38 A. Mezzacappa et al.
Any information about the rebounding inner core would be conveyed to
the outer core via pressure waves that propagate radially outward at the
speed of sound. When these waves reach the point at which the infall is
supersonic—i.e., the “sonic point”—they are swept in as fast as they attempt
to propagate outward. The net result: No information about the rebounding
inner core reaches the infalling outer core, which in turn sets up a density,
pressure, and velocity discontinuity in the flow—i.e., a shock wave. This shock
wave will ultimately be responsible for propagating outward through the star,
disrupting the star in a core collapse supernova. Schematically, the shock wave
is launched and energized by the rebounding inner core piston. (In Fig. 2, the
shock wave is represented schematically by the orange circle in stages 5–7.)
If the shock were to propagate outward without stalling, we would have
what has been called a “prompt” explosion. All of the realistic models com-
pleted to date suggest that this does not occur. Because the shock loses energy
in dissociating the iron nuclei that pass through it as it propagates outward,
the shock is enervated. Nuclei exist in the regions shaded with a light blue in
Fig. 2. The yellow regions in stages 5–7 are regions of shock- compressed and
heated material in which the nuclei are dissociated into nucleons. The shock
loses additional energy in the form of electron neutrinos. The copious pro-
duction of electron neutrinos occurs when the core electrons capture on the
newly dissociation-liberated protons. These neutrinos are initially trapped
but escape when the shock moves out beyond the electron neutrinosphere.
This gives rise to the “electron neutrino burst” in a core collapse supernova,
which is the first of three major phases of the three-flavor neutrino emis-
sion during these events. As a result of these two enervating mechanisms,
the shock stalls in the iron core. How the shock is reenergized in a “delayed
shock mechanism” is currently the central question in core collapse supernova
theory.
At the time the shock stalls, the core configuration is composed of a cen-
tral radiating object: the proto-neutron star (Fig. 3), which will go on to
form a neutron star or black hole. The proto-neutron star has a relatively
cold inner core, composed of unshocked bulk nuclear matter, together with
a hot “mantle” of nuclear matter that has been shocked but not expelled.
The ultimate source of energy in a core collapse supernova is the ∼10
53
erg
of gravitational binding energy associated with the formation of the neutron
star. This gravitational binding energy is released after core bounce over ∼10
seconds in the form of a three-flavor neutrino “pulse.” This marks the sec-
ond phase of the neutrino emission from a core collapse supernova. Electron
neutrinos are produced during stellar core collapse by electron capture on
protons and nuclei, but after bounce, in the hot proto-neutron star mantle,
all three flavors of neutrinos and their antineutrinos are produced and are
emitted as the mantle cools and contracts during its “Kelvin-Helmholtz” cool-
ing phase. The neutrinos are emitted from their respective neutrinospheres.
The neutrinospheres are defined in a way similar to the way the photosphere