Peculiar Chemical Abundances in the Starburst Galaxy M82 and Hypernova Nucleosynthesis

Abstract

X-ray observations have shown that the chemical abundance in the starburst galaxy M82 is quite rich in Si and S compared with oxygen. Such an abundance pattern cannot be explained with any combination of conventional Type I and II supernova yields. Also the energy to heavy element mass ratio of the observed hot plasma is much higher than the value resulted from normal supernovae. We calculate explosive nucleosynthesis in core-collapse hypernovae and show that the abundance pattern and the large ratio between the energy and the heavy element mass can be explained with the hypernova nucleosynthesis. Such hypernova explosions are expected to occur for stars more massive than >~ 20-25 Msun, and likely dominating the starburst, because the age after the starburst in M82 is estimated to be as short as ~ 10^6 - 10^7 yr. We also investigate pair-instability supernovae (~ 150-300 Msun) and conclude that the energy to heavy element mass ratio in these supernovae is too small to explain the observation.