Etudes des proprietes des neutrinos dans les contextes ...

tel-00450051, version 1 - 25 Jan 2010
is the most tightly bound nucleus, there remains no thermonuclear fuel to burn.
The core contracts and the increased temperature causes photo-dissociation of
iron through the process:
γ + 56 Fe → 13 4 He + 4 n (3.1)
This reaction consumes about 124 MeV of energy and reduces the kinetic energy
and pressure of the electrons. Therefore, compression yields a lesser pressure
increase than would occur in the absence of photo dissociation. Electron capture
of nuclei,
e − + N(Z, A) → N(Z − 1, A) + νe
(3.2)
and free protons, via an inverse β-decay process:
e − + p → n + νe
(3.3)
favored by the high electron Fermi energy, additionally reduce the number and
pressure of the electrons. At the onset of collapse, when the density of the iron
core is not too high, the electron neutrinos produced by electron capture leave
freely 1 the core carrying away most of the kinetic energy of the captured electrons
since their mean free path is longer than the radius of the core. In this so-called
capture phase electron neutrinos have a non-thermal spectrum and average energy
that grows from about 12 to about 16 MeV. The luminosity reaches about 10 53
erg s −1 . 2
The value of the Chandrasekhar mass decreases until it becomes smaller than the
core mass, because of the combined effect of iron photo-dissociation and electron
capture, that diminishes the electron pressure. The collapse commences when
the pressure of degenerate relativistic electrons can no longer sustain the weight
of the core. As the nuclear density and temperature increase, the processes
accelerate, favoring the collapse by lowering further the electron pressure. This
collapse fasters until it is halted by hard core nuclear repulsion leading to a
bounce back. In the process of bounce back, whose comprehension has known
recent developments 3 , the stellar envelopes explode causing an intense flash of
neutrinos and photons. In the process of the collapse, the stellar core of about
1 Indeed, if one recalls the formula of mean free path of neutrino in matter from the previous
chapter, one has
l ∼ 1
Nσ ∼
1038cm (N/cm−3 )(E M /GeV2 . (3.4)
)
where N is the number density of target particles which are nucleon with mass M ∼ 1 GeV.
Inside the iron core, the nucleon density is less than 10 10 NA.cm−3 , neutrinos with energy of
the order of 1 MeV will have a mean free path greater than 100 km.
2 51 Because the capture phase is very short (less than about 10 ms), only about 10 ergs are
released before the core bounces.
3Instead of an immediate bounce back, an oscillatory movement of the core appears, called
SASI (Steady Accretion Shock Instability) mode, which may trigger the explosion.
53