\to u +e+ \bar{\nu} \quad  u +e \to d+ \nu }                       $  
                                                  $                                          10^{23}-10^{24}$
  ----------------------------------------------------------------------------------------------------------------------

  : Leading processes of fast neutrino emission in nucleon matter and three models of exotic matter

\[tab-nucore-fast\]

Let us summarize the main neutrino emission mechanisms in the neutron star core. They are strongly affected by baryon superfluidity. More details can be found, e.g., in Refs. [@pethick92; @yls99; @ykgh01].

#### Neutrino emission in nonsuperfluid cores

The major neutrino mechanisms in nucleon matter of the outer core ($\rho \la 2 \, \rho_0$) are modified Urca process and nucleon-nucleon bremsstrahlung. They are listed in Table \[tab-nucore-slow\] (from Ref. [@yp04]), where N denotes a nucleon (n or p). They are relatively weak and produce slow neutrino cooling. The modified Urca process differs from its direct Urca progenitor, described below, by an additional nucleon-spectator that is required to satisfy momentum conservation of reacting particles. All reactions involving electrons can involve muons instead (if available in dense matter).

At higher $\rho$, in the inner core, neutrino emission can be strongly enhanced by new mechanisms (Table \[tab-nucore-fast\], from [@yp04]). The enhancement level greatly depends on the EOS and composition of superdense matter that is most important for the cooling problem. The strongest 