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 ($-2.8 le [Fe/H] le -0.6$). Above $Msim 0.7,msun$, the isochrones are very similar to those which are constructed using an equation of state which includes the analytical Debye-Huckel correction at high temperatures. The absolute magnitude of the main sequence turn-off (mvto) with the OPAL or Debye-Huckel isochrones is about 0.06 magnitudes fainter, at a given age, than mvto derived from isochrones which do not include the Debye-Huckel correction. As a consequence, globular clusters ages derived using mvto are reduced by 6 -- 7% as compared to the ages determined from the standard isochrones. Below $Msim 0.7,msun$, 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.Comment: to appear in ApJ, 8 pages LaTeX, uses aaspptwo.sty. Complete uuencoded postscript file (including figures) available from: ftp://ftp.cita.utoronto.ca/cita/chaboyer/papers/opal.u