Evolution of Single Stars. V. Carbon Ignition in Population I Stars

The results of model computations are presented for Population I stars of 3, 5, 7, 10 and 15 M_odot in the evolutionary phases from helium exhaustion in the centre up to carbon ignition in the centre. The massive models (10 M_odot and 15 M_odot) ignited carbon under the nondegenerate conditions. During the evolutionary phases described here the contraction of the cores of these models was independent of the structure of stellar envelopes. The intermediate mass models (3, 5 and 7 M_odot) formed highly degenerate carbon-oxygen cores that converged to a common evolutionary track. Carbon content was increasing outwards in those cores as a result of helium shell burning at the steadily higher temperatures with the increasing core mass. Carbon was ignited at the density of 3x10^9 g/cm^3 in the centre of 1.39 M_odot core. If there were no mass loss prior to carbon ignition the stars in the mass range of 1.4 M_odot - 8 M_odot should follow this pattern of evolution. Mass loss may increase the lower mass limit. This penetration occurs while the hydrogen burning shell source temporarily disappears after the helium exhaustion in the centre. The convective envelope never reaches an active hydrogen burning shell.