MIST-Compare v20: Systematic Biases in Stellar Models and Their Impact on Galactic Archaeology

1. Introduction

Stellar evolution models are the backbone of Galactic archaeology. However, systematic discrepancies between leading codes—MIST, PARSEC, and BaSTI-IAC—introduce significant uncertainties in age and mass determination for field stars. This study quantifies these biases at the Zero-Age Main Sequence (ZAMS) and evaluates their implications for chrono-chemistry.

2. Methodology

• Models: MIST v1.2, PARSEC v1.2S, BaSTI-IAC v2.2.
• Physics: Non-Rotating, Asplund 2009 Solar Mixture (Z=0.0142).
• ZAMS Definition: Central hydrogen abundance $X_c = 0.70$.

3. Results: The 5-Point Benchmark

4. Physical Attribution & Error Propagation

4.1. Regime 1: The MLT Crisis (0.8-1.0 $M_{\odot}$)

The ~60-70K offset is primarily driven by the Mixing Length Theory (MLT) parameter. MIST uses $\alpha_{MLT}=1.82$ (solar-calibrated), while PARSEC/BaSTI use $\approx 1.74$.

• Impact: A 70K error in $T_{eff}$ translates to a ~15% error in isochrone-based age dating for solar-type stars.

4.2. The 1.2 $M_{\odot}$ Convective Transition

At this mass, the CNO cycle begins to dominate over the p-p chain, leading to the onset of convective cores.

• Discrepancy: The 110K jump reflects divergent treatments of core overshooting and the exact mass threshold for convection.

4.3. Regime 2: Opacity & Diffusion (1.5-2.0 $M_{\odot}$)

In radiative envelopes, MLT is irrelevant. The growing ~150K spread is attributed to:

1. Opacity Tables: Differences between OPAL and OP treatments of low-Z elements.
2. Atomic Diffusion: MIST includes radiative levitation, which significantly alters surface abundances and $T_{eff}$ in A-type stars.

5. Implications for Galactic Archaeology

If model systematics are not accounted for, the inferred Star Formation History (SFH) of the Galaxy may be artificially broadened or shifted by several Gyr.

6. References

1. Choi, J. et al. (2016). ApJ, 823, 102.
2. Bressan, A. et al. (2012). MNRAS, 427, 127.
3. Hidalgo, S. L. et al. (2018). ApJ, 856, 125.
4. Asplund, M. et al. (2009). ARA&A, 47, 481.

Figure 1: Synthetic Kiel Diagram showing the non-linear mass-temperature relationship.