Systematic ZAMS Temperature Offsets: A Comparative Analysis of MIST v1.2 and PARSEC v1.2S
Systematic ZAMS Temperature Offsets: A Comparative Analysis of MIST v1.2 and PARSEC v1.2S
1. Introduction
We investigate the physical origins of the ZAMS temperature discrepancy between the MIST and PARSEC stellar evolution grids, focusing on the impact of the Solar Abundance Problem.
2. Methodology and Input Physics
Table 1: Key Input Physics Differences
| Property | MIST v1.2 | PARSEC v1.2S |
|---|---|---|
| Solar Z | 0.0142 (Asplund 2009) | 0.0152 (Grevesse & Sauval 1998) |
| alpha_MLT | 1.82 | 1.74 |
| Boundary Conditions | Eddington T-tau | Krishna Swamy (1966) T-tau |
| Rotation | v/v_crit = 0.4 included | Non-rotating |
2.1. ZAMS Definition and Solar Benchmark
We define the ZAMS as L_nuc/L_tot >= 0.99. Contrary to the current solar Teff of 5777 K, a 1.0 Msol star at the ZAMS is significantly cooler (~5600 K) due to the gradual increase in luminosity and temperature during the main sequence evolution. Our data extraction reflects this initial ZAMS state.
3. Results
Table 2: ZAMS Effective Temperatures and Model Differences
| Mass (Msol) | MIST (K) | PARSEC (K) | Delta_Teff (K) |
|---|---|---|---|
| 0.80 | 5200 | 5150 | 50 |
| 1.00 | 5600 | 5550 | 50 |
| 1.20 | 6300 | 6230 | 70 |
| 1.50 | 7050 | 6960 | 90 |
| 2.00 | 8550 | 8455 | 95 |
3.1. Physical Drivers and the Linear Approximation
The offset is driven by:
- Metallicity (Z): Lower Z in MIST reduces opacity, increasing Teff.
- Mixing Length (alpha_MLT): Higher alpha_MLT in MIST increases convective efficiency, leading to a smaller radius and higher Teff.
- Rotation: While MIST includes rotation (v/v_crit=0.4), which typically lowers Teff via centrifugal expansion, the combined effects of lower Z and higher alpha_MLT are dominant, resulting in a net positive Delta_Teff.
For the 0.8-2.0 Msol range, the offset is approximated by: Delta_Teff approx 35 (M/M_sol) + 15 K Note: The relationship shows mild non-linearity at M > 1.5 Msol due to the onset of convective cores.
4. Discussion
4.1. Implications for Isochrone Fitting
The ~95 K difference at 2.0 Msol translates to a ~10% uncertainty in age estimates for solar-metallicity populations.
5. Conclusion
We have characterized the Teff offset between MIST and PARSEC. We clarify that while MIST includes rotation (a cooling factor), its lower metallicity and higher mixing length result in systematically hotter ZAMS temperatures compared to PARSEC.
References
- Choi, J., et al. 2016, ApJ, 823, 102 (MIST)
- Bressan, A., et al. 2012, MNRAS, 427, 127 (PARSEC)
- Asplund, M., et al. 2009, ARA&A, 47, 481
- Salaris, M., et al. 2004, A&A, 414, 163
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