Session C10: Neutron Star Theory

Live

Recent observations of neutron stars in a variety of astrophysical systems provides a new handle on the equation of state of extremely dense matter. Building on recent nonparametric studies of with GW170817, we investigate how to self-consistently incorporate theoretical uncertainties from chiral effective field theory at low densities with nonparametric extensions to high densities. Nonparametric equation of state inference offers the best chance to avoid modeling systematics and other less-than-obvious prior beliefs and has already enabled novel studies of, e.g., whether the cores of neutron stars undergo strong phase transitions without the need to specify the precise form of the phase transition a priori. Coupling this extreme model freedom at high densities with carefully quantified theoretical uncertainty at low densities, we investigate astrophysical observations' impact on our belief that strong phase transitions exist as well as the maximum density up to which chiral effective field theory agrees with the data. In this way, neutron star observations can directly determine when effective field theory may begin to break down, rather than assuming a particular theoretical model from the start.?   

Live

Neutron stars are natural laboratories for the study of dense matter. Their densities vary from $\sim 1g/cm^3$ in the atmosphere to $\sim 10g/cm^3$ in the core. As the density of matter increases, atomic nuclei disintegrate into nucleons and, eventually, the nucleons themselves disintegrate into quarks. The transition between these phases can vary steep first order to smooth crossover, depending on certain conditions. As the matter in the inner core of neutron stars (NS) and protoneutron stars (PNS) is very dense but strongly interacting, it cannot be described by first principle theories. We choose Chiral Mean Field Model (CMF) to describe neutron stars. It is a quantum relativistic model that describes hadrons (nucleons and hyperons) and 3 light flavors of quarks interacting via meson exchange, as a way to describe the attractive and repulsive components of the strong force.   

Live

There exist approximate universal relations connecting properties of neutron stars (NSs) that are insensitive to the equation of state with important applications on probing fundamental physics.To date, analytic works on universal relations for realistic NSs are lacking, which may lead to a better understanding of their origin.In this talk, we focus on I (moment of inertia)-Love (related to tidal deformability) universal relation and derive related approximate relations analytically.To achieve this, we construct analytic slowly-rotating/tidally-deformed NS solutions starting from an extended Tolman VII model that accurately describes non-rotating realistic NSs, allowing us to extract I and the Love number.The field equations are solved analytically by expansion about the Newtonian limit and keeping up to 6th order in stellar compactness (C).Based on these, we mathematically show the O(10%) equation-of-state variation in the I-C and Love-C relations and O(1%) variation in the I-Love relation as previously found numerically.Our new analytic relations agree better with numerical results of realistic NSs(especially I-C and Love-C) than the analytic relations for constant density stars.Our results provide a mathematical explanation of the amount of universality in the above relations.   

Live

We describe a relativistic multifluid dynamics appropriate for application to neutron star cores at finite temperatures based on Carter's convective variational procedure. The model includes seven fluids, accounting for both normal and superfluid/superconducting neutrons and protons, leptons and entropy. Vortex lines and flux tubes, mutual friction, vortex pinning, heat conduction and viscosity are incorporated into the model after the basic hydrodynamics is described. The detailed vortex line/flux tube contributions to the equations of motion are found by comparison to a mesoscopic calculation which accounts for individual vortex lines/flux tubes. We find that the magnetic $H$-field inside a neutron star differs from that given in previous astrophysical works, but is in agreement with condensed matter physics literature. This also has a subtle effect on the Maxwell equations inside neutron stars, which could have implications for phenomena involving neutron star magnetic fields.   

Live

We report on recent molecular dynamics simulations studying thermal fluctuations in nuclear pasta in the inner crusts of neutron stars. Large scale simulations of `lasagna' at a range of temperatures have resolved power law fluctuations in surface curvature and a first-order melting phase transition to a disordered phase. We also resolve topological fluctuations in the pasta at temperatures slightly below the melting temperature which may have implications for annealing pasta as the neutron star cools following a supernova. These results may constrain the maximum size of `domains' in nuclear pasta and may have implications for the transport properties and shear moduli of the inner crust.   

Maximum mass and universal relations of hadron-quark hybrid stars

Hadron-quark hybrid stars are compact stars with a deconfined quark core surrounded by hadronic matter, which are compatible with the observation of the gravitational wave event GW170817. To better understand the properties of possible binary neutron star merger remnants, it is crucial to properly characterize the solution space of hybrid stars. We construct equilibrium models of uniformly and differentially rotating hybrid stars using equations of state (EOSs) with a first-order phase transition. Contrary to the case of purely hadronic stars, we find that the ratio of the maximum possible mass of uniformly rotating configurations to the Tolman-Oppenheimer-Volkoff limit mass is not EOS-independent. Hence, some of the constraints placed on the nuclear EOS from GW170817 do not apply to hadron-quark EOSs. We also present our findings on universal relations and maximum mass for both uniformly and differentially rotating hybrid stars.   

Reading oscillation modes from waveforms in numerical relativity simulations of rotating neutron stars

The next generation of higher sensitivity gravitational wave detectors and advanced electromagnetic observations will open the door to prove the nature of matter inside of neutron stars, among it the equation of state above nuclear density. An important component of this endeavor is understanding the connection between fundamental oscillation modes in neutron stars and modes in the corresponding gravitational radiation. We present results of perturbed, rapidly rotating neutron star models in full general relativity as a source of gravitational radiation. From a series of non-linear numerical relativity simulations of uniformly rotating polytropes, we calculate the f-mode frequency and decaying time of oscillations from the extracted gravitational waveform. We discuss the ability of numerical relativity codes to provide estimates of oscillation modes from neutron star binary mergers.