Evolutionary models of the Crab Nebula's progenitor.

Evolutionary models of the Crab Nebula's progenitor are reviewed. Both single massive stars and close binary systems are discussed. A model of close binary origin is possible if two intermediate mass stars evolve to form a helium-rich common envelope in which double degenerate cores merge to form a neutron star. Massive single star models are compared with the elemental abundances of the Crab Nebula. The abundances of the Crab Nebula seem to be a result of combined effects of helium shell burning, convective mixing of hydrogen-rich and helium layers (dredge-up), and presupernova mass loss. In particular, carbon abundance of the Crab Nebula is consistent with stars of mass around 9 solar masses.