The surface carbon and nitrogen abundances in models of ultra metal-poor   stars
  We investigate whether the observed high number of carbon- and nitrogen-enhanced extremely metal-poor stars could be explained by peculiar evolutionary properties during the core He flash at the tip of the red giant branch. For this purpose we compute a series of detailed stellar models expanding upon our previous work; in particular, we investigate if during the major He flash the penetration of the helium convective zone into the overlying hydrogen-rich layers can produce carbon- and nitrogen-rich abundances in agreement with current spectroscopic observations. The dependence of this phenomenon on selected model input parameters, such as initial metallicity and treatment of convection is examined in detail. 
H. Schlattl (1), M. Salaris (1), S. Cassisi (2), A. Weiss (3) ((1)   ARI, Liverpool JMU, UK;(2) INAF, OAC Teramo, Italy; (3)...