[Si vii]/[Ca viii] flux ratio with respect to U for Cloudy simulations for the central source as the ionizing source. In green we depict the measured ratio for the averaged nodule, in red the measured ratio for IR-1b with the 3-σ confidence interval. For nodule IR-1 (i.e. 30 pc), U =  −3.35 and for IR-1b (i.e. 14 pc), U =  −2.69. For log U <  −2.75, the Cloudy precision is insufficient to provide confident values: the [Si vii] and [Ca viii] fluxes quickly tend to zero.