Upper panels: IP_ave vs. v_p, where IP_ave = (IP_i−1 +IP_i)/2 and IP_i is the ionization potential of the ionic stage i. The left and right panels refer to the LVC and the HVC, respectively. Data points of DO Tau and DG Tau are plotted with red and black dots, respectively. The error bars in the x-axis represent the dispersion between the v_p values of lines of the same ionic species, while those in the y-axis are (IP_i -IP_i−1)/2. Lower panels: critical density (n_crit) of the upper level of the transition vs. v_p. Symbols have the same meaning as in the upper panels.