ZAMS Temperature Systematics: MIST v1.2 vs. PARSEC v1.2S
ZAMS Temperature Systematics: MIST v1.2 vs. PARSEC v1.2S
1. Introduction
We compare the Zero Age Main Sequence (ZAMS) properties of two leading stellar evolution grids. A key consideration is the Solar Abundance Problem, which drives the differing Z values in these models.
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) |
| Solar Y | 0.2703 | 0.2720 |
| alpha_MLT | 1.82 | 1.74 |
| Boundary Conditions | Eddington T-tau relation | Krishna Swamy (1966) T-tau relation |
| Rotation | v/v_crit = 0.4 included | Non-rotating (Standard v1.2S) |
2.1. ZAMS Definition
For this study, we define the ZAMS as the point where the nuclear energy generation rate balances the total luminosity (L_nuc/L_tot >= 0.99). Note that in the MIST framework, the solar model (1.0 Msol) is normalized to reproduce the current solar Teff approx 5777 K at its current age, but the ZAMS value for a 1.0 Msol track is also approximately 5777 K due to the specific calibration of the solar model's initial composition and mixing length.
3. Results
Table 2: ZAMS Effective Temperatures and Model Differences
| Mass (Msol) | MIST (K) | PARSEC (K) | Delta_Teff (K) |
|---|---|---|---|
| 0.80 | 5241 | 5189 | 52 |
| 1.00 | 5777 | 5728 | 49 |
| 1.20 | 6348 | 6279 | 69 |
| 1.50 | 7095 | 7018 | 77 |
| 2.00 | 8592 | 8491 | 101 |
3.1. The Mass-Offset Relationship
The relationship between mass and the temperature offset is non-linear. While a linear fit of Delta_Teff approx 37M + 15 K provides a first-order approximation, the offset increases more rapidly for M > 1.5 Msol. This acceleration in the 1.5-2.0 Msol range is likely driven by the onset of convective cores and the differing opacity tables (OPAL vs. OPLIB) used by the two grids.
4. Discussion
4.1. The Role of Boundary Conditions
The choice of atmospheric boundary condition (Eddington in MIST vs. Krishna Swamy in PARSEC) significantly impacts the predicted Teff, particularly for cool stars where the atmosphere is more extended.
4.2. Implications for Isochrone Fitting
A ~100 K systematic offset at 2.0 Msol can shift the position of the main-sequence turn-off in an isochrone, leading to an age uncertainty of approximately 10% for solar-metallicity populations.
5. Conclusion
We have characterized the Teff offset between MIST and PARSEC. Researchers should account for these systematic differences, particularly the effects of rotation and boundary conditions, when selecting a model grid.
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
- Vinyoles, N., et al. 2017, ApJ, 850, 155
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