This study considers the onset of stress-free Boussinesq thermal convection in rotating spherical shells with aspect ratio $\eta=r_i/r_o=0.9$ ($r_i$ and $r_o$ being the inner and outer radius), Prandtl numbers ${ \rm Pr} \in[10^{ -4},10^{ -1}]$, and Taylor numbers ${ \rm Ta}\in[10^{ 4},10^{ 12}]$. We are particularly interested in the form of the convective cell pattern that develops, and in its time scales, since this may have observational consequences. For a fixed ${ \rm Ta}<10^{ 9}$ and by decreasing ${ \rm Pr}$ from 0.1 to $10^{ -4}$ a transition between spiralling columnar (SC) and equatorially-attached (EA) modes, and a transition between EA and equatorially antisymmetric or symmetric polar (AP/SP) weakly multicellular modes are found. The latter modes are preferred at very low ${ \rm Pr}$. Surprisingly, for ${ \rm Ta}>3\times 10^{ 9}$ the unicellular polar modes become also preferred at moderate ${ \rm Pr}\sim10^{ -2}$ because two new transition curves between EA and AP/SP and between AP/SP and SC modes are born at a triple-point bifurcation. The dependence on ${ \rm Pr}$ and ${ \rm Ta}$ of the transitions is studied to estimate the type of modes, and their critical parameters, preferred at different stellar regimes.
Categories: astro-ph.SR astro-ph.HE physics.flu-dyn
