  Hydrodynamics and explosive nucleosynthesis in bipolar supernova explosions
are examined to account for some peculiar properties of hypernovae as well as
peculiar abundance patterns of metal-poor stars. The explosion is supposed to
be driven by bipolar jets which are powered by accretion onto a central
remnant. We explore the features of the explosions with varying progenitors'
masses and jet properties. The outcomes are different from conventional
spherical models. (1) In the bipolar models, Fe-rich materials are ejected at
high velocities along the jet axis, while O-rich materials occupy the central
region whose density becomes very high as a consequence of continuous accretion
from the side. This configuration can explain some peculiar features in the
light curves and the nebular spectra of hypernovae. (2) Production of $^{56}$Ni
tends to be smaller than in spherical thermal bomb models. To account for a
large amount of $^{56}$Ni observed in hypernovae, the jets should be initiated
when the compact remnant mass is still smaller than $2-3\msun$, or the jets
should be very massive and slow. (3) Ejected isotopes are distributed as
follows in order of decreasing velocities: $^{64}$Zn, $^{59}$Co, $^{56}$Fe,
$^{44}$Ti, and $^{4}$He at the highest velocities, $^{55}$Mn, $^{52}$Cr,
$^{32}$S, and $^{28}$Si at the intermediate velocities, and $^{24}$Mg, $^{16}$O
at the lowest velocities. (4) The abundance ratios (Zn, Co)/Fe are enhanced
while the ratios (Mn, Cr)/Fe are suppressed. This can account for the abundance
pattern of extremely metal-poor stars. These agreements between the models and
observations suggest that hypernovae are driven by bipolar jets and have
significantly contributed to the early Galactic chemical evolution.

Astrophysics