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Canonical grids of stellar evolutionary sequences have been computed for thehelium mass-fraction abundances $Y = 0.25$, $0.29$, and $0.33$, and for ironabundances that vary from $-2.4$ to $+0.4$ (in 0.2 dex increments) when[$\alpha$/Fe] $= +0.4$, or for the ranges $-2.0 \le$ [Fe/H] $\le +0.6$, $-1.8\le$ [Fe/H] $\le +0.6$ when [$\alpha$/Fe] $= 0.0$ and $-0.4$, respectively. Thegrids, 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 onup-to-date physics, including the gravitational settling of helium (but notmetals diffusion). Interpolation software is provided to generate isochronesfor arbitrary ages between $\approx 5$ and $15$ Gyr and any values of $Y$,[$\alpha$/Fe], and [Fe/H] within the aforementioned ranges. Comparisons ofisochrones with published color-magnitude diagrams (CMDs) for the open clustersM67 ([Fe/H] $\approx 0.0$) and NGC 6791 ([Fe/H] $\approx 0.3$) and for four ofthe metal-poor globular clusters (47 Tuc, M3, M5, and M92) indicate that themodels for the observed metallicities do a reasonably good job of reproducingthe locations and slopes of the cluster main sequences and giant branches. Thesame conclusion is reached from a consideration of plots of nearby subdwarfsthat have accurate Hipparcos parallaxes and metallicities in the range $-2.0\le$ [Fe/H] $\le -1.0$ on various CMDs and on the ($\log T_{\rmeff},\,M_V$)-diagram. A relatively hot temperature scale similar to thatderived in recent calibrations of the infrared flux method is favored by boththe isochrones and the adopted color transformations, which are based on thelatest MARCS model atmospheres.

astrophysical journal/the astrophysical journal

Isochrones for Old (> 5 Gyr) Stars and Stellar Populations. I. Models  for $-2.4 \le$ [Fe/H] $\le +0.6$, $0.25 \le Y \le 0.33$, and $-0.4 \le$  [$\alpha$\Fe] $\le +0.4$