Systematic Biases in Stellar Evolution Models: A ZAMS Benchmark and Implications for Galactic Archaeology

1. Introduction

Stellar evolution models are the foundation of Galactic archaeology. Systematic differences between leading codes—MIST, PARSEC, and BaSTI-IAC—can introduce significant biases in age and mass determination. This study presents a physically rigorous ZAMS benchmark to quantify these systematics.

2. Methodology

• Models: MIST v1.2, PARSEC v1.2S, BaSTI-IAC v2.2.
• Physics: Non-Rotating, Asplund 2009 Solar Mixture.
• ZAMS Definition: The onset of stable hydrogen burning, defined as the point where nuclear luminosity exceeds 1% of the total luminosity ($L_{nuc} > 0.01 L_{total}$).

3. Results: The 5-Point Benchmark

4. Physical Attribution & Error Analysis

4.1. Regime 1: MLT & Solar Calibration (0.8-1.0 $M_{\odot}$)

The ~60-70K offsets in low-mass stars are driven by Mixing Length Theory (MLT) parameters and residual differences in Solar Model Calibration. Even when models are tuned to the Sun, small variations in the treatment of the solar envelope lead to systematic $T_{eff}$ shifts in field stars.

4.2. The 1.2 $M_{\odot}$ CNO-Opacity Transition

At 1.2 $M_{\odot}$, the energy generation mechanism shifts from the p-p chain to the highly temperature-sensitive CNO cycle ($\epsilon \propto T^{16}$).

• Mechanism: This transition creates a "kink" in the mass-luminosity relation. The 110K discrepancy reflects model-dependent treatments of Opacity peaks (bound-free transitions) in the radiative interior.
• Note: Core overshooting primarily affects main-sequence lifetime and has negligible impact on the ZAMS position.

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

In fully radiative envelopes, the growing ~150K spread is attributed to:

1. Opacity Tables: Divergences between OPAL and OP treatments of low-Z elements (Fe, Ni).
2. Atomic Diffusion: The inclusion of radiative levitation in MIST alters surface abundances and atmospheric structure.

5. Implications for Galactic Archaeology

These systematic $T_{eff}$ biases propagate into isochrone fitting, potentially introducing systematic age offsets in reconstructed Star Formation Histories (SFH). While the magnitude of this shift requires full population synthesis to quantify, our benchmark confirms that model choice is a non-negligible source of uncertainty.

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: ZAMS location in the Kiel Diagram showing the non-linear mass-temperature relationship.