Line strength, age, and IMF sensitivity. From left to right and top to bottom, we show the age dependence of the Mgb, H_βO, Nad, and TiO₂ spectral features, respectively. Solid lines correspond to the MILES α-variable models and dashed ones indicate their (adapted) super-young extension. The gray shaded area in the background shows the unmodified super-young model predictions for a Milky Way-like IMF (see main text for more details). Orange lines are predictions for a bottom-heavy IMF (i.e., steeper than the Milky Way standard), blue lines for a top-heavy (i.e., flatter) IMF, and black lines for the reference Salpeter IMF. The rapid increase in the equivalent width of the indices (but H_βO) at ages of ∼10 Myr is caused by the sudden appearance of red supergiant stars. Predictions are shown at the native resolution of the MILES models (2.51 Å).