  Two-dimensional (2D) hydrodynamical simulations of progenitor evolution of a
23 solar mass star, close to core collapse (about 1 hour, in 1D), with
simultaneously active C, Ne, O, and Si burning shells, are presented and
contrasted to existing 1D models (which are forced to be quasi-static).
Pronounced asymmetries, and strong dynamical interactions between shells are
seen in 2D. Although instigated by turbulence, the dynamic behavior proceeds to
sufficiently large amplitudes that it couples to the nuclear burning. Dramatic
growth of low order modes is seen, as well as large deviations from spherical
symmetry in the burning shells. The vigorous dynamics is more violent than that
seen in earlier burning stages in the 3D simulations of a single cell in the
oxygen burning shell, or in 2D simulations not including an active Si shell.
Linear perturbative analysis does not capture the chaotic behavior of
turbulence (e.g., strange attractors such as that discovered by Lorenz), and
therefore badly underestimates the vigor of the instability. The limitations of
1D and 2D models are discussed in detail. The 2D models, although flawed
geometrically, represent a more realistic treatment of the relevant dynamics
than existing 1D models, and present a dramatically different view of the
stages of evolution prior to collapse. Implications for interpretation of
SN1987A, abundances in young supernova remnants, pre-collapse outbursts,
progenitor structure, neutron star kicks, and fallback are outlined. While 2D
simulations provide new qualitative insight, fully 3D simulations are needed
for a quantitative understanding of this stage of stellar evolution. The
necessary properties of such simulations are delineated.
