2010ApJ...720L...1I

There have been suggestions that the abundance of extremely metal-poor (EMP) stars can be reproduced by hypernovae (HNe), not by normal supernovae (SNe). However, recently it was also suggested that if the innermost neutron-rich or proton-rich matter is ejected, the abundance patterns of ejected matter are changed, and normal SNe may also reproduce the observations of EMP stars. In this Letter, we calculate explosive nucleosynthesis with various Y_e and entropy, and investigate whether normal SNe with this innermost matter, which we call the "hot-bubble" component, can reproduce the abundance of EMP stars. We find that neutron-rich (Y_e= 0.45-0.49) and proton-rich (Y_e= 0.51-0.55) matter can increase Zn/Fe and Co/Fe ratios as observed, but tend to overproduce other Fe-peak elements. In addition, we find that if slightly proton-rich matter with 0.50 ≤ Y_e< 0.501 with s/k_b∼ 15-40 is ejected as much as ∼0.06 M_☉, even normal SNe can reproduce the abundance of EMP stars, though it requires fine-tuning of Y_e. On the other hand, HNe can more easily reproduce the observations of EMP stars without fine-tuning. Our results imply that HNe are the most likely origin of the abundance pattern of EMP stars.