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Y²(Yonsei-Yale) Isochrones

    We have constructed a new set of isochrones, called the Y² Isochrones, that represent an update of the Revised Yale Isochrones (RYI), using improved opacities and equations of state.  Helium diffusion and convective core overshoot have also been taken into consideration.  This first set of isochrones (Paper 1) was for the scaled solar mixture. Now the completing sets (for twice and four-times a-enhanced) are released through Paper 2. Two additional significant features of these isochrones are that (1) the stellar models start their evolution from the pre-main sequence birthline instead of from the zero-age main sequence, and (2) the color transformation has been performed using both the latest table of Lejeune et al., and the older, but now modified, Green et al.table.  The isochrones have performed well under the tests conducted thus far.  The reduction in the age of the Galactic globular clusters caused by this update in stellar models alone (assuming [a/Fe]=0.3) is approximately 20-25% relative to RYI-based studies. When post-RGB evolutionary stages are included, we find that the ages of globular clusters derived from integrated colors are consistent with the isochrone fitting ages (Paper 1).