Reaction rates for O-18(alpha, gamma)Ne-22, Ne-22(alpha, gamma)Mg-26, and Ne-22(alpha, n)Mg-25 in stellar helium burning and s-process nucleosynthesis in massive stars

New experimental results in O-18(Li-6, d) and Ne-22(Li-6, d) alpha transfer reaction studies have been utilized to calculate the resonance strength of low-energy resonances in the alpha-capture on O-18 and Ne-22. The present results imply a significant increase of the reaction rates for O-18(alpha, gamma)Ne-22 and Ne-22(alpha, gamma)Mg-26. The Ne-22(alpha, n)Mg-25 rate might be strongly enhanced by a possible resonance at 633 keV. The respective uncertainties are presented as a function of temperature. The influence of the proposed reaction rates on the s-process nucleosynthesis during core helium burning has been investigated in the range 15 less than M/solar mass less than 30. The resulting uncertainties are compared to those related to the modeling of core helium burning in massive stars. This comparison indicates that the s-process may be a useful constraint for the evolution and convection in massive stars, provided the persistent problem of the 633 keV resonance in the Ne-22(alpha, n)Mg-25 channel can be solved.