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 from 3000Â Ã to 1.34 \mu m at a constant resolution of FWHM=1Â Ã and a sampling of 0.2 { \AA } , for overall metallicities in the range 0.005 \leq Z \leq 0.048 and for stellar population ages between 3 and 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 iron abundances ( [ Fe/H ] = - 0.5 , 0.0 and 0.2 ) and two different \alpha -element-to-iron abundance ratios ( [ \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 Sloan Digital Sky Survey Data Release 4 .
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 .