Similar to Figures 3 and 4, but showing the variation of UV spectral flux as a function of [Fe/H]. The synthetic spectra were calculated for fixed parameters (T_eff=4500 K, log g=3.0). Upper row: variation for the [O/Fe] ratios of +0.2,+0.4,+0.8, and +1.2. Lower row: same as the upper row but for [Mg/Fe] ratios. As [Fe/H] decreases, the strength of the OH molecular band, Mg II doublet, and Mg I line gradually increases, with the Mg II doublet showing a more significant increase. When [Fe/H]<−2.8, the strength of the Mg II doublet surpasses that of the Mg I line, and as [Fe/H] decreases further, the strength difference between the two increases.