Toward better age estimates for stellar populations: The Y-2 isochrones for solar mixture

We have constructed a new set of isochrones, called the Y-2 Isochrones, tha t represent an update of the Revised Yale Isochrones (RYI), using improved opacities and equations of state. Helium diffusion and convective core over shoot have also been taken into consideration. This first set of isochrones is for the scaled solar mixture. A subsequent paper will consider the effe cts of alpha -element enhancement, believed to be relevant in many stellar systems. Two additionally significant features of these isochrones are that (1) the stellar models start their evolution from the pre-main-sequence bi rthline instead of from the zero-age main sequence and (2) the color transf ormation 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 pe rformed well under the tests conducted thus far. The reduction in the age o f the Galactic globular clusters caused by this update in stellar models al one is approximately 15% relative to RYI-based studies. When the suggested modification for the alpha -element enhancement is made as well, the total age reduction becomes approximately 20%. When post-RGB evolutionary stages are included, we find that the ages of globular clusters derived from integ rated colors are consistent with the isochrone fitting ages.