Effective astrophysical S factor for nonresonant reactions - art. no. 045801

We have made an asymptotic expansion of the effective S factor, S-eff, for nonresonant reactions of charged particles. The resulting analytic formulas can generate approximations to S-eff which contain derivatives, with respe ct to the center of mass energy E, of the astrophysical S factor, S(E), up to any desired order. Thus our approximation incorporates better the center -of-mass energy dependence in S(E) than previous approximations which inclu ded only the first or second derivatives. The range of applicability of the method is discussed. We numerically investigate our expressions for the re action Be-7(p,gamma)B-8.