Modeling results for different cavity sizes for three of the four sources. The plots show the ¹³CO intensity cuts of the data (black) and the models (colors) for different density drops δ_gas2. The central panel uses the gas cavity size r_cavgas of the final model (see Table 3), the left panel the results for a 5 AU smaller gas cavity, and the right panel the results for a 5 AU larger cavity. For SR21 the 7 AU radius remains unresolved in the ALMA data so the radius is not explored. The plots reveal that the gas cavity radius is determined to within 5 AU uncertainty.