Neutron stars are one of the most compact objects in the universe. They typically have one-two solar masses and radii of about ten kilometers. They are believed to have an onion-like configuration, with an atmosphere and two main layers, the crust and the core. The characterization of these layers is, however, still a matter of intense analysis.
One possible way to extract information on the structure of neutron stars is to study the dynamical processes that occur in their interior and the associated transport coefficients. A transport coefficient refers to the response of a system to some external perturbation. For example, the shear and bulk viscosities are associated to the resistance of the system to a deformation due to the shear and tensile stress, respectively. Other transport coefficients, such as the thermal or electrical conductivities, are related to the conduction of heat or electric current.
The determination of the transport mechanisms inside neutron stars depends crucially on their structure and it is fundamental for understanding, among others, the cooling, the damping of certain hydrodynamical modes (the r-modes) as well as the relaxation processes after accretion in binary systems or glitches.
Cooling of neutron stars
Neutron stars are formed at very high interior temperatures in the core of a supernova explosion. After an initial thermal relaxation stage, neutrino emission dominates the cooling of neutron stars until photon emission overtakes . The thermal relaxation period, which lasts for the first hundred years, crucially depends on the thermal conductivity of the crust. As a consequence, the cooling of young neutron stars turns out to be very sensitive to the physics of the crust .
R-mode oscillations are toroidal hydrodynamical modes that occur in rotating stars, the Coriolis force acting as their restoring force. These modes are generally unstable through their coupling to gravitational radiation emission. The r-modes, however, can be damped due to dissipative phenomena so that the star can rotate without losing angular momentum. The dissipative or viscous damping strongly depends on the interplay and characterization of the crust and the core of neutron stars .
Relaxation after accretion in binary systems or glitches
The physics of the crust determines its relaxation after the deposit of heat by accretion in a binary system . Moreover, the structure of the core is fundamental for the analysis of the heat conduction mechanisms that take place in pulsars in their relaxation after a sudden increase of the rotation speed (glitch) .
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Author: Laura Tolos