Canonical grids of stellar evolutionary sequences have been computed for the helium mass-fraction abundances Y = 0.25 , 0.29 , and 0.33 , and for iron abundances that vary from -2.4 to +0.4 ( in 0.2 dex increments ) when [ \alpha /Fe ] = +0.4 , or for the ranges -2.0 \leq [ Fe/H ] \leq + 0.6 , -1.8 \leq [ Fe/H ] \leq + 0.6 when [ \alpha /Fe ] = 0.0 and -0.4 , respectively .
The grids , which consist of tracks for masses from 0.12 { \cal M _ { \odot } } to 1.1– 1.5 { \cal M _ { \odot } } ( depending on the metallicity ) are based on up-to-date physics , including the gravitational settling of helium ( but not metals diffusion ) .
Interpolation software is provided to generate isochrones for arbitrary ages between \approx 5 and 15 Gyr and any values of Y , [ \alpha /Fe ] , and [ Fe/H ] within the aformentioned ranges .
Comparisons of isochrones with published color-magnitude diagrams ( CMDs ) for the open clusters M 67 ( [ Fe/H ] \approx 0.0 ) and NGC 6791 ( [ Fe/H ] \approx 0.3 ) and for four of the metal-poor globular clusters ( 47 Tuc , M 3 , M 5 , and M 92 ) indicate that the models for the observed metallicities do a reasonably good job of reproducing the locations and slopes of the cluster main sequences and giant branches .
The same conclusion is reached from a consideration of plots of nearby subdwarfs that have accurate Hipparcos parallaxes and metallicities in the range -2.0 \mathrel { \hbox { \raise 2.58 pt \hbox { $ < $ } \kern - 7.74 pt \lower 2.15 pt \hbox { $ \sim% $ } } } [ Fe/H ] \mathrel { \hbox { \raise 2.58 pt \hbox { $ < $ } \kern - 7.74 pt \lower 2.15 pt \hbox { $ \sim$ } } } % -1.0 on various CMDs and on the ( \log { T _ { eff } } , M _ { V } ) -diagram .
A relatively hot temperature scale similar to that derived in recent calibrations of the infrared flux method is favored by both the isochrones and the adopted color transformations , which are based on the latest MARCS model atmospheres .