Calculated rotational temperatures, T_rot, plotted M_env, T_bol, and L_bol, for ¹²CO (top), ¹³CO (middle), C¹⁸O lines (bottom). The median excitation temperatures of ~70 K, 48 K and 37 K for ¹²CO, ¹³CO and C¹⁸O, respectively, are indicated with the green dashed lines. Typical error bar for each of the T_rot values is shown in the upper left plot, represented in black circle data point. In the C¹⁸O plots, blue arrows indicate the upper limits for a number of sources. These figures are compared with disk evolution models of Harsono et al. (2013, magenta solid lines), which represents all three models discussed in the text. Magenta arrows also show the direction of time. The figures show that the excitation temperature does not change with the increasing luminosity, envelope mass or density, confirming that Class 0 and I sources have similar excitation conditions.