Giorgio Almirante "Fraction superfluide dans la croûte interne des étoiles à neutrons"

mardi 14 oct. 2025, 14:00 → 16:00 Europe/Paris

100/0-MXX - Salle des Conseils (IJCLab)

Fraction superfluide dans la croûte interne des étoiles à neutrons / Superfluid fraction in the inner crust of neutron stars

Abstract :

Neutron stars are fascinating astrophysical objects, containing matter at densities that exceeds the density of atomic nuclei. Among the most puzzling phenomena associated with them are pulsar glitches. Pulsars are rapidly rotating neutron stars, emitting beams of radiation from their magnetic poles and acting as the most precise clocks in the Universe, even surpassing the accuracy of atomic clocks on Earth. Occasionally, however, they exhibit sudden increases in their rotational frequency, events known as glitches. These unexpected spin-ups are thought to arise from complex interactions between different internal components of the star. Several theoretical models have been proposed to explain glitches, most of them relying on superfluidity in parts of the star. Superfluidity, the possibility of a fluid to flow without viscosity at very low temperature, has been studied on Earth in liquid helium and ultracold atoms. It is believed that also matter under extreme conditions inside neutron stars exhibits this fascinating phenomenon. Despite decades of effort, many uncertainties remain, particularly concerning the deepest layers of the neutron star core. In the outer core, it is expected that extremely neutron-rich nuclear matter coexists with electrons and, at higher densities, muons. The crust is thought to be a lattice of nuclear clusters immersed in an electron gas. The transition from the outer to the inner crust is where neutrons begin to "drip" from nuclei, eventually leading to a superfluid neutron gas between the nuclear clusters. The exotic phases within the inner crust are central to the star thermal and hydrodynamic behavior. This Thesis focuses on a quantum mechanical description of the inner crust. Modeling this requires solving the nuclear many-body problem using realistic interactions and accounting for the crystal structure and periodicity, as well as the presence of superfluid neutron matter. A central aspect of this work is determining the superfluid density, which is a quantity directly related to the effective mass of nuclear clusters moving through the neutron gas. This parameter influences the thermodynamics and hydrodynamics of neutron stars and has observable consequences, such as in the thermal evolution and pulsar glitches. In this Thesis, Hartree-Fock-Bogoliubov calculations with Bloch boundary conditions are used to model the inner crust. In order to get the most reliable results, modern energy density functionals are implemented, together with a realistic pairing interaction. For the extraction of the superfluid density, a framework based on a Galilean transformation is constructed, allowing one to get this quantity within a time-independent calculation. Together with fully self-consistent numerical results, this Thesis provides also a derivation of the expression for the superfluid density in the Bardeen-Cooper-Schrieffer approximation. This allows one to evaluate this quantity directly from the single-particle band structure, thus offering a benchmark for complete results and providing insight into the interplay between periodicity and superfluidity. The goal of this Thesis is to clarify the role of superfluidity and provide a more precise determination of the superfluid density, contributing to our understanding of glitch mechanisms and more generally the behavior of neutron star matter.