Extremely metal-deficient stars - I. Evolution of 10 M stars to C-ignition

Abstract

The evolution of 10 M⊙ star models of Y = 0.20 and Z = 10−8, 10−6, 10−5 and 0.0004 has been followed from the main sequence phase to carbon ignition. A previous evolution of a Z = 0, 10 M⊙ model has been also extended to C-ignition. The main results may be summarized as follows: (i) A peculiar dependence of H-exhausted cores on Z has been found, with a minimum mass size when Z = 10−8. As a consequence, in a very metal-poor stellar population the upper mass limit for degenerate carbon ignition results larger than in more metal-rich stars. We find that this upper mass limit increases by about 2 M⊙ (up to ∼9.5 M⊙) if Z decreases from 0.0004 to 10−8 by mass. (ii) The upper mass limit for C-shell ignition has been found larger than in previous works. (iii) All along the studied phases Z = 0, Z = 10−8 and Z = 10−6 star models never become red giants. (iv) The layers surrounding the He cores in stars with Z < 10−6 are sensitively He-enriched. As a consequence an additional amount of He has to be added to the yields derived by bare He-core evolutions.