  We present models of alpha Centauri A and B implementing an entropy
calibration of the mixing-length parameter alpha_MLT, recently developed and
successfully applied to the Sun (Spada et al. 2018, ApJ, 869, 135). In this
technique the value of alpha_MLT in the 1D stellar evolution code is calibrated
to match the adiabatic specific entropy derived from 3D radiation-hydrodynamics
simulations of stellar convective envelopes, whose effective temperature,
surface gravity, and metallicity are selected consistently along the
evolutionary track. The customary treatment of convection in stellar evolution
models relies on a constant, solar-calibrated alpha_MLT. There is, however,
mounting evidence that this procedure does not reproduce the observed radii of
cool stars satisfactorily. For instance, modelling alpha Cen A and B requires
an ad-hoc tuning of alpha_MLT to distinct, non-solar values. The
entropy-calibrated models of alpha Cen A and B reproduce their observed radii
within 1% (or better) without externally adjusted parameters. The fit is of
comparable quality to that of models with freely adjusted alpha_MLT for alpha
Cen B (within 1 sigma), while it is less satisfactory for alpha Cen A (within ~
2.5 sigma). This level of accuracy is consistent with the intrinsic
uncertainties of the method. Our results demonstrate the capability of the
entropy calibration method to produce stellar models with radii accurate within
1%. This is especially relevant in characterising exoplanet-host stars and
their planetary systems accurately.

Solar and Stellar Astrophysics;Earth and Planetary Astrophysics