D4000_n and Hδ_A as a function of stellar age derived from a grid of synthetic spectra (BC03 model) are plotted assuming stellar metallicity: log (Z/Z_⊙) = 0.4, log (Z/Z_⊙) = 0.0, log (Z/Z_⊙) = −0.4, log (Z/Z_⊙) = −0.7. The model assumes Chabrier IMF and one single stellar burst with timescales τ = 0.1,0.2,0.3 Gyr. Dashed lines correspond to τ = 0.1, and 0.3 Gyr, while the solid ones to τ = 0.2 Gyr. Gray areas correspond to the ranges of D4000_n and Hδ_A obtained for VIPERS stacked spectra of passive red galaxies.