Thermal Evolution of Neutron Stars

Neutron stars (NSs) are astrophysical objects produced by the collapse of a stellar core in a supernova explosion when the degenerate pressure of neutrons halts the contraction. Under such conditions, matter is to a first approximation made of a plasma of nucleons ad electrons. Such a system is governed by strong nuclear interactions, the collective properties of which are poorly known as yet. This leaves still open the question of what is the equation of state of mater at ultra-high densities. Many possible forms for this equation have been proposed, some of them predicting the presence of \emph{exotic states} of matter, i.e., the formation of mesons, strange particles or even deconfined quarks at very high densities (several times the nuclear saturation density). The main tool to get information about the internal structure of a NS is the study of the reactions occurring in its interior. These produce neutrinos and photons, that subtract energy causing to the cooling of the star, that is born very hot after the collapse (T~10^11 K). Observations can provide the star age and surface temperature, allowing one to reconstruct its thermal history and hence inferring information about the physical processes at work in the interior. This work considers the main processes involved in the thermal evolution of an isolated NS, focusing on the different neutrino emission channels and on the effects of th presence of a superfluid phase in the core. Different reactions produce different thermal evolutions; in particular two main cooling scenarios can be identified, fast cooling and slow cooling. Albeit observations do not provide conclusive evidences, the comparison of models with data is not consistent with fast cooling at present. This put constraints on the equation of state, disfavouring those ones which predict the presence of exotic phases in the inner core.