Synthetic ice spectra to which a photostellar spectrum at five different effective temperatures was added, containing the column densities listed in Table 2 of McClure et al. (2023) in black (with ${\text{N}}_{{\text{CH}}_3\text{CN}}=0$), the previously estimated 5σ threshold in red (computed without including the photosphere contribution, see Fig. 6), and an example of the column density needed to proceed to the identification of the 4.45 µm CH₃CN feature in blue. Each spectrum presents its own effective temperature for the stellar model used (from top to bottom: T_eff = 3500, 4500, 5500, 6000, and 6500 K), to which we applied an extinction law for different A_V (25, 33, 40, 42, and 45 mag respectively).