Column density of the relativistic electrons reproducing the radio emission level expected from a fixed value of the power released by the CBOs. The calculations were performed considering different combinations of stellar parameters capable of providing the same value of L_CBO; this explains the different lengths of the colored bars, which represent the derived values of n_r × l (Table 2). The gray line is the best fit of the two parameters. Open square symbol indicates the column densities that reproduce the observed radio spectra of the stars studied in Paper I.