Hδ/Fe I λ4045 vs. Ca II as a function of age and metallicity. The top left panel shows the index evolution for [Fe/H] = −0.4. The open squares are the index values for the stellar population at a given age labelled in gigayears. This curve is reproduced as a dotted line in the other panels for comparison purposes. The other panels display (solid lines) the index trajectories for other metallicities as labelled in the figure.