The Study Of Some Issues In Nuclear Symmetry Energy And Compact Stars

Nowadays,the density dependence of nuclear symmetry energy is still not well known,especially at suprasaturation densities,knowledge on the density dependence of symmetry energy plays an important role in understanding nuclear physics and astrophysics,heavy –ion reactions have been used as the unique means to probe the density dependent of nuclear symmetry energy.Neutron stars are compact objects related to many branches of nuclear physics and astrophysics.By comparing the theoretical cooling curves with the observations,we can explore the internal structure and equation of state of dense matter,in turn,we can give a constraint of nuclear symmetry energy.Recently,some peculiar over-luminous type Ia supernovae have been observed,this brings the topic of super-Chandrasekhar white dwarfs in lime-light.In this work,we investigated the three issues.The study of the density dependence of the nuclear symmetry energy from heavy-ion reactions was based on the Boltzmann-Uehling-Uhlenbeck(BUU)transport model.There are two main works about the nuclear symmetry energy: One is to study the sensitivity of the symmetry energy observables in different density regions.Another is to probe the momentum dependence of symmetry potential by the pre-equilibrium free neutron-proton ratio n/p.Our results indicate that effects of symmetry energy in the sub-normal densities is small and so does it in the maximal baryon-density region,which increasing the difficulty of constraining nuclear symmetry energy at supradensities.Besides,due to the momentum dependence of the symmetry potential is crucial to determine the nuclear symmetry energy,we probe the momentum dependent symmetry potential with the pre-equilibrium free n/p,it shows that only a certain momentum range is important for an observable,n/p is not sensitive to the maximal momentum region.Therefore,we need to input reasonable density and momentum dependent symmetry potential for the different magnitude of incident beam energy of heavy-ion collisions.The current experimental data on the symmetry energy/potential is not enough,more experimental data at high densities and momenta on the symmetry energy/potential are needed to constrain the high density symmetry energy.The cooling of neutron stars was studied with use of the general relativistic stellar evolutionary equations.There have been reported observational results of soft X-ray transients(SXRTs)during quiescence in low mass X-ray binaries,the emergent radiation flux may depend on the neutron star structure,which opens an important possibility to explore the internal structure and the equation of state of dense matter by comparing numerical results with the observations.We take into account strong pion condensation and the effect of superfluidity,by comparing the results of observations of several SXRTs,in particular the two sources containing the codest(SAX J1808.4-3658)and the hottest(Aql X-1)neutrons stars,it shows that neutrino emission by strong pion condensation can explain quiescent X-ray luminosity of SAX J1808.4-3658 and we don't need direct Urca processes concerning nucleons/hyperons,besides,the cooling curves of isolated neutron stars was studied,it shows that the two types of neutron stars(isolated neutron star and transient accreting neutron star)can be well explained by tuning the critical temperature distribution.The quiescent light curve of accreting neutron star MAXI J0556-332 was studied,it's the hottest transient accreting neutron star at the beginning of quiescence,it's thought that a shallow heat source is needed in the shallow outer crust,but the physical reason is not clear,we perform stellar evolutionary calculations with considering the compressional heating,crustal heating and the nuclear energy generation due to hot CNO cycle,the quiescent light curve of MAXI J0556-332 can be well reproduced as a whole.Finally,the super-Chandrasekhar white dwarfs was studied.Recent observations of some peculiar type Ia supernova required the progenitor masses lie in the range 2.1-2.8M,it's far higher than the Chandrasekhar limit,so the strongly magnetized white dwarfs(SMWD)have been proposed,it shows that the new mass limit of white dwarfs reaches 2.58 M,but the author calculate it in the framework of Newtonian theory,as it has a smaller size compare with the usual one,we check the general relativistic effects and find that it can't be neglected.Besides,as the SMWD suffers severe stability issues,we address this problem by exploiting the effects of electric field inside the white dwarfs.