Surface effects and turbulent pressure. Assessing the Gas-$\Gamma_1$ and Reduced-$\Gamma_1$ empirical models

TL;DR

This paper evaluates empirical models for turbulent pressure in stellar oscillation calculations, deriving them from turbulence equations and analyzing their assumptions and limitations in modeling surface effects.

Contribution

It derives the gas-$ ext{Gamma}_1$ and reduced-$ ext{Gamma}_1$ models from turbulence equations, clarifying their physical basis and assumptions.

Findings

Both models originate from turbulent pressure source and divergence terms.

They are compatible with the adiabatic approximation.

Several questionable assumptions are identified in the models.

Abstract

The use of the full potential of stellar seismology is made difficult by the improper modeling of the upper-most layers of solar-like stars and their influence on the modeled frequencies. Our knowledge on these \emph{surface effects} has improved thanks to the use of 3D hydrodynamical simulations but the calculation of eigenfrequencies relies on empirical models for the description of the Lagrangian perturbation of turbulent pressure: the reduced-$\Gamma_1$ model (RGM) and the gas-$\Gamma_1$ model (GGM). Starting from the fully compressible turbulence equations, we derive both the GGM and RGM models using a closure to model the flux of turbulent kinetic energy. It is found that both models originate from two terms: the source of turbulent pressure due to compression produced by the oscillations and the divergence of the flux of turbulent pressure. It is also demonstrated that they are both compatible with the adiabatic approximation but also imply a number of questionable assumptions mainly regarding mode physics. Among others hypothesis, one has to neglect the Lagrangian perturbation of the dissipation of turbulent kinetic energy into heat and the Lagrangian perturbation of buoyancy work.