A Technical Note on the MIST-PARSEC ZAMS Temperature Systematics

1. Introduction

The choice of stellar evolution grid introduces systematic uncertainties in isochrone fitting. This note compares MIST and PARSEC, highlighting the role of the Solar Abundance Problem, which led to the different solar metallicity scales adopted by these two grids.

2. Physical Drivers and Input Physics

Table 1: Key Input Physics Differences

2.1. Low-Mass Stars: Opacity and Convection

For stars < 1.2 Msol, MIST's lower Z and alpha_MLT differences drive the Teff offset. Lower metallicity reduces envelope opacity, while the higher alpha_MLT increases convective efficiency, both leading to a hotter ZAMS.

2.2. High-Mass Stars: The Role of Core Size

For stars > 1.5 Msol, the convective envelope vanishes. The increasing offset (up to 101 K at 2.0 Msol) is largely driven by differences in opacity tables (MIST uses OPAL/OPLIB while PARSEC's implementation varies) and the resulting convective core sizes, which alter the L-M relation and consequently the Teff.

3. Results

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

Table 2: ZAMS Effective Temperatures and Model Differences

3.1. Linear Fit

For the 0.8-2.0 Msol range, the offset is described by: Delta_Teff approx 37 (M/M_sol) + 15 K Note: The maximum residual is ~12 K at 2.0 Msol. Users should be aware that this linear approximation breaks down at higher masses where convective core physics dominates.

4. Discussion

4.1. Implications for Age Dating

The ~100 K difference at 2.0 Msol translates to a ~10% uncertainty in age estimates (Salaris et al. 2004).

4.2. Limitations and Future Work

We emphasize that this comparison is limited to solar metallicity and the ZAMS phase. Future work should include non-solar metallicities and the effects of rotation.

5. Conclusion

We have provided a clear, quantitative comparison of MIST and PARSEC ZAMS temperatures.

References

1. Choi, J., et al. 2016, ApJ, 823, 102 (MIST)
2. Bressan, A., et al. 2012, MNRAS, 427, 127 (PARSEC)
3. Asplund, M., et al. 2009, ARA&A, 47, 481
4. Salaris, M., et al. 2004, A&A, 414, 163
5. Vinyoles, N., et al. 2017, ApJ, 850, 155