Bipolar supernova explosions: Nucleosynthesis and implications for abundances in extremely metal-poor stars

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 assumed to be driven by bipolar jets that are powered by accretion onto a central remnant. The energy injection rate by the jets is assumed to be proportional to the accretion rate, i.e., (E) over dot(jet) = alpha(M) over dot c(2). 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 Ni-56 tends to be smaller than in spherical thermal bomb models. To account for a large amount of 56Ni observed in hypernovae, the jets should be initiated when the compact remnant mass is still smaller than 2 - 3 M-., or they should be very massive and slow. (3) Ejected isotopes are distributed as follows in order of decreasing velocities: Zn-64, Co-59, Fe-56, Ti-44, and He-4 at the highest velocities; Mn-55, Cr-52, S-32, and Si-28 at the intermediate velocities; and Mg-24 and O-16 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.