Etudes des proprietes des neutrinos dans les contextes ...

tel-00450051, version 1 - 25 Jan 2010
3.1 General description of core-collapse supernovae
3.1.1 A qualitative picture
The types of supernovae
For historical reasons, SNe are divided into different types because of their spectroscopic
characteristics near maximum luminosity and by the properties of the
light curve, which depend on the composition of the envelope of the SN progenitor
star. The two wide categories called type I and type II are characterized by
the absence or presence of hydrogen lines. However, the most important physical
characteristic is the mechanism that generates the supernova, which distinguishes
SNe of type Ia from SNe of type Ib, Ic and II.
We are here interested about the latter type, simply because they produce a huge
flux of neutrinos of all types. These SNe are generated by the collapse of the core
of massive stars (M 8M⊙), which leaves a compact remnant.
Historically, the study of SNe was initiated by W. Baade and F. Zwicky in the
early 1930s. They already suggested that the source of the enormous quantity
of energy released in SNe is the gravitational collapse of a star to a neutron star
and that SNe may be sources of cosmic rays. In the early 40’s, Gamow and
Schoenberg were the first to speculate that neutrino emission would be a major
effect in the collapse of a star.
The core-collapse mechanism
As stars evolve, like the Sun, they first get their energy burning hydrogen to
helium. The Helium being heavier, settles to the core of the star. The duration
of this process depends mainly on the mass of the star. Indeed, the process last
longer for a less massive star, and shorter for more massive ones. Towards the
end of the hydrogen burning period, a period of gravitational contraction heats
up the core and starts the phase of Helium burning to carbon. The carbon being
heavier, will similarly to Helium with hydrogen before, settles to the center with
respectively Helium then Hydrogen floating above. The process of contraction
and heating repeats itself towards the end of the helium burning phase when
carbon will start burning to neon. Similar processes then lead from neon to
oxygen, and from oxygen to silicon. If a star is more massive than 10 − 11 M⊙,
silicon burning can start at T ≃ 3.4 × 10 9 K giving rise to iron.
At that time, the star has an onion-like structure, with an iron core surrounded by
shells composed of elements with decreasing atomic mass. At this point the iron
core has a mass of about 1 solar mass, a radius of a few thousand km, a central
density of about 10 10 g·cm −3 , a central temperature of about 1 MeV, and its
weight is sustained by the pressure of degenerate relativistic electrons. Since iron
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