On the evolution of massive stars through the core carbon-burning phase

Results are presented for detailed calculations of the evolution of model stars with Population I compositions and masses of 15 and 25 solar masses from the main-sequence phase to the core carbon-burning phase. Up to 1100 mass shells, a full reaction network in the later stages of evolution, and energy losses due to a direct electron-neutrino interaction are included in the computations. Those results which have a bearing on the observable properties of massive stars include the following: (1) objects near 15 solar masses will ignite and burn carbon as red supergiants, but objects as massive as 25 solar masses will ignite carbon in a region considerably to the blue of the red-supergiant branch; (2) there are effective upper limits to the mass and period of classical Cepheids which are determined by the intersection in the H-R diagram of a core carbon-burning band and a theoretical pulsational-instability strip; (3) stars near 15 solar masses will experience enhancement in surface abundance of hydrogen-burning products toward the end of their redward transit of the Hertzsprung gap at the end of core helium burning; and (4) stars of 25 solar masses will develop such enhancement, if at all, only after their evolution has become so rapid as to render them unobservable.