New Paradigm of Stellar Convection

forthcoming

Tension between solar observations and theoretical and numerical models is shifting the paradigm of stellar convection: simulations produce dominant large-scale flows that are absent in the Sun and the supergranulation scale which according to helioseismology is dominant in the Sun is not captured by simulations. This discrepancy, named the convective conundrum, is the most likely reason why solar differential rotation and global dynamo cannot be reproduced by current simulations. Canonical theory of convection, adopted in many models, assumes an unstable temperature gradient and local driving of convection leading to large-scale flows. More recent models relaxing this assumption suggest that surface cooling is the main driver of convection leading to non-local flows in the form of cool entropy rain. In this scenario the bulk of the convection zone is weakly stably stratified in stark contrast to traditional mixing length picture. Recent solar observations also suggest fast small-scale plumes and weakly stable stratification. Simulations of stellar convection are limited by computational resources such that fully realistic models of stars are unreachable in the foreseeable future. This is most likely why surface driving of convection is too weak in current simulations. In the NeoCon project this is addressed via a solar observation-inspired subgrid-scale model incorporating entropy rain to reach the highly non-local regime of convection conjectured to exist in stars. This has paradigm-shifting consequences for theories of stellar convection and dynamos with potential ramifications for stellar structure and evolution. In addition to capturing convection accurately, another crucial requirement is ensuring the correct force balance in stellar interiors with strong multiscale magnetic fields. Strong magnetism is another possible solution to the convective conundrum which is addressed in the NeoCon project by comprehensive study of magnetic back-reaction on convection.