Stars as Laboratories for Fundamental Physics - MPP Theory Group

Supernova Neutrinos 405
Hayes, and Fryxell 1995). For the first time 2- and 3-dimensional calculations
have become possible. They reveal a large-scale convective
overturn (Fig. 11.5) which helps at revitalizing the shock because it
brings hot material from depths near the neutrino sphere quickly up
to the region immediately behind the shock, and cooler material down
to the neutrino sphere where it absorbs energy from the neutrino flow.
Successful explosions can be obtained for amounts of neutrino heating
where 1-dimensional calculations did not succeed. The sharp transition
between failed and successful explosions as a function of neutrino heating
that was found in 1-dimensional calculations (Fig. 11.4) is smoothed
out, but neutrino heating still plays a pivotal role at obtaining a successful
and sufficiently energetic explosion.
In summary, then, the current standard picture of SN explosions is
a modification and synthesis of the Colgate and Johnson (1960) shockdriven
and the Colgate and White (1966) neutrino-driven explosions.
At the present time there may still remain a quantitative problem at
obtaining enough neutrino energy deposition behind the shock wave to
guarantee a successful and sufficiently energetic explosion. It remains
to be seen if this scenario withstands the test of time, or if a novel
ingredient will have to be invoked in the future.
11.1.4 Nucleosynthesis
The universe began in a hot “big bang” which allowed for the formation
of nuclei from the protons and neutrons originally present in thermal
equilibrium with the ambient heat bath. The primordial abundances
“froze out” at about 22−24% helium, the rest hydrogen, and a small
trace of other light elements such as lithium. The present-day distribution
of elements was bred from this primeval mix mostly by nuclear
processes in stars; they eject some of their mass at the end of their lives
(Chapter 2), returning processed material to the interstellar medium
from which new stars and planets are born.
However, the normal stellar burning processes can produce elements
only up to the iron group which have the largest binding energy per
nucleon. Thus, the heavy elements must have been produced by different
processes at different sites. It has long been thought that nuclei
with A > ∼ 70 were predominantly made by neutron capture, notably
the s- and r- (slow and rapid) processes (Burbidge et al. 1957;
Cameron 1957; Clayton 1968; Meyer 1994). The site for the occurrence
of the r-process has remained elusive for the past three decades,
although many different suggestions have been made. The crux is that