The role of Z=0 AGB stars on the early chemical enrichment
  We present theoretical evolutionary models for Z=0 stars in the mass range 4< M/Mo <8 from the pre-main sequence up to the AGB phase. Contrary to previous calculations we found that these stars develop normal thermal pulses and third dredge-up episodes. Special attention is devoted to analyze the chemical enrichment in the envelope due to the above mechanisms. As a consequence, we show that these stars become carbon and nitrogen rich. Using different IMFs proposed in the literature for the Population III stars, we study their contribution to the pre-galactic chemical enrichment. It is found that Z=0 AGB stars could significantly contribute to 7Li, 12C and 14N and produce extreme non-solar 24Mg/25Mg/26Mg ratios. However, the net contribution is very sensitive to the IMF adopted and to the fraction of primordial matter which goes into stars. 
Inma Dominguez, Carlos Abia, Oscar Straniero, Alessandro Chieffi and   Marco Limongi