Impact of the uncertainties of 3α and 12C(α, γ)16O reactions on the He-burning phases of low- and intermediate-mass stars

Aims. We aim to estimate the impact on the stellar evolution of the uncertainties in the 3 alpha and the C-12(alpha, gamma)O-16 reaction rates, taking into account the recent improvements in their precision.Methods. We calculated models of low- and intermediate-mass stars for different values of 3 alpha and C-12(alpha, gamma)O-16 reaction rates. The 3 alpha reaction rate was varied up to +/- 24% around the reference value, while the C-12(alpha, gamma)O-16 reaction rate was varied by up to +/- 35%, taking into account different recent values for these quantities available in the literature. The models were calculated with the FRANEC evolutionary code for two different initial chemical compositions, namely, Y = 0.246, Z = 0.0001, and Y = 0.28, Z = 0.015 to represent different stellar populations. A M = 0.67 M-circle dot model was chosen as representative of the first class (halo ancient stars), while for the second composition (disk stars), the M = 1.5 M-circle dot and M = 2.5 M-circle dot models were considered. The impact of 3 alpha and C-12(alpha, gamma)O-16 reaction rates on the central He-burning lifetime and the asymptotic giant branch (AGB) lifetime, as well as the mass of the C/O core at the central He exhaustion and the internal C and O abundances, was investigated.Results. A variation of the C-12(alpha, gamma)O-16 reaction rates within its nominal error resulted in marginal differences in the analysed features among the three considered stellar masses, except for the C/O abundances. The central He-burning lifetime changed by less than 4%, while the AGB lifetime was affected only at the 1% level. The internal C and O abundances showed greater variation, with a change of about 15%. The uncertainty in the 3 alpha reaction rate mainly influences the C and O central abundances (up to 10%) for all the models considered, and the AGB lifetime for intermediate mass stars (up to 5%). Most of the investigated features were affected by less than 2%.Conclusions. The current uncertainty in the explored reaction rates has a negligible effect on the predicted evolutionary time scale with respect to other uncertainty sources. On the other hand, the variability in the chemical profile left at the end of the shell He-burning phase is still relevant. We also checked that there is no interaction between the effects of the two reaction rates, as would be expected in the case of small perturbations.