρ_{\rm c}$ in our catalogue is $\sim 5.8$ times the nuclear saturation density $ρ_{\rm sat}$, which is much less than the expected maximum value $13 ρ_{\rm sat}$. We argue that the $ρ_{\rm c}$-values of at most a small fraction of compact stars could be much larger than $5.8 ρ_{\rm sat}$. Besides, we find that the constraints on EoS models from accurate radius measurements could be significantly biased for some of our pulsars, if stellar $spinning$ configurations are not used to compute the theoretical radius values.

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arXiv2016

Nuclear matter properties from local chiral interactions with $Δ$ isobar intermediate states

Using two-nucleon and three-nucleon interactions derived in the framework of chiral perturbation theory (ChPT) with and without the explicit $Δ$ isobar contributions, we calculate the energy per particle of symmetric nuclear matter and pure neutron matter in the framework of the microscopic Brueckner-Hartree-Fock approach. In particular, we present for the first time nuclear matter calculations using the new fully local in coordinate-space two-nucleon interaction at the next-to-next-to-next-to-leading-order (N3LO) of ChPT with $Δ$ isobar intermediate states (N3LO$Δ$) recently developed by Piarulli et al. [arXiv:1606:06335]. We find that using this N3LO$Δ$ potential, supplemented with a local N2LO three-nucleon interaction with explicit $Δ$ isobar degrees of freedom, it is possible to obtain a satisfactory saturation point of symmetric nuclear matter. For this combination of two- and three-nucleon interactions we also calculate the nuclear symmetry energy and we compare our results with the empirical constraints on this quantity obtained using the excitation energies to isobaric analog states in nuclei and using experimental data on the neutron skin thickness of heavy nuclei, finding a very good agreement with these empirical constraints in all the considered nucleonic density range. In addition, we find that the explicit inclusion of $Δ$ isobars diminishes the strength of the three-nucleon interactions needed the get a good saturation point of symmetric nuclear matter. We also compare the results of our calculations with those obtained by other research groups using chiral nuclear interactions with different many-body methods, finding in many cases a very satisfactory agreement.

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arXiv2016

Fast spinning strange stars: possible ways to constrain interacting quark matter parameters

For a set of equation of state (EoS) models involving interacting strange quark matter, characterized by an effective bag constant (B_eff) and a perturbative QCD corrections term (a_4), we construct fully general relativistic equilibrium sequences of rapidly spinning strange stars for the first time. Computation of such sequences is important to study millisecond pulsars and other fast spinning compact stars. Our EoS models can support a gravitational mass (M_G) and a spin frequency at least up to approximately 3.0 solar mass and approximately 1250 Hz respectively, and hence are fully consistent with measured M_G and spin frequency values. This paper reports the effects of B_eff and a_4 on measurable compact star properties, which could be useful to find possible ways to constrain these fundamental quark matter parameters, within the ambit of our EoS models. We confirm that a lower B_eff allows a higher mass. Besides, for known M_G and spin frequency, measurable parameters, such as stellar radius, radius-to-mass ratio and moment of inertia, increase with the decrease of B_eff. Our calculations also show that a_4 significantly affects the stellar rest mass and the total stellar binding energy. As a result, a_4 can have signatures in evolutions of both accreting and non-accreting compact stars, and the observed distribution of stellar mass and spin and other source parameters. Finally, we compute the parameter values of two important pulsars, PSR J1614-2230 and PSR J1748-2446ad, which may have implications to probe their evolutionary histories, and for constraining EoS models.

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arXiv2016

The Hyperon Puzzle in Neutron Stars

The so called "hyperon puzzle", i.e. the difficulty to reconcile the measured masses of neutron stars (NSs) with the presence of hyperons in their interiors, is one of the hot topics in astrophysics which is stimulating copious experimental and theoretical research in hypernuclear physics. After illustrating the origin of the hyperon puzzle, I discuss some of its possible solutions, and particularly those related to the role of hyperonic two- and three-body interactions on the equation of state of dense matter. Afterward, I discuss a possibility to circumvent the hyperon puzzle allowing for the presence of strangeness in NSs in the form of deconfined strange quark matter, and thus considering the so called quark stars, i.e. hybrid stars or strange stars. Finally I discuss the astrophysical consequences of the possible conversion process of an hadronic star to a quark star.

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arXiv2016

Quark matter nucleation in neutron stars and astrophysical implications

A phase of strong interacting matter with deconfined quarks is expected in the core of massive neutron stars. We investigate the quark deconfinement phase transition in cold (T = 0) and hot beta-stable hadronic matter. Assuming a first order phase transition, we calculate and compare the nucleation rate and the nucleation time due to quantum and thermal nucleation mechanisms. We show that above a threshold value of the central pressure a pure hadronic star (HS) (i.e. a compact star with no fraction of deconfined quark matter) is metastable to the conversion to a quark star (QS) (i.e. a hybrid star or a strange star). This process liberates an enormous amount of energy, of the order of 10^{53}~erg, which causes a powerful neutrino burst, likely accompanied by intense gravitational waves emission, and possibly by a second delayed (with respect to the supernova explosion forming the HS) explosion which could be the energy source of a powerful gamma-ray burst (GRB). This stellar conversion process populates the QS branch of compact stars, thus one has in the Universe two coexisting families of compact stars: pure hadronic stars and quark stars. We introduce the concept of critical mass M_{cr} for cold HSs and proto-hadronic stars (PHSs), and the concept of limiting conversion temperature for PHSs. We show that PHSs with a mass M < M_{cr} could survive the early stages of their evolution without decaying to QSs. Finally, we discuss the possible evolutionary paths of proto-hadronic stars.

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arXiv2015

Comparative study of three-nucleon force models in nuclear matter

We calculate the energy per particle of symmetric nuclear matter and pure neutron matter using the microscopic many-body Brueckner-Hartree-Fock (BHF) approach and employing the Argonne V18 (AV18) nucleon-nucleon (NN) potential supplemented with two different three-nucleon force models recently constructed to reproduce the binding energy of $^3$H, $^3$He and $^4$He nuclei as well as the neutron-deuteron doublet scattering length. We find that none of these new three-nucleon force models is able to reproduce simultaneously the empirical saturation point of symmetric nuclear matter and the properties of three- and four-nucleon systems.

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arXiv2013

Quark deconfinement transition in neutron stars with the field correlator method

A phase of strong interacting matter with deconfined quarks is expected in the core of massive neutron stars. In this article, we perform a study of the hadron-quark phase transition in cold (T = 0) neutron star matter and we calculate various structural properties of hybrid stars. For the quark phase, we make use of an equation of state (EOS) derived with the field correlator method (FCM) recently extended to the case of nonzero baryon density. For the hadronic phase, we consider both pure nucleonic and hyperonic matter, and we derive the corresponding EOS within a relativistic mean field approach. We make use of measured neutron star masses, and particularly the mass $M = 1.97 \pm 0.04 \, M_\odot$ of PSR J1614 -2230 to constrain the values of the gluon condensate $G_2$, which is one of the EOS parameters within the FCM. We find that the values of $G_2$ extracted from the mass measurement of PSR J1614 -2230 are consistent with the values of the same quantity derived within the FCM from recent lattice QCD calculations of the deconfinement transition temperature at zero baryon chemical potential. The FCM thus provides a powerful tool to link numerical calculations of QCD on a space-time lattice with measured neutron star masses.

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arXiv2013

A link between measured neutron star masses and lattice QCD data

We study the hadron-quark phase transition in neutron star matter and the structural properties of hybrid stars using an equation of state (EOS) for the quark phase derived with the field correlator method (FCM). We make use of the measured neutron star masses, and particularly the mass of PSR J1614-2230, to constrain the values of the gluon condensate $G_2$ which is one of the EOS parameter within the FCM. We find that the values of $G_2$ extracted from the mass measurement of PSR J1614-2230 are fully consistent with the values of the same quantity derived, within the FCM, from recent lattice quantum chromodynamics (QCD) calculations of the deconfinement transition temperature at zero baryon chemical potential. The FCM thus provides a powerful tool to link numerical calculations of QCD on a space-time lattice with neutron stars physics.

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arXiv2012

Quark matter nucleation with a microscopic hadronic equation of state

The nucleation process of quark matter in cold (T = 0) stellar matter is investigated using the microscopic Brueckner-Hartree-Fock approach to describe the hadronic phase, and the MIT bag model, the Nambu-Jona-Lasinio, and the Chromo Dielectric models to describe the deconfined phase of quark matter. The consequences of the nucleation process for neutron star physics are outlined. Hyperonic stars are metastable only for some of the quark matter equations of state considered. The effect of an hyperonic three body force on the metastability of compact stars is estimated, and it is shown that, except for the Nambu-Jona-Lasinio model and the MIT bag model with a large bag pressure, the other models predict the formation of hybrid stars with a maximum mass not larger than \sim 1.62 M\odot .

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arXiv2012

Chiral model approach to quark matter nucleation in neutron stars

The nucleation process of quark matter in both cold and hot dense hadronic matter is investigated using a chiral approach to describe the quark phase. We use the Nambu-Jona-Lasinio and the Chromo Dielectric models to describe the deconfined phase and the non-linear Walecka model for the hadronic one. The effect of hyperons on the transition phase between hadronic and quark matter is studied. The consequences of the nucleation process for neutron star physics are outlined.

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arXiv2011

Estimation of the effect of hyperonic three-body forces on the maximum mass of neutron stars

A model based on a microscopic Brueckner--Hartree--Fock approach of hyperonic matter supplemented with additional simple phenomenological density-dependent contact terms is employed to estimate the effect of hyperonic three-body forces on the maximum mass of neutron stars. Our results show that although hyperonic three-body forces can reconcile the maximum mass of hyperonic stars with the current limit of $1.4-1.5 M_\odot$, they are unable to provide the repulsion needed to make the maximum mass compatible with the observation of massive neutron stars, such as the recent measurements of the unusually high masses of the millisecond pulsars PSR J1614-2230 ($1.97 \pm 0.04 M_\odot$) and PSR J1903+0327 ($1.667 \pm 0.021 M_\odot$).

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arXiv2011

Effects of quark matter nucleation on the evolution of proto-neutron stars

(Abridged) A phase of strong interacting matter with deconfined quarks is expected in the core of massive neutron stars. If this deconfinement phase transition is of the first order then it will be triggered by the nucleation of a critical size drop of the stable quark phase in the metastable hadronic phase. Within these circumstances it has been shown that cold pure hadronic compact stars above a threshold value of their gravitational mass are metastable with respect to the "decay" to quark stars (compact stars made at least in part of quark matter). This stellar conversion process liberates a huge amount of energy, and it could be the energy source of some of the long GRBs. The main goal of the present work is to establish whether a newborn hadronic star (proto-hadronic star) could survive the early stages of its evolution without "decaying" to a quark star. To this aim, we study the nucleation process of quark matter in hot beta-stable hadronic matter, with and without trapped neutrinos. We calculate and compare the nucleation rate and the nucleation time due to thermal and quantum nucleation mechanisms. We compute the crossover temperature above which thermal nucleation dominates the finite temperature quantum nucleation mechanism. We next discuss the consequences of quark matter nucleation for the physics and the evolution of proto-neutron stars. We introduce the new concept of limiting conversion temperature and critical mass M_cr for proto-hadronic stars, and we show that proto-hadronic stars with a mass M < M_cr could survive the early stages of their evolution without decaying to a quark star. We extend the concept of maximum mass of a "neutron star" with respect to the classical one introduced by Oppenheimer & Volkoff to account for the existence of two distinct families of compact stars (hadronic stars and quark stars) as predicted by the present scenario.

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arXiv2010

An analytic parametrization of the hyperonic matter equation of state

An analytic parametrization of the hyperonic matter equation of state based on microscopic Brueckner-Hartree-Fock calculations has been constructed using the realistic Argonne V18 nucleon-nucleon potential plus a three-body force of Urbana type, and three models of the hyperon-nucleon interaction: the Nijmegen soft-core models NSC89 and NSC97e, and the most recent meson-exchange potential of the Juelich group. The construction of this parametrization is based on a simple phase-space analysis and reproduces with good accuracy the results of the microscopic calculations with a small number of parameters. This parametrization allows for rapid calculations that accurately mimic the microscopic results, being therefore, very useful from a practical point of view.

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arXiv2004

Quark deconfinement and implications for the radius and the limiting mass of compact stars

We study the consequences of the hadron-quark deconfinement phase transition in stellar compact objects when finite size effects between the deconfined quark phase and the hadronic phase are taken into account. We show that above a threshold value of the central pressure (gravitational mass) a neutron star is metastable to the decay (conversion) to a hybrid neutron star or to a strange star. The "mean-life time" of the metastable configuration dramatically depends on the value of the stellar central pressure. We explore the consequences of the metastability of ``massive'' neutron stars and of the existence of stable compact quark stars (hybrid neutron stars or strange stars) on the concept of limiting mass of compact stars. We discuss the implications of our scenario on the interpretation of the stellar mass and radius extracted from the spectra of several X-ray compact sources. Finally, we show that our scenario implies, as a natural consequence a two step-process which is able to explain the inferred ``delayed'' connection between supernova explosions and GRBs, giving also the correct energy to power GRBs.

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