Photoionization model predictions of the R23 (top) and R3 (bottom) indexes based on the binary evolution SEDs (BPASS). The model tracks (dashed lines) span the ionization parameter log(U) from −3.5 (thin lines) to −0.5 (thick lines) with a step of 0.5 dex. The gas density is fixed to 100 cm⁻³. The models encompass the scatter of data points and the dependency on the EW(Hβ) by changing the ionization parameter. The datapoints are those collected by Nakajima et al. (2022) for which accurate metallicities were measured with the direct T_e method (figures adapted from Nakajima et al. 2022).