Energy conservation and gravity waves in sound-proof treatments of stellar interiors: Part II Lagrangian constrained analysis

TL;DR

This paper extends sound-proof models for stellar interiors, introducing a generalized pseudo-incompressible model that conserves energy and accurately captures low-frequency phenomena across different stratified regimes.

Contribution

It develops a new energy-conserving pseudo-incompressible model (GPI) applicable to both convection and radiative zones in stars, extending previous anelastic models.

Findings

GPI model conserves energy unlike some other models

GPI accurately captures low-frequency phenomena in stratified stellar regions

Recommendations provided for implementing GPI in low-Mach number codes

Abstract

The speed of sound greatly exceeds typical flow velocities in many stellar and planetary interiors. To follow the slow evolution of subsonic motions, various sound-proof models attempt to remove fast acoustic waves whilst retaining stratified convection and buoyancy dynamics. In astrophysics, anelastic models typically receive the most attention in the class of sound-filtered stratified models. Generally, anelastic models remain valid in nearly adiabatically stratified regions like stellar convection zones, but may break down in strongly sub-adiabatic, stably stratified layers common in stellar radiative zones. However, studying stellar rotation, circulation, and dynamos requires understanding the complex coupling between convection and radiative zones, and this requires robust equations valid in both regimes. Here we extend the analysis of equation sets begun in Brown Vasil & Zweibel 2012, which studied anelastic models, to two types of pseudo-incompressible models. This class of models has received attention in atmospheric applications, and more recently in studies of white-dwarf supernovae progenitors. We demonstrate that one model conserves energy but the other does not. We use Lagrangian variational methods to extend the energy conserving model to a general equation of state, and dub the resulting equation set the Generalized Pseudo-Incompressible (GPI) model. We show that the GPI equations suitably capture low frequency phenomena in both convection and radiative zones in stars and other stratified systems, and we provide recommendations for converting low-Mach number codes to this equation set.