Neutron Star Structure from Static and Rotating Equilibrium Configurations

• Hyukjin Kwon (Institute of Science Tokyo)

Room: Room 320, Baird Hall Neutron stars are fascinating compact objects that provide extreme conditions for studying nuclear matter. By formulating equations of state (EoS) based on assumptions for each region within the star, we can investigate the internal structure of neutron stars. Although much of this research relies on the Tolman–Oppenheimer–Volkoff (TOV) equation—which treats the Einstein field equations under spherical symmetry—observations reveal that neutron stars rotate. Some of them spin at remarkably high rates, known as millisecond pulsars, leading to deformation from perfect sphericity into ellipsoidal shapes. To obtain accurate theoretical predictions, these rotational effects must be incorporated. Solving the axisymmetric Einstein equations directly cannot be reduced to simple ordinary differential equations, and thus requires numerical methods. One such approach, proposed in 1989 by Komatsu, Eriguchi, and Hachisu and known as the KEH method, transforms these complex differential equations into integral form, enabling stable solutions for the internal structure of rotating neutron stars. In this study, we employ the Skyrme energy density functional to describe nuclear interactions and apply the KEH method to explore the internal structure of rotating neutron stars. We then discuss how our theoretical predictions compare with observational data and how the resulting constraints on the EoS are influenced by the inclusion of rotation.

Superfluid Band Calculation for Neutron Star Inner Crusts

• Kenta Yoshimura (Institute of Science Tokyo)

Room: Room 320, Baird Hall In the inner crust of neutron stars, a Coulomb lattice of nuclei exists, immersed in a sea of superfluid neutron gas. The interplay between these nuclear crystals and the background neutrons may significantly alter nuclear dynamics, a phenomenon known as the "entrainment" effect, which is crucial for understanding several astronomical phenomena. In our study, we have developed new self-consistent calculations that fully account for both superfluid effects and band structure effects. We have extracted the "effective mass" of free neutrons through the real-time method. In this presentation, we will show the formalism and methodology of our calculations, as well as further extensions towards comprehensive simulations of the subnuclear properties of neutron star matter.