CO (left panel) and H₂O (right panel) rotational diagrams for Ser SMM3 (Class 0) and L1489 (Class I) calculated using the total flux in lines measured in the PACS field-of-view. The base 10 logarithm of the number of emitting molecules from a level u, , divided by the degeneracy of the level, g_u, are written on the Y-axis. Two-component fits in the CO diagrams cover the transitions below and above E_u/k_B ~ 1700 K (CO 24–23) for the “warm” and “hot component” (see text). Each data point corresponds to one observed transition of a molecule. The limited number of lines observed in the line spectroscopy mode is responsible for the gaps in the otherwise linearly spaced CO diagrams. The error bars reflect the uncertainties in the fit.