54 research outputs found

### 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 ÃŽÂ²-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 unâ€™equazione di stato ispirata dal MIT bag model per gli adroni, mentre
lâ€™equazione di stato della fase metastabile e' ottenuta usando una teoria di
campo relativistica in approssimazione di campo medio. La temperatura fa
sÄ±' 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' lâ€™uno o lâ€™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

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