We present a new set of isochrones in which the effect of the \alpha -element enhancement is fully incorporated .
These isochrones are an extension of the already published set of Y ^ { 2 } Isochrones ( Yi et al .
2001 : Paper 1 ) , constructed for the scaled-solar mixture .
As in Paper 1 , helium diffusion and convective core overshoot have been taken into account .
The range of chemical compositions covered is 0.00001 \leq Z \leq 0.08 .
The models were evolved from the pre-main-sequence stellar birthline to the onset of helium burning in the core .
The age range of the full isochrone set is 0.1 – 20 Gyr , while younger isochrones of age 1 – 80 Myr are also presented up to the main-sequence turn-off .
Combining this set with that of Paper 1 for scaled-solar mixture isochrones , we provide a consistent set of isochrones which can be used to investigate populations of any value of \alpha -enhancement .
We confirm the earlier results of Paper 1 that inclusion of \alpha -enhancement effects further reduces the age estimates of globular clusters by approximately 8 % if [ \alpha /Fe ] =+0.3 .
It is important to note the metallicity dependence of the change in age estimates ( larger age reductions in lower metallicities ) .
This reduces the age gap between the oldest metal-rich and metal-poor Galactic stellar populations and between the halo and the disk populations .
We also investigate whether the effects of \alpha -enhancement can be mimicked by increasing the total metal abundance in the manner proposed by Salaris and collaborators .
We find such simple scaling formulae are valid at low metallicities but not at all at high metallicities near and above solar .
Thus it is essential to use the isochrones rigorously computed for \alpha -enhancement when modeling metal-rich populations , such as bright galaxies .
The isochrone tables , together with interpolation routines have been made available via internet .