Evolutionary stellar population synthesis with MILES - II. Scaled-solar and \alpha-enhanced models

We present models that predict spectra of old- and intermediate-aged stellar populations at 2.51\AA\ (FWHM) with varying [\alpha/Fe] abundance. The models are based on the MILES library and on corrections from theoretical stellar spectra. The models employ recent [Mg/Fe] determinations for the MILES stars and BaSTI scaled-solar and \alpha-enhanced isochrones. We compute models for a suite of IMF shapes and slopes, covering a wide age/metallicity range. Using BaSTI, we also compute "base models" matching The Galactic abundance pattern. We confirm that the \alpha-enhanced models show a flux excess with respect to the scaled-solar models blue-ward $\sim$4500\AA, which increases with age and metallicity. We also confirm that both [MgFe] and [MgFe]' indices are [\alpha/Fe]-insensitive. We show that the sensitivity of the higher order Balmer lines to [\alpha/Fe] resides in their pseudo-continua, with narrower index definitions yielding lower sensitivity. We confirm that the \alpha-enhanced models yield bluer (redder) colours in the blue (red) spectral range. To match optical colours of massive galaxies we require both \alpha-enhancement and a bottom-heavy IMF. The comparison of Globular Cluster line-strengths with our predictions match the [Mg/Fe] determinations from their individual stars. We obtain good fits to both full spectra and indices of galaxies with varying [\alpha/Fe]. Using thousands of SDSS galaxy spectra we obtain a linear relation between a proxy for the abundance, [Z$_{\rm Mg}$/Z$_{\rm Fe}$]$_{\rm SS(BaSTI)}$, using solely scaled-solar models and the [Mg/Fe] derived with models with varying abundance ([Mg/Fe]=0.59[Z$_{\rm Mg}$/Z$_{\rm Fe}$]$_{\rm SS(BaSTI)}$). Finally we provide a user-friendly, web-based facility, which allows composite populations with varying IMF and [\alpha/Fe] ( http://miles.iac.es ).