Case of parallel (left) and perpendicular (right) shocks on protostellar surfaces: ionisation fraction, x, versus magnetic field strength, B, for the parameters specified in the third line of Table 1 (R_sh = 2 × 10^-2 AU). The colour map shows the values of E_max when the conditions imposed by Eqs. (4) and (18) are simultaneously verified. The cyan contour delimits the region where E_max gets its maximum asymptotic value shown in GeV in each subplot. Vertically hatched regions refer to combinations of parameters corresponding to strong wave damping (R< 1). The solid white star in the left panel shows the values of B and x considered for the evaluation of the emerging spectrum (models in Fig. 4).