A new set of stellar tracks and isochrones, which focusses on low mass stars, is available for download. These models update the low mass end of the Yonsei-Yale models described in the next section.
(1) to take care of the (relatively) low temperatures and high densities in the atmospheres of such stars, the SCVH equation of state
(Saumon et al. 1995)
is ramped with the OPAL 2005 equation of state below 5000 K;
(2) as the grey atmosphere approximation becomes increasingly inadequate at about 0.6 Msun and moving towards lower masses, the surface boundary
conditions have been specified based on the BT-Settl PHOENIX non-grey atmospheric models
(Allard et al., 2011).
Theoretical isochrones, along with synthetic magnitudes obtained from the color--effective temperature transformations of
Lejeune et al. (1998)
and VandenBerg & Clem (2003), are also available.
Evolutionary tracks for models in the range 0.10--1.25 Msun, with various values of the mixing length parameter alpha_MLT = 0.5, 1.0, 1.875, 3.0,and various chemical composition ([Fe/H] = +0.3, 0.0, -0.5, -1.0, -1.5, enrichment parameter Delta Y /Delta Z = 1.48, alpha-enhancement [alpha/Fe] = 0; the Grevesse & Sauval 1998 ((Z/X)_sun = 0.0230 is assumed) are available. Diffusion of helium and of the heavy elements is taken into account; core overshooting is ignored. The evolutionary tracks cover the PMS and the post-MS up to 13 Gyr and/or the sub giant phase (if present). Along with the standard input physics incorporated in the non-rotational version of YREC (Demarque et al., 2008), these new calculations feature two major improvements which are especially relevant to models of mass lower than 0.6 Msun:
We have constructed a new set of isochrones, called the Y2 Isochrones, that represent an update of the Revised Yale Isochrones (RYI), using improved opacities and equations of state. Helium diffusion and convective core overshoot have also been taken into consideration. This first set of isochrones (Paper 1) was for the scaled solar mixture. Now the completing sets (for twice and four-times a-enhanced) are released through Paper 2. Two additional significant features of these isochrones are that (1) the stellar models start their evolution from the pre-main sequence birthline instead of from the zero-age main sequence, and (2) the color transformation has been performed using both the latest table of Lejeune et al., and the older, but now modified, Green et al. table. The isochrones have performed well under the tests conducted thus far. The reduction in the age of the Galactic globular clusters caused by this update in stellar models alone (assuming [a/Fe]=0.3) is approximately 20-25% relative to RYI-based studies. When post-RGB evolutionary stages are included, we find that the ages of globular clusters derived from integrated colors are consistent with the isochrone fitting ages (Paper 1).