Minimum values of expected surface magnetic fields (in Gauss) as a function of location in the HR diagram. This plot is based on evolutionary models between 5 M_⊙ and 120 M_⊙ for solar metallicity (some shown as dotted lines). Surface magnetic fields are calculated scaling the equipartition fields in the FeCZ (see Fig. 1) with ρ^2/3 (see discussion in Sect. 4). The full drawn black line roughly corresponds to the zero age main sequence. Circles correspond to the observed stars in the Cohen et al. (1997) sample, with symbol size coding different range of L_x/L_bol, as explained in the legend. The two star symbols correspond to the location of the 20 and 60 M_⊙ models discussed in the text and in C09.