Example TARDIS model spectra including yttrium (blue) and excluding yttrium (red) but otherwise composed of all r-process elements (33 ≤ Z ≤ 92) with solar r-process abundance ratios (Lodders et al. 2009; Bisterzo et al. 2014) for three different ejecta velocities. The three bottom panels show the difference in the spectral shape between the yttrium-free and yttrium-containing model for each characteristic velocity. Only when the characteristic ejecta velocity has decreased below ∼0.2c (at about 3–4 days post merger) does the P Cygni feature become apparent, because it no longer merges with lower-wavelength yttrium features.