Hunting for Giant Cells in Deep Stellar Convective Zones Using Wavelet Analysis

Abstract

We study the influence of stratification on stellar turbulent convection near the stellar surface and in depth by carrying out 3D high resolution hydrodynamic simulations with the ASH code. Four simulations with different radial density contrast corresponding to different aspect ratio for the same underlying 4 Myr 0.7M ⊙ pre-main sequence star model are thus performed. We highlight the existence of giant cells which are embedded in the complex surface convective patterns using a wavelet and time correlation analysis. Next, we study their properties such as their lifetime, aspect ratio and spatial extension in the different models both in latitude and depth according to the density contrast. We find that these giants cells have a lifetime larger than the stellar period with a typical longitudinal width of 490 Mm and a latitudinal extension increasing with the radial density contrast overpassing 50 • of latitude for the thickest convective zone. Their rotation rate is much larger than the local differential rotation rate increasing also with the radial density contrast. However, their spatial coherence as a function of depth decrease with the density contrast due to the stronger shear present in these more stratified cases.

Convection and differential rotation properties of G and K stars computed with the ASH code

“…A similarly "banded" differential rotation pattern is evident in a number of previously published ASH simulations (e.g. Bessolaz & Brun 2011;Browning 2008). The differential rotation in the 0.7 and 0.9 M cases are the most solar-like, with the fastest angular rotation rate at the equator and slowest at higher latitudes.…”

“…However, the 1.1 M case exhibits anti-solar rotation, where the slowest angular velocity is at the equator. A reversal of the sense of differential rotation was also observed in the simulations of Bessolaz & Brun (2011) to be an effect of the thickness of the convection zone. In that study, the stellar structure and luminosity was held fixed, while only the thickness of the convection zone was varied.…”

“…This code has been extensively tested and used for computing several aspects of the solar interior (e.g., Browning et al 2006;Brun & Toomre 2002;Brun et al 2004;DeRosa et al 2002;Miesch et al 2006), rapidly rotating young stars (Ballot et al 2007;Brown 2009;Brown et al 2008Brown et al , 2011, the convective cores of massive stars (Browning et al 2004a,b;Featherstone et al 2009), fully convective low mass stars (Browning 2008), red giant stars (Brun & Palacios 2009), and pre-main-sequence stars (Bessolaz & Brun, in this volume;Bessolaz & Brun 2011). We briefly describe the basic aspects of the code here, but the reader can find further details of the code in those previous works (see especially, Brun et al 2004;Clune et al 1999).…”

“…The convective patterns and characteristic eddy sizes are influenced strongly by the density scale height, as well as by the contrast between horizontal and vertical velocity (see, e.g., Bessolaz & Brun 2011). Near the surface, where the density scale height is smallest, the shell slices exhibit typical convective patterns characterized by hot, broad/patchy upflows surrounded by cool, narrow downflow lanes (e.g., Cattaneo et al 1991;Miesch et al 2008).…”

Hunting down giant cells in deep stellar convective zones

“…After having detected a large scale signal with our wavelet analysis (see Bessolaz & Brun 2011 for details), a time correlation analysis similar to the work of Miesch et al (2008) is performed both on the full convective flow and on the reconstructed largest spatial scales of the wavelet analysis. This helps to quantify the lifetime of these large scale structures detected over two stellar periods for each model (see Fig.…”