The12C(α, γ)16O Reaction Rate and the Evolution of Stars in the Mass Range 0.8 ≤ M/M⊙ ≤ 25

We discuss the influence of the C-12(alpha, gamma)O-16 reaction rate on the central He burning of stars in the mass range 0.8-25 M., as well as its effects on the explosive yields of a 25 M. star of solar chemical composition. We find that the central He burning is only marginally affected by a change in this cross section within the currently accepted uncertainty range. The only (important) quantity that varies significantly is the amount of C left by the He burning. Since the C-12(alpha, gamma)O-16 is efficient in a convective core, we have also analyzed the influence of the convective mixing in determining the final C abundance left by the central He burning. Our main finding is that the adopted mixing scheme does not influence the final C abundance provided the outer border of the convective core remains essentially fixed (in mass) when the central He abundance drops below similar or equal to0.1 dex by mass fraction; vice versa, even a slight shift (in mass) of the border of the convective core during the last part of the central He burning could appreciably alter the final C abundance. Hence, we stress that it is wiser to discuss the advanced evolutionary phases as a function of the C abundance left by the He burning rather than as a function of the efficiency of the C-12(alpha, gamma)O-16 reaction rate. Only a better knowledge of this cross section and/or the physics of the convective motions could help in removing the degeneracy between these two components. We also prolonged the evolution of the two 25 M. stellar models up to the core collapse and computed the Dnal explosive yields. Our main results are that the intermediate-light elements, Ne, Na, Mg, and Al (which are produced in the C convective shell), scale directly with the C abundance left by the He burning because they depend directly on the amount of available fuel (i.e., C and/or Ne). All the elements whose final yields are produced by any of the four explosive burnings (complete explosive Si burning, incomplete explosive Si burning, explosive O burning, and explosive Ne burning) scale inversely with the C abundance left by the He burning because the mass-radius relation in the deep interior of a star steepens as the C abundance reduces. We confirm previous findings according to which a low C abundance (similar or equal to0.2 dex by mass fraction) is required to obtain yields with a scaled solar distribution.