The Y 2 Isochrones for α‐Element Enhanced Mixtures

Abstract

We present a new set of isochrones in which the effect of the α-element enhancement is fully incorporated. These isochrones are an extension of the already published set of Y 2 Isochrones (our Paper I), constructed for the scaled-solar mixture. As in Paper I, helium diffusion and convective core overshoot have been taken into account. The range of chemical compositions covered is 0.00001 ≤ Z ≤ 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 I for scaled-solar mixture isochrones, we provide a consistent set of isochrones that can be used to investigate populations of any value of α-enhancement. We confirm the earlier results of Paper I that inclusion of α-enhancement effects further reduces the age estimates of globular clusters by approximately 8% if [α/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 α-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 α-enhancement when modeling metal-rich populations, such as bright galaxies. The isochrone tables, together with interpolation routines have been made available via internet.