The origin of low [alpha/Fe] ratios in extremely metal-poor stars
  We show that the low ratios of $\alpha$ elements (Mg, Si, and Ca) to Fe
recently found for a small fraction of extremely metal-poor stars can be
naturally explained with the nucleosynthesis yields of core-collapse
supernovae, i.e., $13-25M_\odot$ supernovae, or hypernovae. For the case
without carbon enhancement, the ejected iron mass is normal, consistent with
observed light curves and spectra of nearby supernovae. On the other hand, the
carbon enhancement requires much smaller iron production, and the low
[$\alpha$/Fe] of carbon enhanced metal-poor stars can also be reproduced with
$13-25M_\odot$ faint supernovae or faint hypernovae. Iron-peak element
abundances, in particular Zn abundances, are important to put further
constraints on the enrichment sources from galactic archaeology surveys.
