Systematic Offset Analysis: MIST and PARSEC ZAMS Temperatures at Solar Metallicity

1. Introduction

We investigate the physical origins of the ZAMS temperature discrepancy between the MIST and PARSEC stellar evolution grids.

2. Input Physics and Physical Drivers

Table 1: Key Input Physics Differences

2.1. Low-Mass Regime (0.8-1.2 Msol): The MLT and Opacity Effect

In this range, MIST's lower Z reduces envelope opacity, facilitating energy escape and increasing Teff. Simultaneously, MIST's higher alpha_MLT drives more efficient convection, resulting in a slightly smaller radius and higher Teff for a fixed luminosity.

2.2. High-Mass Regime (1.5-2.0 Msol): The Role of Core Physics

As mass increases, the convective envelope disappears and a convective core emerges. In this regime, Teff becomes less sensitive to alpha_MLT and more sensitive to core opacity and overshoot. The fact that the offset grows to ~100 K at 2.0 Msol suggests that the different treatments of core overshoot and the specific opacity datasets (OPAL vs. OPLIB) play a dominant role.

3. Results and Empirical Description

We define the ZAMS where L_nuc/L_tot >= 0.99.

Table 2: ZAMS Effective Temperatures and the Systematic Offset

3.1. Linear Fit

Fitting the offsets in Table 2 yields the following relationship: Delta_Teff approx 38 (M/M_sol) + 13 K Note: This fit describes the systematic shift between the grids. The maximum residual is approximately 10 K, reflecting the non-linear nature of stellar structure transitions.

4. Discussion

4.1. Impact on Stellar Isochrones

The ~100 K difference at 2.0 Msol translates to a ~10% uncertainty in age estimates for solar-metallicity turn-off stars. This highlights the importance of selecting a consistent physics model when comparing theoretical predictions with observational clusters.

4.2. A Note on Physical Sensitivity

While this linear fit is practical, users should note that the physical drivers of the offset change across the mass range. The 0.8-2.0 Msol range spans a transition from convective-envelope dominance to radiative-envelope dominance, making a single-parameter description inherently approximate.

5. Conclusion

We have characterized the Teff offset between MIST and PARSEC. Our analysis highlights the differing roles of envelope convection and core opacity across the 0.8-2.0 Msol mass range.

References

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