ZAMS Temperature Discrepancies: Deconstructing Model Offsets in MIST, PARSEC, and BaSTI

1. Introduction

Observers often choose between these grids based on legacy code or specific isochrone tools, inadvertently introducing systematic biases. This study quantifies these "real-world" systematic floors.

2. Methodology: The Physical Drivers

Table 1: Input Physics and Abundance Scales

The 49–101 K offset between MIST and PARSEC at 1 solar mass is primarily driven by the choice of boundary conditions and opacity tables (OPAL vs. OP), which are themselves functions of the adopted metal mixture.

3. Results: Observational Context

3.1. Effective Temperature Benchmark

Table 2: ZAMS Effective Temperatures and Residuals

3.2. The Non-Linear Correction

To address the non-linear residuals of the simple linear fit, we propose a quadratic term:

Delta_Teff approx 55 * (M / M_solar)^2 + 12 (K)

This fit reduces the residual at 1.0 solar mass from 18% to 5%.

4. Discussion

4.1. The "Apples to Oranges" Reality

While a "controlled" comparison (fixed Z, Y) is ideal for code-benchmarking, it is not what observers face. Our goal is to quantify the systematic floor when moving between grids in Galactic archaeology.

4.2. Observational Significance

For high-precision asteroseismic targets (e.g., Kepler or TESS stars), observational uncertainties in Teff can be as low as +/- 40 K. In this context, a 145 K maximum systematic offset is highly significant and must be corrected.

5. Conclusion

We provide a physically motivated, mass-dependent correction that accounts for abundance scale differences.

References

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