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.Comment: PHYSICAL REVIEW D (in press). arXiv admin note: substantial text overlap with arXiv:1212.590

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 $\beta$-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.Comment: Accepted for publication in PAS

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.Comment: Minor changes and typos correcte

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.Comment: Invited review paper accepted for publication in EPJ A, Topical Issue on "Exotic Matter in Neutron Stars

Neutron star structure with chiral interactions

We use two-body and three-body nuclear interactions derived in the framework of chiral perturbation theory (ChPT) with and without the explicit isobar contributions to calculate the energy per particle of symmetric nuclear matter and pure neutron matter employing the microscopic Brueckner-Hartree-Fock approach. In particular, we present 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 isobar intermediate states (N3LO) recently developed by Piarulli et al. [1]. We compute the β-equilibrium equation of state and determine the neutron star mass-radius and mass-central density sequences. We find that the adopted interactions are able to provide satisfactory properties of nuclear matter at saturation density as well as to fulfill the limit of two-solar mass for the maximum mass configuration as required by recent observations

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.Comment: 17 pages, 11 figures, 7 tables, accepted for publication in Monthly Notices of the Royal Astronomical Societ

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.Comment: 10 pages, 5 figures, accepted for publication in The Astrophysical Journa

Nucleazione della materia di quark nella materia adronica (densa)

Nel presente lavoro viene affrontato il problema della nucleazione della materia di quark nella materia adronica densa considerando sia il caso di temperatura nulla che di temperatura finita. A tal fine sono inizialmente discusse le equazioni di stato della fase stabile (fase di quark) e di quella metastabile (fase adronica). Per descrivere la fase stabile viene fatto uso di un’equazione di stato ispirata dal MIT bag model per gli adroni, mentre l’equazione di stato della fase metastabile e' ottenuta usando una teoria di campo relativistica in approssimazione di campo medio. La temperatura fa sı' che la nucleazione possa avvenire in due diversi modi: o per via quantis- tica (ossia per effetto tunnel) o per via termica. Una volta determinate le condizioni termodinamiche per cui la transizione di fase puo' aver luogo, si cerca di stabilire in quali range di temperature a dominare e' l’uno o l’altro meccanismo di nucleazione. Viene inoltre brevemente discusso il caso di neutrino trapping a T=0 e il relativo effetto sulle condizioni che determi- nano la transizione di deconfinamento. Infine si considerano alcune possibili applicazioni dei risultati ottenuti agli oggetti compatti