Testing the 3-equation Kuhfuss convection model using the Sun

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85741 Garching, Germany
² Ludwig-Maximillians-Universität München, Geschwister-Scholl-Platz 1, 80539 Munich, Germany
³ Heidelberger Institut für Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany
⁴ Faculty of Comp. Sci. & Appl. Math., Univ. of Applied Sciences, Technikum Wien, Höchstädtplatz 6, A-1200 Wien, Austria
⁵ Wolfgang-Pauli-Institute c/o Faculty of Mathematics, University of Vienna, Oskar-Morgenstern-Platz 1, A-1090 Wien, Austria
⁶ Fakultät für Mathematik, Universität Wien, Oskar-Morgenstern-Platz 1, A-1090 Wien, Austria

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 9 October 2025
Accepted: 26 March 2026

Abstract

Context. Simplified one-dimensional models are necessary to model convection in the context of stellar evolution. By including the non-local effects of convection, turbulent convection models describe convection in a more physical way than does mixing length theory, which is typically used in one-dimensional stellar evolution models. We recently showed that the 1-equation Kuhfuss turbulent convection model is not sufficient to model the solar convective envelope satisfactorily.

Aims. Using the Sun as a benchmark, we test the physically more complete 3-equation Kuhfuss turbulent convection model.

Methods. We calculated a solar calibrated model with the 3-equation Kuhfuss turbulent convection model using the one-dimensional stellar evolution code GARSTEC. We compared the predicted interior structure of the model with helioseismic measurements of the Sun. Furthermore, we investigated how the free parameters and the closure relations of the 3-equation model affect the results.

Results. We find that with the 3-equation model, the temperature gradient at the inner boundary of the convective envelope is modelled more realistically than with the mixing length theory or the 1-equation model. This also improves the agreement for the sound speed profile between the model and the Sun and reduces the asteroseismic surface effect. However, close to the surface, the 3-equation model results in a layer with an unphysical negative temperature gradient. This layer is connected to the closure relations used in the 3-equation model.

Conclusions. Our results demonstrate the capabilities of turbulent convection models and can serve as a next step towards an improved and more realistic modelling of convection in stellar evolution codes.