A note on renormalon models for the determination of alpha_s(M_tau)

28 pages, 5 figures - Subm. to JHEPThe tau hadronic width provides a determination of the strong coupling constant alpha_s at low energies, since it can be related to a weighted integral of the Adler function in the complex energy plane. Using Operator Product Expansion, one sees that the sensitivity to alpha_s comes from the perturbative contribution, which can be obtained by integrating the perturbative expansion of the Adler function. Two different prescriptions proposed to perform this integral, called Fixed-Order Perturbation Theory and Contour-Improved Perturbation Theory (FOPT and CIPT), yield different results for the strong coupling constant. Recently, models for the Adler function based on renormalon calculus have been proposed to determine which of the two methods is the most accurate, by comparing the resulting asymptotic series with the true value of the integral. We discuss the assumptions of such ansatz and the determination of their free parameters. We show that variations of this renormalon ansatz can yield opposite conclusions concerning the comparison of CIPT versus FOPT, and that such models are not constrained enough to provide a definite answer on this issue or to be exploited for a high-precision determination of alpha_s(m_tau^2)