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

Abstract

We discuss the inuence of the 12C(a, c)16O reaction rate on the central He burning of stars in the mass range 0.825 as well as its e ects on the explosive yields of a 25 star of solar chemical M _ , M _ composition. We nd that the central He burning is only marginally a ected by a change in this cross section within the currently accepted uncertainty range. The only (important) quantity that varies signicantly is the amount of C left by the He burning. Since the 12C(a, c)16O is efficient in a convective core, we have also analyzed the inuence of the convective mixing in determining the nal C abundance left by the central He burning. Our main nding is that the adopted mixing scheme does not inuence the nal C abundance provided the outer border of the convective core remains essentially xed (in mass) when the central He abundance drops below ^0.1 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 nal 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 12C(a, c)16O 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 stellar models up to the core collapse and computed the M _ nal 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 nal 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 conrm previous ndings according to which a low C abundance (^0.2 dex by mass fraction) is required to obtain yields with a scaled solar distribution.