We present models that predict spectra of old- and intermediate-aged stellar populations at 2.51 Ã ( FWHM ) with varying [ \alpha /Fe ] Â abundance .
The models are based on the MILES library and on corrections from theoretical stellar spectra .
The models employ recent [ Mg/Fe ] Â 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 Ã , which increases with age and metallicity .
We also confirm that both [ MgFe ] Â and [ MgFe ] ^ { \prime } Â indices are [ \alpha /Fe ] -insensitive .
We show that the sensitivity of the higher order Balmer lines to [ \alpha /Fe ] Â 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 ] Â 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 _ { Mg } / Z _ { Fe } ] _ { SS ( BaSTI ) } , using solely scaled-solar models and the [ Mg/Fe ] Â derived with models with varying abundance ( [ Mg/Fe ] = 0.59 [ Z _ { Mg } / Z _ { Fe } ] _ { SS ( BaSTI ) } ) .
Finally we provide a user-friendly , web-based facility , which allows composite populations with varying IMF and [ \alpha /Fe ] .