2008A&A...479L...9C

Fast stellar rotation is currently the most promising mechanism for producing primary nitrogen in metal-poor massive stars. Chemical evolution models computed with the inclusion of the yields of fast rotating models at a metallicity Z=10^–8 can account for the high N/O abundances observed in normal metal-poor halo stars. If, as believed, intermediate mass stars did not have enough time to contribute to the interstellar medium enrichment at such low metallicities, the above result constitutes a strong case for the existence of fast rotators in the primordial Universe. An important result of stellar models of fast rotators is that large quantities of primary ¹³C are produced. Hence, our goal is to investigate the consequence of fast rotation on the evolution of the ¹²C/¹³C ratio in the interstellar medium at low metallicity. We compute the evolution of the ¹²C/¹³C ratio for the first time at very low metallicities upon the inclusion of fast rotators at Z=10^–8. We predict that, if fast rotating massive stars were common phenomena in the early Universe, the primordial interstellar medium of galaxies with a star formation history similar to the one inferred for our galactic halo should have ¹²C/¹³C ratios between 30-300. Without fast rotators, the predicted ¹²C/¹³C ratios would be ∼ 4500 at [Fe/H]=-3.5, increasing to ∼31000 at around [Fe/H]=-5.0. Current data on very metal-poor giant normal stars in the galactic halo agree better with chemical evolution models including fast rotators. The expected difference in the ¹²C/¹³C ratios, after accounting for the effects of the first dredge-up, between our predictions with/without fast rotators is of the order of a factor of 2-3. However, larger differences (a factor of ∼60-90) are expected for giants at [Fe/H]=-5 or turnoff stars already at [Fe/H]=-3.5. To test our predictions, challenging measurements of the ¹²C/¹³C in more extremely metal-poor giants and turnoff stars are required.