Séminaires au LIRA

The chemical abundances in the most metal poor stars provide information of the nature of the first stars. Among the useful elements there is the 12C/13C isotopic ratio which , however, is difficult to measure and in particular at low metallicity. We show that the CEMP-no stars, Carbon enhanced with no neutron-capture elements, offer a unique opportunity to measure the carbon isotopic ratio at the lowest metallicities. We report on the results of the GTO ESPRESSO program on extremely poor stars where we have studied at very high resolution (R 140000) six of the most extreme metal poor stars known all with Fe/H] < -4.5 and dawn to -7. In all the program CEMP-no stars we succeed in measuring the ratio with values in the range 39