We present the first models allowing one to explore in a consistent way the influence of changes in the alpha-element-to-iron abundance ratio on the high-resolution spectral properties of evolving stellar populations. The models cover the wavelength range 300-1340nm at a resolution of FWHM=1AA, for metallicities in the range 0.005<=Z<=0.048 and stellar population ages 3 to 14 Gyr. These models are based on a recent library of synthetic stellar spectra and a new library of stellar evolutionary tracks, both computed for three different [Fe/H] (-0.5,0.0 and 0.2) and two different [alpha/Fe] (0.0 and 0.4). We expect our fully synthetic models to be primarily useful for evaluating the differential effect of changes in the alpha/Fe ratio on spectral properties such as broad-band colours and narrow spectral features. In addition, we assess the accuracy of absolute model predictions in two ways: first, by comparing the predictions of models for scaled-solar metal abundances [alpha/Fe]=0.0) to those of existing models based on libraries of observed stellar spectra; and secondly, by comparing the predictions of models for alpha-enhanced metal abundances ([alpha/Fe]=0.4) to observed spectra of massive early-type galaxies in the SDSS-DR4. We find that our models predict accurate strengths for those spectral indices that are strongly sensitive to the abundances of Fe and alpha elements. The predictions are less reliable for the strengths of other spectral features, such as those dominated by the abundances of C and N, as expected from the fact that the models do not yet allow one to explore the influence of these elements in an independent way. We conclude that our models are a powerful tool for extracting new information about the chemical properties of galaxies for which high-quality spectra have been gathered by modern surveys.
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