Etudes des proprietes des neutrinos dans les contextes ...

tel-00450051, version 1 - 25 Jan 2010
3.1.2 The neutrino fluxes and the neutrino spheres
To study the characteristics of neutrinos emitted from the supernova, we have
to discuss the mechanism for their production. There are basically two components
to the neutrino flux. The first one occurs during the first few milliseconds
after the post core bounces, when electron gets absorbed into protons to give
neutrinos via processes described in the previous paragraph in Eqs.(3.2) and
(3.3). These are known as the deleptonization neutrinos, which are very energetic.
The second type of neutrino fluxes are created almost simultaneously but
are produced through neutral-current processes which emit neutrinos all along
the cooling phase of the supernova explosion, whose most powerful period occurs
during the first 10-12 seconds.
The post bounce shockwave and the neutronization burst
When the density of the inner part of the core (about 0.8 M⊙) exceeds about
3 × 10 11 g.cm −3 , neutrinos are trapped in the collapsing material leading to an
adiabatic collapse with constant lepton number. During this stage, the inner part
of the core collapses homologously, i.e. it maintains its relative density profile.
The collapse velocity is proportional to the radius with v/r = 400 − 700.s −1 , yet
it remains subsonic. The outer part of the core collapses with supersonic nearly
free-fall velocity. After about one second from the start of instability, the density
of the inner core reaches the density of nuclear matter, about 10 14 g.cm −3 , and
the pressure of degenerate non-relativistic nucleons abruptly stops the collapse.
The inner core settles into hydrostatic equilibrium, while a supersonic shock wave
caused by the halting and rebound of the inner core forms at its surface. The
shock propagates outward through the outer iron core, which is still collapsing,
with an initial velocity of the order of 100 km.sec −1 . The gas that is infalling
at a velocity near free-fall is abruptly decelerated within the shock. Below the
shock it falls much more slowly on the surface of the proto-neutron star, accreting
it. Therefore, the proto-neutron star develops an unshocked core and a shocked
mantle. The core has a radius of the order of 10 km with a density of the order
of 10 14 g.cm −3 , as a nucleus. The mantle has a radius of about 100 km, with a
density decreasing from the nuclear density of the core to about 10 9 g.cm −3 at
the surface of the proto-neutron star, where the density has a steep decrease of
several orders of magnitude.
As the shock propagates through the infalling dense matter of the outer core, its
energy is dissipated by the photo dissociation of nuclei into protons and neutrons.
Thus, the material behind the shock wave is mainly composed of free nucleons.
Free protons have a high electron capture rate, leading to the transformation of
most protons into neutrons, with huge production of electron neutrinos. These
neutrinos pile up behind the shock, which is dense and opaque to them 5 , until
5 Here the mean free path is lesser than 1km while the density is greater than 10 12 g.cm −3 .
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