Evolution of a He Iλ1083.3 nm feature. The vertical line indicates the transition’s rest wavelength. In NLTE, a He I feature would become increasingly strong with time, as the dominant pathways depleting the triplet states are from excited triplet levels that are less populated at lower temperatures. This highlights the differences in the temporal trends, which show a very weak initial but then increasingly strong He line, in contrast to the observed feature that emerges suddenly and then reduces over time.