The Crab Nebula's Composition and Precursor Star Mass
  We present results of new photoionization calculations for investigating gaseous regions that represent potentially expected stages of nuclear processing in the Crab Nebula supernova remnant. In addition to gas resulting from CNO-processing and oxygen-burning, as previously reported, a large component of the nebula appears to be carbon-rich. These results suggest that the precursor star had an initial mass of 9.5 solar masses or more. 
Gordon M. MacAlpine and Timothy J. Satterfield