We present a large , new set of stellar evolution models and isochrones for an \alpha -enhanced metal distribution typical of Galactic halo and bulge stars ; it represents a homogeneous extension of our stellar model library for a scaled-solar metal distribution already presented in Pietrinferni et al .
( 2004 ) .
The effect of the \alpha - element enhancement has been properly taken into account in the nuclear network , opacity , equation of state and , for the first time , the bolometric corrections , and color transformations .
This allows us to avoid the inconsistent use - common to all \alpha -enhanced model libraries currently available - of scaled-solar bolometric corrections and color transformations for \alpha -enhanced models and isochrones .
We show how bolometric corrections to magnitudes obtained for the U,B portion of stellar spectra ( i.e .
not only the Johnson-Cousins filters but also Strömgren u,b magnitudes ) for T _ { eff } \leq 6500 K , are significantly affected by the metal mixture , especially at the higher metallicities .

Our models cover both an extended mass range ( between 0.5 M _ { \odot } to 10 M _ { \odot } , with a fine mass spacing ) and a broad metallicity range , including 11 values of the metal mass fraction Z , corresponding to the range -2.6 \leq [ Fe / H ] \leq 0.05 .
The initial He mass fraction is Y =0.245 for the most metal-poor models and increases with Z according to \Delta Y / \Delta Z =1.4 , to reproduce the initial solar He obtained from the calibration of the solar model .
Models with and without the inclusion of overshoot from the convective cores during the central H burning phase are provided , as well as models with different mass loss efficiencies .

We also provide complete sets of evolutionary models for low-mass , He-burning stellar structures covering the whole metallicity range , to enable synthetic horizontal branch simulations .

We compare our database with several widely used stellar model libraries from different authors , as well as with various observed color magnitude and color-color diagrams ( Johnson-Cousins BVI and near infrared magnitudes , Strömgren colors ) of Galactic field stars and globular clusters .
We also test our isochrones comparing integrated optical colors and Surface Brightness Fluctuation magnitudes with selected globular cluster data .
We find a general satisfactory agreement with the empirical constraints .

This database , used in combination with our scaled-solar model library , is a valuable tool for investigating both Galactic and extra-galactic simple and composite stellar populations , using stellar population synthesis techniques .