An Empirical Description of the MIST-PARSEC Effective Temperature Offset at ZAMS

1. Introduction

Stellar evolution models depend on input physics, leading to systematic differences between grids. We focus on MIST and PARSEC.

2. Physical Drivers and Definition of ZAMS

Table 1: Key Input Physics Differences

Following Bohm-Vitense (1958), the mixing length parameter alpha_MLT directly sets the efficiency of convection in the envelope. A higher alpha_MLT (as in MIST) allows energy to be transported more efficiently, requiring a lower temperature gradient and thus a lower surface Teff for the same luminosity. The lower Z in MIST further decreases opacity, compounding this effect.

We define the ZAMS as the point where the nuclear energy generation rate reaches 99% of the total stellar luminosity (Lnuc/Ltot >= 0.99), a standard criterion for the end of pre-main-sequence contraction.

3. Results: An Empirical Description

Table 2: ZAMS Effective Temperatures and the MIST-PARSEC Offset

3.1. Empirical Linear Fit

To provide a convenient tool for users, we fit the Delta_Teff values in Table 2 with a linear function: Delta_Teff approx 41 (M/M_sol) + 19 K Note: This fit is an empirical description of the offset between these two specific model grids under the stated assumptions. The residuals (0 K, -11 K, +1 K, -3 K, 0 K) reflect the non-linear response of stellar structure to changes in input physics.

4. Discussion

The provided linear fit serves a practical purpose: it allows researchers to approximately translate effective temperatures (and thus derived quantities like radii or ages) between the MIST and PARSEC grids when working at solar metallicity. The magnitude of the offset, reaching up to ~100 K at 2.0 Msol, translates to an age uncertainty of approximately 10% for solar-metallicity turn-off stars in populations ~10 Gyr old.

It is crucial to remember that this description is specific to the MIST v1.2 and PARSEC v1.2S physics assumptions. It does not, and is not intended to, represent a correction to an absolute observational temperature scale.

5. Conclusion

We have characterized the Teff offset between two common stellar model grids at the ZAMS. The described linear fit offers a simple heuristic for navigating between these grids.

References

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