THE OPAL EQUATION OF STATE AND LOW-METALLICITY ISOCHRONES

The Yale stellar evolution code has been modified to use the OPAL equation of state tables (Rogers 1994). Stellar models and isochrones were constructed for low-metallicity systems (-2.8 less than or equal to [Fe/H] less than or equal to -0.6). Above M similar to 0.7 M., the isochrones are very similar to those that are constructed using an equation of state that includes the analytical Debye-Huckel correction at high temperatures. The absolute magnitude of the main-sequence turnoff [M(upsilon)(TO)] with the OPAL or Debye-Huckel isochrones is about 0.06 mag fainter, at a given age, than M(upsilon)(TO) derived from isochrones that do not include the Debye-Huckel correction. As a consequence, globular clusters ages derived using M(upsilon)(TO) are reduced by 6%-7% as compared to the ages determined from the standard isochrones. Below M similar to 0.7 M., the OPAL isochrones are systematically hotter (by approximately 0.04 in B-V) at a given magnitude as compared to the standard, or Debye-Huckel isochrones. However, the lower mass models fall out of the OPAL table range, and this could be the cause of the differences in the location of the lower main sequences.