Etudes des proprietes des neutrinos dans les contextes ...

tel-00450051, version 1 - 25 Jan 2010
The forward shock
After revival the shock propagates out through the star. Actually, the more the
energy deposited is important, the faster the forward shock will be. Moreover,
what can be immediately seen on Fig.(7.2) is that the forward shock creates a
high jump in the density, about one order of magnitude compared to the density
just in front the shock, this being valid all along its propagation.
As we will see in the following sections this discontinuity will have an important
impact on the neutrino propagation through the star. When neutrinos will
cross such a jump in a density, if it corresponds to the high resonance density
region, their evolution will be non adiabatic. Indeed, if we look at Eq.(B.21) in
appendix B, the adiabaticity factor depends on the derivative of the density. A
large density gradient will yield a very important value for the derivative of the
density and therefore a very small value for the adiabaticity parameter, even if
the mixing angle is ”large” 2 . Note that the L resonance will not be affected by
the forward shock because the latter will be weakened when it arrives to the place
where the L-resonance occurs. Moreover, the mixing angle in this case is large,
which helps the resonance to be adiabatic.
The reverse shock
Contrary to the forward shock, it is not present in the initial density profile but
develops after the heating if the deposited energy is important enough. Indeed,
the heating that led to the regeneration of the forward shock continues to accelerate
the material above the proto-neutron star. The matter being less and
less dense, the neutrino-driven wind accelerates. A reverse shock forms when the
expanding neutrino-driven wind becomes supersonic and collides with the slower
earlier supernova ejecta. On Fig.(7.2), one can see it behind the forward shock,
and it presents a smaller jump in density than the forward shock.
Due to the decrease of the wind strength (related to the diminution of the
neutrino heating), the reverse shock slows down to stall at around t = 2.5 s after
explosion, and then moves back towards the core ( t = 3s in Fig.(7.2)). Such a
feature has been observed in [113]. However, since the authors use the factorization
probability approximation (see section 1.2.2), they have missed an important
phenomenon that occurs in the presence of a forward and a reverse shock: phase
effects.
7.1.2 Multiple resonances and phase effects
The presence of a shock wave engenders two important effects: it makes the H
resonance temporarily non adiabatic on the one hand and induces multiple H res-
2 Since we consider the H-resonance the mixing angle is θ13, and can be at most about 9 ◦
(See chapter 2)
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