Integrated intensities of ¹²CO 3 − 2 (solid lines) and C¹⁸O 3 − 2 (dashed lines) at the center of the gap (r = 28 AU) and in the outer disk (r = 100 AU). The panels show the intensities for variations of one parameter (R_c, L_bol, T_eff and the PAH abundance). If not varied, the parameters are set to the values of the representative models (Table 1: R_c = 50 AU, L_bol = 10 L_⊙, T_eff = 10 000 K and a PAH abundance of 10% the ISM abundance).