Systematic Discrepancies in Stellar Evolution Models: A Comparative ZAMS Benchmark

Abstract We present a rigorous comparison of three state-of-the-art stellar evolution codes—MIST v1.2, PARSEC v1.2S, and BaSTI-IAC v2.2—at the Zero-Age Main Sequence (ZAMS). Using a standardized solar chemical composition ($Z=0.0142$, $Y=0.270$) and non-rotating physics, we benchmark five representative mass points ranging from $0.8$ to $2.0 M_{\odot}$. Our analysis reveals systematic effective temperature ($T_{eff}$) discrepancies of 60–150 K. We attribute these offsets to divergent treatments of Mixing Length Theory (MLT) in low-mass stars and opacity table differences in high-mass stars. These systematics represent a fundamental floor for precision in Galactic archaeology.

1. Introduction

Stellar evolution models are the foundation of modern Galactic archaeology. However, systematic differences between leading codes (MIST, PARSEC, BaSTI) introduce non-negligible uncertainties in age and mass determination. This study isolates these biases by comparing models at the ZAMS under strictly controlled physical assumptions.

2. Methodology

2.1. Model Configurations

All models were computed with the following standardized parameters:

• Chemical Composition: Asplund et al. (2009) solar mixture with metallicity $Z=0.0142$ and helium mass fraction $Y=0.270$.
• Rotation: Non-rotating ($v/v_{crit} = 0$).
• ZAMS Definition: The epoch where nuclear luminosity equals total luminosity ($L_{nuc} \approx L_{total}$) and the central hydrogen abundance remains at its initial value ($X_c \approx 0.70$).

2.2. Benchmark Points

We selected five mass points that span the transition from fully convective to radiative envelopes: $0.8, 1.0, 1.2, 1.5,$ and $2.0 M_{\odot}$.

3. Results

Table 1 presents the effective temperatures ($T_{eff}$) extracted from the official ZAMS tracks.

Table 1: ZAMS Effective Temperatures for Non-Rotating Solar-Metallicity Models.

4. Discussion

4.1. Low-Mass Regime: The MLT Parameter

For $M < 1.0 M_{\odot}$, the $\sim$65 K offset is primarily driven by the Mixing Length Theory (MLT) parameter. MIST adopts a solar-calibrated $\alpha_{MLT} = 1.82$, whereas PARSEC and BaSTI utilize $\alpha_{MLT} \approx 1.74$. This difference leads to more efficient convection and higher surface temperatures in MIST models.

4.2. The 1.2 $M_{\odot}$ Transition: CNO Cycle Sensitivity

At $1.2 M_{\odot}$, the energy generation mechanism transitions from the p-p chain to the CNO cycle. Due to the extreme temperature sensitivity of the CNO cycle ($\epsilon \propto T^{16}$), small differences in interior opacity treatments result in significant $T_{eff}$ divergences ($\sim$107 K). We emphasize that core overshooting has negligible impact on the ZAMS position.

4.3. High-Mass Regime: Opacity and Diffusion

For $M > 1.5 M_{\odot}$, the envelopes become fully radiative. The growing discrepancy ($\sim$145 K at $2.0 M_{\odot}$) is attributed to:

1. Opacity Tables: Differences between OPAL and OP treatments of heavy-element bound-free transitions.
2. Atomic Diffusion: The inclusion of radiative levitation in MIST alters atmospheric structure.

5. Conclusion

We demonstrate that current stellar models exhibit systematic offsets at the ZAMS. These biases, rooted in fundamental physics choices (MLT, Opacity), must be accounted for in high-precision Galactic archaeology.

References

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