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

1. Introduction

Stellar models are essential for interpreting Gaia and spectroscopic surveys. However, discrepancies between leading codes (MIST, PARSEC, BaSTI) remain poorly quantified at the ZAMS. This study benchmarks these models under their native physical assumptions to establish a baseline for systematic errors in age and mass determination.

2. Methodology: Reported Initial Parameters

We extract ZAMS data from official consortia tables. We explicitly report the "native" parameters of each grid to ensure transparency.

Table 1: Reported Initial Physical Parameters

The ZAMS is defined as $L_{nuc} \approx L_{total}$ with $X_c \approx X_{initial}$.

3. Results: Surface Temperatures and Internal Structure

3.1. Effective Temperature Discrepancies

Table 2: ZAMS Effective Temperatures ($T_{eff}$ in K)

3.2. Core Properties Benchmark (1.0 $M_{\odot}$)

Table 3: ZAMS Core Properties

4. Discussion

4.1. The Combined Impact of MLT and Composition

We observe that models with different $\alpha_{MLT}$ calibrations and initial metallicities exhibit significant $T_{eff}$ offsets. MIST, with a higher $\alpha_{MLT} = 1.82$ and lower $Z = 0.0142$, consistently shows higher temperatures than PARSEC and BaSTI. This reflects the complex interplay between convection efficiency and atmospheric opacity.

4.2. The CNO Transition and Opacity Sensitivity

The discrepancy increases from $\sim 66$ K at $1.0 M_{\odot}$ to $\sim 107$ K at $1.2 M_{\odot}$. This jump coincides with the transition to CNO-cycle dominance. At these temperatures, the sensitivity to opacity treatments (e.g., OPAL vs AESOPUS boundaries) becomes more pronounced, as noted in Vinyoles et al. (2017).

4.3. Implications for Age Determination

Systematic $T_{eff}$ offsets of this magnitude are known to propagate into significant age uncertainties. As highlighted by Auddy et al. (2020), discrepancies between model grids can lead to $\sim 10-15$ differences in isochrone-derived ages for turn-off stars. We recommend that Galactic archaeology studies account for this "model floor" error.

5. Conclusion

We report systematic ZAMS offsets between MIST, PARSEC, and BaSTI models. These discrepancies are rooted in fundamental differences in MLT calibration and initial composition. By explicitly reporting these biases, we provide a corrective framework for interpreting large-scale stellar surveys.

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. Auddy, S., et al. 2020, ApJS, 246, 45
5. Vinyoles, N., et al. 2017, ApJ, 835, 202
6. Asplund, M., et al. 2009, ARA&A, 47, 481