The Peculiar Evolutionary Behaviour of Population III Stars

Abstract.

The role of metal deficient stars in the scenario of galactic evolution is shortly recalled and the origin of their peculiar evolutionary behaviour is discussed to the light of the physical ingredients governing stellar structures. On this basis we define as Pop III all the metal deficient stars which are predicted to succeed in self producing carbon during their H burning phase. One finds that such Pop III stars are mainly characterised by a decreased upper limit for the appearance of electron degeneracy in the core of H-shell burning structures and by an increased upper limit for the occurrence of degenerate CO cores, both these occurrences being the result of the increased efficiency of pp burning. As a consequence, in Pop III one finds an increased prob ability for C detonation events. Further dynamic instabilities either at the He flash or during the AGB evolution are recalled, discussing the open problems still affecting the present knowledge of Pop III stellar structures.