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SU(2) Chiral Sigma Model Study of Phase Transition in Hybrid Stars  [PDF]
P. K. Jena,L. P. Singh
Physics , 2003, DOI: 10.1142/S0217732303011976
Abstract: We use a modified SU(2) chiral sigma model to study nuclear matter component and simple bag model for quark matter constituting a neutron star. We also study the phase transition of nuclear matter to quark matter with the mixed phase characterized by two conserved charges in the interior of highly dense neutron stars. Stable solutions of Tolman-Oppenheimer-Volkoff equations representing hybrid stars are obtained with a maximum mass of 1.67$M_{\odot}$ and radius around 8.9 km.
Rotating Neutron Stars in a Chiral SU(3) Model  [PDF]
S. Schramm,D. Zschiesche
Physics , 2002, DOI: 10.1088/0954-3899/29/3/307
Abstract: We study the properties of rotating neutron stars within a generalized chiral SU(3)-flavor model. The influence of the rotation on the inner structure and the hyperon matter content of the star is discussed. We calculate the Kepler frequency and moments of inertia of the neutron star sequences. An estimate for the braking index of the associated pulsars is given.
Slowly rotating neutron stars and hadronic stars in chiral SU(3) quark mean field model  [PDF]
Shaoyu Yin,Jiadong Zang,Ru-Keng Su
Physics , 2009, DOI: 10.1140/epja/i2010-10916-0
Abstract: The equations of state for neutron matter, strange and non-strange hadronic matter in a chiral SU(3) quark mean field model are applied in the study of slowly rotating neutron stars and hadronic stars. The radius, mass, moment of inertia, and other physical quantities are carefully examined. The effect of nucleon crust for the strange hadronic star is exhibited. Our results show the rotation can increase the maximum mass of compact stars significantly. For big enough mass of pulsar which can not be explained as strange hadronic star, the theoretical approaches to increase the maximum mass are addressed.
Phase transition and hybrid star in a SU(2) chiral sigma model  [PDF]
P. K. Jena,L. P. Singh
Physics , 2002, DOI: 10.1142/S0217732302009295
Abstract: We use a modified SU(2) chiral sigma model to study nuclear matter at high density using mean field approach. We also study the phase transition of nuclear matter to quark matter in the interior of highly dense neutron stars. Stable solutions of Tolman-Oppenheimer-Volkoff equations representing hybrid stars are obtained with a maximum mass of 1.69 $M_{\odot}$, radii around 9.3 kms and a quark matter core constituting nearly 55-85 % of the star radii.
Neutron stars and strange stars in the chiral SU(3) quark mean field model  [PDF]
P. Wang,S. Lawley,D. B. Leinweber,A. W. Thomas,A. G. Williams
Physics , 2005, DOI: 10.1103/PhysRevC.72.045801
Abstract: We investigate the equations of state for pure neutron matter and strange hadronic matter in $\beta$-equilibrium, including $\Lambda$, $\Sigma$ and $\Xi$ hyperons. The masses and radii of pure neutron stars and strange hadronic stars are obtained. For a pure neutron star, the maximum mass is about $1.8 M_{\mathrm{sun}}$, while for a strange hadronic star, the maximum mass is around $1.45 M_{\mathrm{sun}}$. The typical radii of pure neutron stars and strange hadronic stars are about 11.0-12.3 km and 10.7-11.7 km, respectively.
Neutron stars within the SU(2) parity doublet model  [PDF]
V. Dexheimer,G. Pagliara,L. Tolos,J. Schaffner-Bielich,S. Schramm
Physics , 2008, DOI: 10.1140/epja/i2008-10652-0
Abstract: The equation of state of beta-stable and charge neutral nucleonic matter is computed within the SU(2) parity doublet model in mean field and in the relativistic Hartree approximation. The mass of the chiral partner of the nucleon is assumed to be 1200 MeV. The transition to the chiral restored phase turns out to be a smooth crossover in all the cases considered, taking place at a baryon density of just $2\rho_0$. The mass-radius relations of compact stars are calculated to constrain the model parameters from the maximum mass limit of neutron stars. It is demonstrated that chiral symmetry starts to be restored, which in this model implies the appearance of the chiral partners of the nucleons, in the center of neutron stars. However, the analysis of the decay width of the assumed chiral partner of the nucleon poses limits on the validity of the present version of the model to describe vacuum properties.
Modeling Hybrid Stars with an SU(3) non-linear sigma model  [PDF]
Rodrigo Picanco Negreiros,V. A. Dexheimer,S. Schramm
Physics , 2010, DOI: 10.1103/PhysRevC.82.035803
Abstract: We study the behavior of hybrid stars using an extended hadronic and quark SU(3) non-linear sigma model. The degrees of freedom change naturally, in this model, from hadrons to quarks as the density/temperature increases. At zero temperature, we reproduce massive neutron stars containing a core of hybrid matter of 2 km for the non-rotating case and 1.18 km and 0.87 km, in the equatorial and polar directions respectively, for stars rotating at the Kepler frequency (physical cases lie in between). The cooling of such stars is also analyzed.
Chiral Symmetry Restoration and Deconfinement to Quark Matter in Neutron Stars  [PDF]
V. A. Dexheimer,S. Schramm
Physics , 2009, DOI: 10.1016/j.nuclphysbps.2010.02.051
Abstract: We describe an extension of the hadronic SU(3) non-linear sigma model to include quarks. As a result, we obtain an effective model which interpolates between hadronic and quark degrees of freedom. The new parameters and the potential for the Polyakov loop (used as the order parameter for deconfinement) are calibrated in order to fit lattice QCD data and reproduce the QCD phase diagram. Finally, the equation of state provided by the model, combined with gravity through the inclusion of general relativity, is used to make predictions for neutron stars.
Compact stars in a SU(3) Quark-Meson Model  [PDF]
Andreas Zacchi,Rainer Stiele,Juergen Schaffner-Bielich
Physics , 2015, DOI: 10.1103/PhysRevD.92.045022
Abstract: The recent observations of the massive pulsars PSR J1614-2230 and of PSR J0348+0432 with about two solar masses implies strong constraints on the properties of dense matter in the core of compact stars. Effective models of QCD aiming to describe neutron star matter can thereby be considerably constrained. In this context, a chiral quark-meson model based on a SU(3) linear $\sigma$-model with a vacuum pressure and vector meson exchange is discussed in this work. The impact of its various terms and parameters on the equation of state and the maximum mass of compact stars are delineated to check whether pure quark stars with two solar masses are feasible within this approach. Large vector meson coupling constant and a small vacuum pressure allow for maximum masses of two or more solar masses. However, pure quark stars made of absolutely stable strange quark matter, so called strange stars, turn out to be restricted to a quite small parameter range.
Neutron star properties in a chiral SU(3) model  [PDF]
M. Hanauske,D. Zschiesche,S. Pal,S. Schramm,H. St?cker,W. Greiner
Physics , 1999, DOI: 10.1086/309052
Abstract: We investigate various properties of neutron star matter within an effective chiral $SU(3)_L \times SU(3)_R$ model. The predictions of this model are compared with a Walecka-type model. It is demonstrated that the importance of hyperon degrees are strongly depending on the interaction used, even if the equation of state near saturation density is nearly the same in both models. While the Walecka-type model predicts a strange star core with strangeness fraction $f_S \approx 4/3$, the chiral model allows only for $f_S \approx 1/3$ and predicts that $\Sigma^0$, $\Sigma^+$ and $\Xi^0$ will not exist in star, in contrast to the Walecka-type model.
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