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. Low-Mass Regime: The MLT Calibration

For $M < 1.0 M_{\odot}$, the $\sim 65$ K offset is driven by MLT parameters. MIST's higher $\alpha_{MLT} = 1.82$ yields more efficient convection and higher $T_{eff}$ compared to PARSEC/BaSTI ($\alpha_{MLT} \approx 1.74$). This aligns with findings by Joyce & Chaboyer (2018).

4.2. The Role of Metallicity and Opacity

We acknowledge that MIST's lower $Z$ (0.0142) contributes to its higher $T_{eff}$. Following the scaling $T_{eff} \propto Z^{-0.1}$, the $\sim 7$ difference in $Z$ between MIST and PARSEC accounts for a portion of the offset. However, residual differences are attributed to Opacity Table treatments (OPAL vs OP/AESOPUS) in the stellar envelopes. As noted by Vinyoles et al. (2017), opacity uncertainties remain a primary source of divergence in solar-like models.

4.3. ZAMS and the Negligibility of Diffusion

We clarify that Atomic Diffusion has negligible impact on ZAMS properties, as the ZAMS represents the onset of stable hydrogen burning ($t \approx 0$) before significant chemical settling occurs. Future discrepancies in evolved phases will be dominated by diffusion and overshooting.

4.4. Quantifying the Impact on Galactic Archaeology

Using the mass-luminosity relation $L \propto M^{3.5}$ and $\tau_{MS} \propto M/L$, a systematic $T_{eff}$ offset of 100 K translates to an uncertainty in derived ages of $\sim 10-15$ for solar-metallicity turn-off stars. This represents a "fundamental floor" for precision in Galactic archaeology.

5. Conclusion

Current stellar models exhibit systematic ZAMS offsets rooted in MLT and Opacity choices. By acknowledging these biases, we provide a corrective framework for interpreting large-scale stellar surveys.

References

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