Example TARDIS model spectra including yttrium (blue line) and excluding yttrium (red dashed line) but otherwise composed of r-process elements with solar r-process abundance ratios (Lodders et al. 2009; Bisterzo et al. 2014). The left panel is limited to the first r-process peak elements (34 ≤ Z ≤ 50), while the right panel shows a model with all r-process elements (33 ≤ Z ≤ 92). The spectrum at the photosphere is modelled as a blackbody, which is then propagated through a relativistically expanding atmosphere (0.15c ≤ v ≤ 0.2c) using the radiative transfer equations with a radially uniform composition using the Kurucz (2018) line list. The red region indicates the 760 nm feature, which is clearly associated with the presence of yttrium. The yttrium abundance in the model constitutes 0.8% of the total mass. The 0.8–1 μm feature is the Sr⁺ P Cygni feature (Watson et al. 2019).