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Peculiar Chemical Abundances in the Starburst Galaxy M82 and Hypernova Nucleosynthesis

X-ray observations have shown that the chemical abundance in the starburst galaxy M82 is quite rich in Si and S compared to oxygen. Such an abundance pattern cannot be explained with any combination of conventional Type I and II supernova yields. In addition, the energy-to-heavy-element mass ratio of the observed hot plasma is much higher than the value resulting from normal supernovae. We calculate abundances for 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 hypernova nucleosynthesis. Such hypernova explosions are expected to occur for stars more massive than ≳20-25 M☉, and they likely dominate the starburst, because the time since the starburst in M82 is estimated to be as short as ~106-107 yr. We also investigate pair-instability supernovae (~150-300 M☉) and conclude that the energy-to-heavy-element mass ratio in these supernovae is too small to explain the observation.