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Recently, the study of globular cluster (GC) CMDs has shown that some of themharbor multiple populations with different chemical compositions and/or ages.In the first case, the most common candidate is a spread in the initial heliumabundance, but this quantity is difficult to determine spectroscopically due tothe fact that helium absorption lines are not present in cooler stars, whereasfor hotter GC stars gravitational settling of helium becomes important. As aconsequence, indirect methods to determine the initial Y among populations arenecessary. For that reason, in this series of papers, we investigate theeffects of a Y enrichment in populations covering the range of GCmetallicities. In this first paper, we present the theoretical evolutionarytracks, isochrones, and ZAHB loci calculated with the Princeton-Goddard-PUC(PGPUC) stellar evolutionary code, which has been updated with the most recentinput physics and compared with other theoretical databases. The chemicalcomposition grid covers 9 Z ranging from Z=1.60x10^-4 to 1.57x10^-2, 7 Y fromY=0.230 to 0.370, and an alpha-element enhancement of [alpha/Fe]=0.3. Theeffects 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, splitsin the SGB being more prominent for lower ages or higher metallicities, splitsin the lower red giant branch being more prominent for higher ages or highermetallicities, 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 isan increase of L for low Teff, which is affected in different degrees dependingon the age of the GC being studied. Finally, the ZAHB morphology distributiondepending on the age explains how for higher GC metallicities a population withhigher helium abundance could be hidden at the red ZAHB locus.

Effects of Helium Enrichment in Globular Clusters I.Theoretical Plane with PGPUC stellar evolution code