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The effective temperature (Teff) distribution of stellar evolution modelsalong the red giant branch (RGB) is sensitive to a number of parametersincluding the overall metallicity, elemental abundance patterns, the efficiencyof convection, and the treatment of the surface boundary condition. Recentlythere has been interest in using observational estimates of the RGB Teff toplace constraints on the mixing length parameter, a_MLT, and possible variationwith metallicity. Here we use 1D MESA stellar evolution models to explore thesensitivity of the RGB Teff to the treatment of the surface boundary condition.We find that different surface boundary conditions can lead to +/- 100 Kmetallicity-dependent offsets on the RGB relative to one another in spite ofthe fact that all models can reproduce the properties of the Sun. Moreover, fora given atmosphere T-tau relation, we find that the RGB Teff is also sensitiveto the optical depth at which the surface boundary condition is applied in thestellar model. Nearly all models adopt the photosphere as the location of thesurface boundary condition but this choice is somewhat arbitrary. We compareour models to stellar parameters derived from the APOGEE-Kepler sample of firstascent red giants and find that systematic uncertainties in the models due totreatment of the surface boundary condition place a limit of ~100 K below whichit is not possible to make firm conclusions regarding the fidelity of thecurrent generation of stellar models.

On the Red Giant Branch: Ambiguity in the Surface Boundary Condition Leads to ~100 K Uncertainty in Model Effective Temperatures