  Recently, the study of globular cluster (GC) CMDs has shown that some of them
harbor multiple populations with different chemical compositions and/or ages.
In the first case, the most common candidate is a spread in the initial helium
abundance, but this quantity is difficult to determine spectroscopically due to
the fact that helium absorption lines are not present in cooler stars, whereas
for hotter GC stars gravitational settling of helium becomes important. As a
consequence, indirect methods to determine the initial Y among populations are
necessary. For that reason, in this series of papers, we investigate the
effects of a Y enrichment in populations covering the range of GC
metallicities. In this first paper, we present the theoretical evolutionary
tracks, isochrones, and ZAHB loci calculated with the Princeton-Goddard-PUC
(PGPUC) stellar evolutionary code, which has been updated with the most recent
input physics and compared with other theoretical databases. The chemical
composition grid covers 9 Z ranging from Z=1.60x10^-4 to 1.57x10^-2, 7 Y from
Y=0.230 to 0.370, and an alpha-element enhancement of [alpha/Fe]=0.3. The
effects of different helium abundances that can be observed in isochrones are:
splits in the MS, differences in the L and Teff of the turn off point, splits
in the SGB being more prominent for lower ages or higher metallicities, splits
in the lower red giant branch being more prominent for higher ages or higher
metallicities, differences in L of the RGB bump (with small changes in Teff),
and differences in L at the RGB tip. At the ZAHB, when Y is increased there is
an increase of L for low Teff, which is affected in different degrees depending
on the age of the GC being studied. Finally, the ZAHB morphology distribution
depending on the age explains how for higher GC metallicities a population with
higher helium abundance could be hidden at the red ZAHB locus.
