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11.1 Neutrino emission processes – an overview

Table 6: Main neutrino emission processes in neutron star crusts. Symbols: γ stands for a photon or a plasmon; (A, Z ) stands for a nucleus with charge number Z and mass number A; lepton symbol x = e,μ,τ; neutron quasiparticle (neutron-like elementary excitation) in superfluid neutron gas is denoted by ^n.
Process name Reaction
e− e+ pair annihilation e− + e+ −→ νx + ¯νx
Plasmon decay γ −→ ν + ¯ν x x
Photoneutrino emission − − γ + e −→ e + νx + ¯νx
Electron synchrotron radiation e− −→ e− + νx + ¯νx
Electron-nucleus Bremsstrahlung − − e + (A, Z) −→ e + (A, Z ) + νx + ¯νx
Cooper pair formation ^n + ^n −→ νx + ¯νx

There is a great wealth of neutrino emission processes in hot neutron star crusts. These processes are associated with weak interaction involving electrons, positrons, nuclei, and free nucleons. As soon as the crust becomes neutrino-transparent, which occurs in about a minute after neutron star birth, these neutrinos freely leave the crust, taking away thermal energy, and contributing in this way to crust cooling. Let us notice that, because of a finite thermal equilibration timescale, during the first few decades of a neutron star’s life the thermal evolution of the crust is decoupled from that of the liquid core.

In this section we do not pretend to give a complete review of neutrino emission processes. We will limit ourselves to a brief presentation of six main neutrino emission mechanisms, listed in Table 6. Some other neutrino emission processes are briefly mentioned in Section 11.8. We will dwell on the qualitative features of main contributions to the neutrino emissivity of the crust, Q ν, which is defined as the energy radiated in neutrinos (in erg) from 1 cm3 during 1 s. In particular, we will discuss, at the qualitative level, the density and temperature dependences of the contributions from a given process. A reader interested in a more complete and detailed description of neutrino emission processes from the crust is referred to the review of Yakovlev et al. [428Jump To The Next Citation Point].

Processes reviewed in Sections 11.211.6 have been studied mainly at BBB = 0. The effects of BBB on Q ν were calculated only for some processes, and will be briefly mentioned at the end of the corresponding sections. The presence of BBB makes possible the synchrotron radiation of electrons, considered in Section 11.7.

A summary of the ρ − T dependence of different contributions to Q ν, and a discussion of their relative importance in different regions of the ρ − T plane, are presented in Section 11.9.

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