Effects of nuclear symmetry energy on properties of neutron star crust.

摘要

Neutron stars (NSs) are the end point of the evolution of a star with between about 8 and 25 solar masses. The star's core collapses to form an object with about 1.5 times the mass of our Sun and a radius of 10 km. The average density of matter in a neutron star is comparable with that of the nuclei of atoms which provides us with an excellent laboratory for examining nuclear theory under conditions not obtainable here on Earth. Most nuclei found on earth have an asymmetry of 0.5, meaning that they contain roughly an even number or protons and neutrons. In the inner crust of the NSs, the nuclei become extremely neutron rich, having an extremely low asymmetry. At the transition between the solid and liquid core, nuclei can form cylindrical, slab and bubble structures (so-called pasta phases). NSs are observed to undergo gamma ray flares which have oscillations in the X-ray tail of their lightcurve. These oscillations are thought to be caused by torsional oscillations in the crust which depends on the shear modulus (rigidity) of the crust (Duncan, 1998; Steiner and Watts, 2009). Nuclear symmetry energy encodes the energy cost of transforming a proton to a neutron from symmetric nuclear matter. To study the structure of the inner crust and how symmetry energy affects the nuclei, we devise a compressible liquid drop model following the tactics introduced by Baym, Bethe and Pethick (1971) and elaborated on by many scientists that implores the use of Skyrme-Hartree-Fock (Chabanat et al., 1997) and Modified Skyrme-Like Interaction (Chen et al., 2009) equations of state (EOS) of bulk nuclear matter, as well as effects of surface symmetry energy (Danielewicz, 2001). Using this model, we present a study of the effects of nuclear and surface symmetry energy on the pasta regime and of the lower and upper bound observational effects of the pasta regime on the frequency of the torsional modes and the maximum quadrupole ellipticity sustainable by the crust.