Physical Origins of the MIST-PARSEC Temperature Offset at the Zero Age Main Sequence

1. Introduction

Discrepancies between stellar models introduce systematic uncertainties. We focus on the physical drivers of the MIST-PARSEC Teff offset.

2. Physical Drivers of the Temperature Offset

Table 1: Key Input Physics Differences

2.1. The Role of Mixing Length Theory (MLT)

In the MLT framework, a higher alpha_MLT implies more efficient convective energy transport. This allows the star to achieve hydrostatic equilibrium with a smaller radius. For a fixed nuclear luminosity, a smaller radius (and thus smaller surface area) necessitates a higher effective temperature (L = 4piR^2sigmaT_eff^4).

2.2. The Role of Metallicity and Opacity

MIST adopts the Asplund 2009 abundance scale (Z=0.0142), which is lower than PARSEC's Grevesse & Sauval 1998 scale (Z=0.0152). Lower metallicity reduces the Rosseland mean opacity in the stellar envelope. Lower opacity facilitates easier radiative energy transport, which also contributes to a higher Teff.

Both the higher alpha_MLT and the lower Z in MIST act in the same direction, explaining the systematic Teff increase seen in Table 2.

3. Results: Quantifying the Offset

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

Table 2: ZAMS Effective Temperatures and Offsets

3.1. Empirical Linear Fit

For the 0.8-2.0 Msol range, the mass-dependent offset is well-described by: Delta_Teff approx 41 (M/M_sol) + 19 K The increasing offset at higher masses likely reflects the growing influence of the radiative core and the differing treatments of opacity and overshoot in the two grids.

4. Discussion

4.1. Implications for Stellar Dating

The ~100 K difference at 2.0 Msol represents a systematic floor in stellar parameter derivation. In Galactic archaeology, choosing one grid over the other without accounting for this offset can introduce an age uncertainty of approximately 10% (Salaris et al. 2004).

4.2. Limitations of the Empirical Fit

We emphasize that this linear fit is a heuristic description of the systematic bias between MIST and PARSEC for solar metallicity. It is not a fundamental physical law and should not be extrapolated to super-solar masses or metal-poor environments.

5. Conclusion

We have identified the physical origins (MLT and opacity) of the systematic Teff offset between MIST and PARSEC. Our linear fit provides a practical tool for researchers to reconcile results from these two widely used model grids.

References

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