Ca II IRT sample spectra in the different regimes. In each panel, the spectra (black line) are compared with the template (red line). The enhancement factor, i.e. the integrand of Eq. (1), is reported below the spectrum, and the integral producing α is outlined in orange. Upper left panel: LA subgiant with . Upper right panel: LA dwarf with . Lower left panel: HA dwarf with . Lower right panel: accreting star (one in the Upper Scorpius association reported in Table 2 and Fig. 8) in the VHA regime.