Convection Theory and Sub-photospheric Stratification

TL;DR

This paper re-examines stellar surface and sub-surface convection using mixing-length theory, proposing new constraints and physical interpretations that improve the integration of 3D hydrodynamic simulations into stellar models.

Contribution

It introduces a hydrodynamically realistic dissipation length into MLT and links turbulence scales to physical convection features, advancing stellar convection modeling.

Findings

New constraint on solar models from dissipation length

Connection between turbulence scales and convective velocity

Physical interpretation of MLT parameters

Abstract

As a step toward a complete theoretical integration of 3D compressible hydrodynamic simulations into stellar evolution, convection at the surface and sub-surface layers of the Sun is re-examined, from a restricted point of view, in the language of mixing-length theory (MLT) . Requiring that MLT use a hydrodynamically realistic dissipation length gives a new constraint on solar models. While the stellar structure which results is similar to that obtained by YREC and Garching models, the theoretical picture differs. A new quantitative connection is made between macro-turbulence, micro-turbulence, and the convective velocity scale at the photosphere, which has finite values. The "geometric parameter" in MLT is found to correspond more reasonably with the size of the strong downward plumes which drive convection (Stein and Nordlund 1998), and thus has a physical interpretation even in MLT. Use of 3D simulations of both adiabatic convection and stellar atmospheres will allow the determination of the dissipation length and the geometric parameter (i.e., the entropy jump), with no astronomical calibration. A physically realistic treatment of convection in stellar evolution will require additional modifications beyond MLT, including effects of kinetic energy flux, entrainment (the most dramatic difference from MLT found by Meakin and Arnett 2007), rotation, and magnetic fields (Balbus 2009}.