Peculiar Chemical Abundances in the Starburst Galaxy M82 and Hypernova Nucleosynthesis

X-ray observations have shown that the chemical abundance in the starburstgalaxy M82 is quite rich in Si and S compared with oxygen. Such an abundancepattern cannot be explained with any combination of conventional Type I and IIsupernova yields. Also the energy to heavy element mass ratio of the observedhot plasma is much higher than the value resulted from normal supernovae. Wecalculate explosive nucleosynthesis in core-collapse hypernovae and show thatthe abundance pattern and the large ratio between the energy and the heavyelement mass can be explained with the hypernova nucleosynthesis. Suchhypernova explosions are expected to occur for stars more massive than >~ 20-25Msun, and likely dominating the starburst, because the age after the starburstin M82 is estimated to be as short as ~ 10^6 - 10^7 yr. We also investigatepair-instability supernovae (~ 150-300 Msun) and conclude that the energy toheavy element mass ratio in these supernovae is too small to explain theobservation.Comment: 11 pages, 6 figures, To appear in the Astrophysical Journal 578, 200