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In kilonovae, freshly-synthesized $r$-process elements imprint features onoptical spectra, as observed in AT2017gfo, the counterpart to the GW170817binary neutron star merger. However, measuring the $r$-process compositions ofthe merger ejecta is computationally challenging. Vieira et al. (2023)introduced Spectroscopic $r$-Process Abundance Retrieval for Kilonovae (SPARK),a software tool to infer elemental abundance patterns of the ejecta, andassociate spectral features with particular species. Previously, we appliedSPARK to the 1.4 day spectrum of AT2017gfo and inferred its abundance patternfor the first time, characterized by electron fraction $Y_e=0.31$, asubstantial abundance of strontium, and a dearth of lanthanides and heavierelements. This ejecta is consistent with wind from a remnant hypermassiveneutron star and/or accretion disk. We now extend our inference to spectra at2.4 and 3.4 days, and test the need for multicomponent ejecta, where westratify the ejecta in composition. The ejecta at 1.4 and 2.4 days is describedby the same single blue component. At 3.4 days, a new redder component withlower $Y_e=0.16$ and a significant abundance of lanthanides emerges. This newredder component is consistent with dynamical ejecta and/or neutron-rich ejectafrom a magnetized accretion disk. As expected from photometric modelling, thiscomponent emerges as the ejecta expands, the photosphere recedes, and theearlier bluer component dims. At 3.4 days, we find an ensemble of lanthanides,with the presence of cerium most concrete. This presence of lanthanides hasimportant implications for the contribution of kilonovae to the $r$-processabundances observed in the Universe.

Spectroscopic r-Process Abundance Retrieval for Kilonovae II:  Lanthanides in the Inferred Abundance Patterns of Multi-Component Ejecta from  the GW170817 Kilonova