  We have constructed a new set of isochrones, called the Y^2 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
is for the scaled solar mixture. A subsequent paper will consider the effects
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 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 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 integrated
colors are consistent with the isochrone fitting ages. Isochrones are available
from http://achee.srl.caltech.edu.
