When a neutron star forms, the neutrinos that are present are primarily generated during the stellar collapse that precedes the formation of the neutron star. This collapse occurs in massive stars during a supernova explosion.
During the core collapse of a massive star, the tremendous gravitational forces cause the core to reach extreme densities and temperatures. These conditions lead to a rapid increase in the number of interactions between particles and the production of enormous amounts of energy.
One of the key processes that occurs during a supernova is the conversion of protons into neutrons, a process called neutronization. This conversion happens through the emission of electron neutrinos. As the core collapses, protons and electrons combine to form neutrons and electron neutrinos:
p + e⁻ → n + νe
The newly formed neutrons combine with the preexisting protons to create the dense neutron-rich matter characteristic of neutron stars. During this process, a large number of electron neutrinos are produced and emitted from the collapsing core.
It is important to note that the neutrinos are not confined to the neutron star itself but are emitted and carry away a significant fraction of the gravitational potential energy released during the supernova explosion. These neutrinos stream out into space, carrying away energy and angular momentum from the collapsing core. Neutrinos have a weak interaction with matter, which allows them to escape the dense environment of the collapsing star without being significantly absorbed.
The study of neutrinos emitted during supernova events provides valuable insights into the dynamics of stellar collapses, and their detection is a crucial area of research in astrophysics.