MESA Isochrones and Stellar Tracks. II. Models with alpha-enhanced chemical composition

Abstract

We update and expand the MESA Isochrones and Stellar Tracks (MIST) database to include variations in the alpha-capture elements, specifically [alpha/Fe]=-0.2, 0, +0.2, +0.4, and +0.6 for -3 <= [Fe/H] <= +0.5. Variations in [alpha/Fe] are included in a self-consistent manner from the stellar interior models to the synthetic spectra used to translate these models in the observational plane. We describe a number of updates to the physics utilized in these models as well as new information provided by the models. We validate the models with comparisons to other stellar evolution models including the previous generation of MIST and other models from the literature. MIST data products including stellar evolutiont tracks, isochrones, and bolometric correction tables can be obtained from the MIST project website, https://mist.science. All necessary files to reproduce MIST models are available from Zenodo.

Figures (8)

• Figure 1: The Red Giant Branch Bump (RGBB) data sample compiled by Nataf2013 is compared with Paper 1 (left) and this paper (right) at 12 Gyr. Paper 1 models have only scaled-solar composition. The main difference between the two panels is the calibration of $f_{\rm ov,env}$ (see Section \ref{['sec:mixing']} for details).
• Figure 2: MIST isochrones at 10 Myr (upper left), 100 Myr, 1 Gyr, and 10 Gyr (lower right). The left panel shows $Z=0.0017$ ([Fe/H] $\approx-1$) and the right panel shows $Z=0.016$ ([Fe/H] $\approx 0$), for both Paper 1 (red) and this paper (black). In this and subsequent figures, the pre-MS evolution has been omitted from the younger ages for clarity.
• Figure 3: MIST isochrones at 10 Myr, 100 Myr, 1 Gyr, and 10 Gyr, showing the effect of ${\rm[\alpha/Fe]}$ enhancements across the range $-0.2$ to $+0.6$ in the theoretical HR diagram. The left panel show $[{\rm Fe/H}] = -2.0$ and the right panel shows $[{\rm Fe/H}] = -0.5$ isochrones.
• Figure 4: MIST isochrones at 10 Myr, 100 Myr, 1 Gyr, and 10 Gyr. The red line is the $\alpha$-enhanced model with [Fe/H]=$-0.5$ (left) or $-0.25$ (right) while the blue line is made from a scaled-solar composition but interpolated to the same total Z as the red curve in each panel. For comparison, the gray lines are scaled-solar models with the same [Fe/H] as the red lines.
• Figure 5: Isochrones at 8 Gyr and $[{\rm Fe/H}]=-0.5$ for three different HST-WFC3 CMDs. The panels demonstrate the effect of varying [$\alpha$/Fe] on the bolometric corrections and the underlying isochrones. The left column shows the effect of varying [$\alpha$/Fe] in the BCs while keeping the underlying isochrone fixed at [$\alpha$/Fe] = +0.2. The middle panel shows the effect of varying [$\alpha$/Fe] in the isochrones while leaving the BCs fixed at [$\alpha$/Fe] = +0.2. The right panel shows the combined effect of varying both components together.