Abstract

This project aims at a better understanding of the thermal and magnetic field evolution of neutron stars, focusing particularly on the following issues: 1. Heating of old neutron stars: One of the consequences of the magnetic field in neutron stars is their continuous loss of angular momentum. This causes them to contract, inducing beta decays that release energy in the form of neutrinos, but also as heat. This “rotochemical heating” process can make old neutron stars “shine” in the ultraviolet at a detectable level, for very long times. The ongoing exploration of this process will be continued by more sophisticated modeling that includes superfluid neutrons and/or superconducting protons with density-dependent, anisotropic energy gaps as predicted by theoretical models. We will also construct a realistic model for the atmosphere of these stars in order to know how their spectral energy distribution differs from a blackbody. Finally, we will assess the viability of verifying these models with observations done with optical telescopes located in Chile. 2. Stellar magnetic equilibria and their stability: Upper main-sequence stars, white dwarfs, and neutron stars have persistent magnetic fields dominated by a few low-order multipole components. Motivated by recent numerical simulations and physical arguments, we will study the stability conditions of hydromagnetic equilibria in a stably stratified, conducting fluid involving poloidal and toroidal field components that can be written by explicit, analytical expressions. This will allow us to constrain the internal structure and strength of magnetic fields that can persist over most of the lifetime of these stars. The results will also be extended to superconducting neutron stars, and will have implications for the energetics of magnetars, precession of pulsars, gravitational wave emission, and precision astroseismology of white dwarfs. 3. Magnetic field evolution in neutron stars: The equilibria mentioned above are valid only as long as the matter can be treated as a single fluid. In neutron stars, this assumption is violated by the formation of a solid crust, and, over long times, by Hall drift and ambipolar diffusion, processes by which neutrons, protons, and electrons (and perhaps other particles) move with respect to each other, and by beta decays that convert particles of one kind into another. These mechanisms can cause neutron star magnetic fields to decay and release their energy, which is surely relevant in magnetars, and perhaps also in more weakly magnetized objects. This will be modeled through analytical approximations and progressively more refined numerical simulations, allowing us to learn in what way and how fast the field can decay. We also intend to study the dynamics of the magnetic flux during the phase transition in which the protons in a neutron star become superconducting. 4. Coupled thermo-magnetic evolution: Several of the processes controlling the evolution of the magnetic field are temperature-dependent, particularly beta decays and inter-particle collisions, which determine the ambipolar diffusion velocity and the electric resistivity. On the other hand, the dissipation of magnetic energy releases heat that affects the thermal evolution. We will construct a model that follows the coupled evolution of the temperature, magnetic field, and departure from beta equilibrium in a self-consistent way, and compare it to observational data on magnetars, pulsars, isolated thermal emitters, and other classes of neutron stars.