Calculated rotational temperatures, Trot, plotted Menv, Tbol, and Lbol, 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 Trot 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.
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