Behavior of the R = H₂CO(3_0,3−2_0,2)/H₂CO(3_2,2−2_2,1) integrated intensity ratio. The dotted contours represent the R values from the LVG model for (see Sect. 4.1.2) and go from 2 to 10 in unit steps. The red solid contours are the observed R values, plotted here for cores 3-s, 12-s, 13-s, and 19-s for viewing simplicity. The vertical gray dotted line is the upper limit value obtained with the LVG model. The effect of the optically thin/thick turnover is also seen, which introduces the largest uncertainty in the T_k estimation.