Hunting for giant cells in deep stellar convective zones using wavelet analysis

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 $0.7 M_\odot$ 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\degree 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.