125 research outputs found
Dense hadronic matter in neutron stars
The existence of stars with masses up to and the hints of the
existence of stars with radii smaller than km seem to require, at the
same time, a stiff and a soft hadronic equation of state at large densities. We
argue that these two apparently contradicting constraints are actually an
indication of the existence of two families of compact stars: hadronic stars
which could be very compact and quark stars which could be very massive. In
this respect, a crucial role is played, in the hadronic equation of state, by
the delta isobars whose early appearance shifts to large densities the
formation of hyperons. We also discuss how recent experimental information on
the symmetry energy of nuclear matter at saturation indicate, indirectly, an
early appearance of delta isobars in neutron star matter.Comment: 6 pages, 3 figures, prepared for proceedings of the conference
"Excited QCD 2014", 2-8 February, Bjelasnica Mountain, Sarajev
Quark deconfinement and the duration of short Gamma Ray Bursts
We propose a model for short duration gamma-ray bursts (sGRBs) based on the
formation of a quark star after the merger of two neutron stars. We assume that
the sGRB central engine is a proto-magnetar, which has been previously invoked
to explain the plateau-like X-ray emission observed following both long and
short GRBs. Here, we show that: i) a few milliseconds after the merger it is
possible to form a stable and massive star made in part of quarks; ii) during
the early cooling phase of the incompletely formed quark star, the flux of
baryons ablated from the surface by neutrinos is large and it does not allow
the outflow to achieve a bulk Lorentz factor high enough to produce a GRB; iii)
after the quark burning front reaches the stellar surface, baryon ablation
ceases and the jet becomes too baryon poor to produce a GRB; iv) however,
between these two phases a GRB can be produced over the finite timescale
required for the baryon pollution to cease; a characteristic timescale of the
order of s naturally results from the time the conversion front
needs to cover the distance between the rotational pole and the latitude of the
last closed magnetic field line; v) we predict a correlation between the
luminosity of the sGRB and its duration, consistent with the data; vi) our
model also predicts a delay of the order of ten seconds between the time of the
merger event and the sGRB, allowing for the possibility of precursor emission
and implying that the jet will encounter the dense cocoon formed immediately
after the merger.Comment: 4 pages, 3 figure
The scenario of two families of compact stars 1. Equations of state, mass-radius relations and binary systems
We present several arguments which favor the scenario of two coexisting
families of compact stars: hadronic stars and quark stars. Besides the well
known hyperon puzzle of the physics of compact stars, a similar puzzle exists
also when considering delta resonances. We show that these particles appear at
densities close to twice saturation density and must be therefore included in
the calculations of the hadronic equation of state. Such an early appearance is
strictly related to the value of the L parameter of the symmetry energy that
has been found, in recent phenomenological studies, to lie in the range
MeV. We discuss also the threshold for the formation of deltas and
hyperons for hot and lepton rich hadronic matter. Similarly to the case of
hyperons, also delta resonances cause a softening of the equation of state
which makes it difficult to obtain massive hadronic stars. Quark stars, on the
other hand, can reach masses up to as predicted by
perturbative QCD calculations. We then discuss the observational constraints on
the masses and the radii of compact stars. The tension between the precise
measurements of high masses and the indications of the existence of very
compact stellar objects (with radii of the order of km) is relieved when
assuming that very massive compact stars are quark stars and very compact stars
are hadronic stars. Finally, we discuss recent interesting measurements of the
eccentricities of the orbits of millisecond pulsars in low mass X-ray binaries.
The high values of the eccentricities found in some cases could be explained by
assuming that the hadronic star, initially present in the binary system,
converts to a quark star due to the increase of its central density.Comment: 11 pages, 9 figures, prepared for the 2015 EPJA Topical Issue on
"Exotic Matter in Neutron Stars". Revised versio
Softening of the equation of state of matter at large densities and temperatures: chiral symmetry restoration vs. quark deconfinement
We discuss two models for describing the behavior of matter at large
densities and intermediate temperatures. In both models a softening of the
equation of state takes place due to the appearance of new degrees of freedom.
The first is a hadronic model in which the softening is due to chiral symmetry
restoration. In the second model the softening is associated with the formation
of clusters of quarks in the mixed phase. We show that both models allow a
significant softening but, in the first case the bulk modulus is mainly
dependent on the density, while in the mixed phase model it also strongly
depends on the temperature. We also show that the bulk modulus is not vanishing
in the mixed phase due to the presence of two conserved charges, the baryon and
the isospin one. Only in a small region of densities and temperatures the
incompressibility becomes extremely small. Finally we compare our results with
recent analysis of heavy ion collisions at intermediate energies.Comment: 4 pages, 4 figures, editorially accepted versio
Quantum uncertainty in weakly non-ideal astrophysical plasma
Galitskii and Yakimets showed that in dense or low temperature plasma, due to quantum uncertainty effect, the particle distribution function over momenta acquires a power-like tail even under conditions of thermodynamic equilibrium. We show that in weakly non-ideal plasmas, like the solar interior, both non-extensivity and quantum uncertainty should be taken into account to derive equilibrium ion distribution functions and to estimate nuclear reaction rates and solar neutrino fluxes. The order of magnitude of the deviation from the standard Maxwell-Boltzmann distribution can be derived microscopically by considering the presence of random electrical microfield in the stellar plasma. We show that such a nonextensive statistical effect can be very relevant in many nuclear astrophysical problems
Can very compact and very massive neutron stars both exist?
The existence of neutron stars with masses of ∼2M⊙ requires a stiff equation of state at high densities. On the other hand, the necessary appearance also at high densities of new degrees of freedom, such as hyperons and Δ resonances, can lead to a strong softening of the equation of state with resulting maximum masses of ∼1.5M⊙ and radii smaller than ∼10 km. Hints for the existence of compact stellar objects with very small radii have been found in recent statistical analyses of quiescent low-mass X-ray binaries in globular clusters. We propose an interpretation of these two apparently contradicting measurements, large masses and small radii, in terms of two separate families of compact stars: hadronic stars, whose equation of state is soft, can be very compact, while quark stars, whose equation of state is stiff, can be very massive. In this respect an early appearance of Δ resonances is crucial to guarantee the stability of the branch of hadronic stars. Our proposal could be tested by measurements of radii with an error of ∼1 km, which is within reach of the planned Large Observatory for X-ray Timing satellite, and it would be further strengthened by the discovery of compact stars heavier than ∼2M⊙
Kaons production at finite temperature and baryon density in an effective relativistic mean field model
We investigate the kaons production at finite temperature and baryon density
by means of an effective relativistic mean-field model with the inclusion of
the full octet of baryons. Kaons are considered taking into account of an
effective chemical potential depending on the self-consistent interaction
between baryons. The obtained results are compared with a minimal coupling
scheme, calculated for different values of the anti-kaon optical potential.Comment: 3 pages, contribution presented to the International Conference on
Exotic Atoms and Related Topic
FIRST experiment: Fragmentation of Ions Relevant for Space and Therapy
Nuclear fragmentation processes are relevant in different fields of basic research and applied physics and are of particular interest for tumor therapy and for space radiation protection applications. The FIRST (Fragmentation of Ions Relevant for Space and Therapy) experiment at SIS accelerator of GSI laboratory in Darmstadt, has been designed for the measurement of different ions fragmentation cross sections at different energies between 100 and 1000 MeV/nucleon. The experiment is performed by an international collaboration made of institutions from Germany, France, Italy and Spain. The experimental apparatus is partly based on an already existing setup made of the ALADIN magnet, the MUSIC IV TPC, the LAND2 neutron detector and the TOFWALL scintillator TOF system, integrated with newly designed detectors in the interaction Region (IR) around the carbon removable target: a scintillator Start Counter, a Beam Monitor drift chamber, a silicon Vertex Detector and a Proton Tagger for detection of light fragments emitted at large angles (KENTROS). The scientific program of the FIRST experiment started on summer 2011 with the study of the 400 MeV/nucleon 12C beam fragmentation on thin (8mm) carbon targe
- …