Evidence of mixing in extremely metal-poor giants

Abstract

Thirty five extremely metal-poor (EMP) giants (22 of them with a metallicity [Fe/H] ≤ -3.0) have been observed with UVES at the VLT. In these stars the abundance of lithium carbon and nitrogen and also the 12C/13C ratio have been measured. It is shown that, in about half of them, the carbon abundance and the lithium abundance are very low, on the contrary the nitrogen abundance is high. Moreover the value of the 12C/13C ratio is close to the equilibrium value of the CN cycle. All these phenomena imply a mixing between the surface of the star and the H burning layers. We also study the influence of the mixing on the oxygen, sodium and aluminum abundance. © 2006 Springer-Verlag Berlin Heidelberg.