We have obtained [Mg/Fe] measurements for 76.3% of the stars in the MILES spectral library used for understanding stellar atmospheres and stellar populations in galaxies and star clusters. These abundance ratios were obtained through (1) a compilation of values from the literature using abundances from high-resolution spectroscopic studies and (2) a robust spectroscopic analysis using the MILES mid-resolution optical spectra. All the [Mg/Fe] values were carefully calibrated to a single uniform scale, by using an extensive control sample with results from high-resolution spectra. The small average uncertainties in the calibrated [Mg/Fe] values (respectively 0.09 and 0.12 dex with methods (1) and (2)) and the good coverage of the stars with [Mg/Fe] over stellar atmospheric parameter space of the library will permit the building of new simple stellar populations (SSPs) with empirical $alpha$-enhancements. These will be available for a range of [Mg/Fe], including both sub-solar and super-solar values, and for several metallicities and ages. These models will open up new prospects for testing and applications of evolutionary stellar population synthesis.
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