IMPLICATIONS OF HELIUM DIFFUSION FOR GLOBULAR-CLUSTER ISOCHRONES AND LUMINOSITY FUNCTIONS

Grids of evolutionary sequences for masses from 0.5 to 1.0 M. and metallicities [Fe/H] = -2.26, -1.26, and -0.65 (with [O/Fe] = 0.75, 0.55, and 0.30, respectively, and appropriate helium abundances) have been computed both with and without the inclusion of diffusion. These have been interpolated to produce isochrones, for ages from 10 to 18 Gyr, and associated luminosity functions, for several power-law mass functions. The resultant age-luminosity relations indicate that the gravitational settling of helium can reduce the age at a given turnoff luminosity by about 10%, with considerably larger effects on turnoff temperatures and colors. Indeed, the latter seem to cause some difficulties for the comparison of predicted and observed color-magnitude diagrams; however, this cannot be regarded as a strong argument against diffusion in view of present uncertainties in our understanding of stellar T(eff)'s. Differences in the shapes of the luminosity functions from canonical models and those which included diffusion were found to be so small as to be observationally undetectable. One important prediction of the diffusion calculations is that bright red giants, and hence horizontal-branch stars, will have lower surface helium abundances than those expected from standard evolutionary theory. Since this would imply a fainter horizontal branch by almost-equal-to 0.05 mag, a calibration of the magnitude difference between the HB and the turnoff may provide, when it can be done reliably, a means to derive star cluster ages that is less sensitive to uncertainties associated with the gravitational settling process. Extensive tables of tracks, isochrones, and luminosity functions are provided for general use.