  Magnetic fields of late-type stars are presumably generated by a dynamo
mechanism at the interface layer between the radiative interior and the outer
convective zone. The Rossby number, which is related to the dynamo process,
shows an observational correlation with activity. It represents the ratio
between the rotation period of the star and the local convective turnover time.
The former is well determined from observations but the latter is estimated by
an empirical iterated function depending on the color index (B-V) and the
mixing-length parameter. We computed the theoretical Rossby number of stellar
models with the TGEC code, and analyze its evolution with time during the main
sequence. We estimated a function for the local convective turnover time
corresponding to a mixing-length parameter inferred from a solar model, and
compare our results to the estimated Rossby number of 33 solar analogs and
twins, observed with the spectropolarimeters ESPaDOnS@CFHT and Narval@LBT.
