Stars as Laboratories for Fundamental Physics - MPP Theory Group

Supernova Neutrinos 399
beyond the sonic point at the edge of the inner core which now encompasses
about 0.8 M ⊙ . As material continues to fall onto the inner core
at supersonic velocities a shock wave builds up at the sonic point which
is at the edge of the inner core, not at its center. As more material
moves in, more and more energy is stored in this shock wave which
almost immediately begins to propagate outward into the collapsing
outer part of the iron core—see the thick solid line in Fig. 11.1. Assuming
that enough energy is stored in the shock wave it will eventually
eject the stellar mantle outside of what was the iron core. The rebound
or “bounce” of the collapse turns the implosion of the core into an
explosion of the outer star—a SN occurs.
This “bounce and shock” scenario of SN explosions was first proposed
by Colgate and Johnson (1960) and then elaborated by a number
of authors (see Brown, Bethe, and Baym 1982 and references therein).
In practice, however, the story of SN explosions appears to be more
complicated than this “prompt explosion scenario.” Neutrino losses
and the dissociation of the iron material through which the shock wave
propagates dissipate much of the shock’s energy so that in typical calculations
it stalls and eventually recollapses. It is currently believed that
the energy deposition by neutrinos revives the shock wave, leading to
the “delayed explosion scenario” detailed in Sect. 11.1.3 below.
11.1.2 Deleptonization and Cooling
After core bounce and the formation of a shock wave the next dramatic
step in the evolution of the core is when the outward propagating
shock breaks through the “neutrino sphere,” i.e. the shell within which
neutrinos are trapped, most effectively by the coherent scattering on
heavy nuclei. As the passage of the shock wave dissociates these nuclei,
it is easier for neutrinos to escape. Moreover, the protons newly
liberated from the iron nuclei allow for quick neutronization by virtue
of e − + p → n + ν e , causing a short ν e burst which is often called the
“prompt ν e burst” or “deleptonization burst” (phase No. 2 in Figs. 11.1
and 11.3). However, the material which is quickly deleptonized encompasses
only a few tenths of a solar mass so that most of the leptons
remain trapped in the inner core (Fig. 11.2).
At this stage, the object below the shock has become a “protoneutron
star.” It has a settled inner core within the radius where the
shock wave first formed and which consists of neutrons, protons, electrons,
and neutrinos (lepton fraction Y L ≈ 0.35). The protoneutron
star also has a bloated outer part which has lost a large fraction of