Theoretical zero-age main sequences applied to the Pleiades, Praesepe,and Hyades star clusters.

Since it is the constraints placed on theoretical models by precise observations of stars that eventually lead to an improved understanding of stellar physics, an attempt is presently made to determine how well the most up-to-date stellar interior models can reproduce the observations of two of the best observed star clusters: the Pleiades and the Praesepe. The simplest possible comparisons are carried out, namely matching the computed zero-age main sequences (ZAMSs) with the unevolved stars in each cluster. The fits obtained to these data effectively preclude values of the ratio of the mixing length to pressure scale height ratio (alpha) lower than 1.2, as viable choices of the mixing length parameter. An alpha value of about 1.5 in the construction of stellar models is instead supported. The preferred ZAMS loci were then fitted to the Hyades C-M diagram, yielding a distance modulus of more than 3.2 mag.