A new framework of multidimensional pulsating stellar envelopes I.: Properties of turbulent convection in static RR Lyrae envelope models with SPHERLS

TL;DR

This paper develops a new multidimensional framework for modeling turbulent convection in RR Lyrae stars, revealing significant size-dependent structural differences and providing insights for improving stellar pulsation models.

Contribution

It introduces a comprehensive 2D modeling approach for stellar convection, comparing it with traditional 1D models to enhance understanding of convective processes in RR Lyrae stars.

Findings

Convective zone structure varies with model size, especially below 9°.

Horizontal resolution used is sufficient for large eddy simulation.

Convective flux is partly driven by hydrogen ionization energy transport.

Abstract

Context. The one-dimensional treatment of turbulent convection had large successes until the early 2000s. However, the recent abundance and precision of observational data shows that this problem is far from solved. A modern approach should be developed by using multidimensional models. Aims. We established a new theoretical framework for comparison between one-dimensional and multidimensional convection models by mapping the two-dimensional structure of the convective zone and optimizing the modeling parameters of the SPHERLS code. Methods. We constructed a series of static envelope models for the same RR Lyrae stars, but with different horizontal sizes and resolutions. We then used a series of statistical methods to quantify the sizes of convective eddies, map the energy cascade, and describe the different structural parts of the convective zone. These include integral length scales, Fourier series, and the determination of the convective flux through horizontal averaging. Results. The structure of the convective zone depends significantly on the model size, below an angular size of $9^\circ$. The horizontal resolution of earlier studies is adequate to describe the granulation pattern in the large eddy simulation approach. In quasi-static RR Lyrae stars, the convective zone consists of two distinct dynamically unstable regions that are loosely connected. Approximately half of the convective flux is supplied by the transport of ionization energy in the partial hydrogen ionization zone. Conclusions. The 2D models presented in this work with the described size and resolution parameters can be used for comparison against 1D models. The structure of the convective zone urges reconsideration of some recent approaches to describe the convective flux currently used in radial stellar pulsation codes, which will be addressed in a separate paper.