Ionisation degree of Sr in LTE as a function of temperature and electron density. We overlay the approximate locations in parameter landscape of AT2017gfo with time. The lines indicate the evolution modelled as constraints from reverberation effects within a single epoch. The temperature error bars are for the part of the ejecta with line-of-sight expansion (Sneppen 2023), and the density assumes a spherically symmetric, homogeneous, and homologously expanding high-velocity (0.2c ≤ v ≤ 0.3c) ejecta component of 0.01 M_⊙. The electron density can only be estimated to an order of magnitude, but regardless of the exact n_e, a rapid step change in ionisation state occurs over a narrow interval in temperature around 4500 K. The 1 μm feature is not detected in observations prior to 1 day (marked in red), but from 1.17 days and forward, the feature is present (marked in blue).