Princeton University Star Formation/ISM Rendezvous (SFIR)

Large-scale convective flows called giant cells were once thought to transport the Sun’s luminosity in the solar convection zone, but recent observations do not detect giant cells. The absence of giant cells is referred to as the “Solar Convective Conundrum” and calls into question our fundamental understanding of stellar envelope convection. In place of large-scale flows, some authors have suggested the solar luminosity may instead be transported by small droplets of rapidly falling, low entropy fluid. This “entropy rain” could propagate as dense vortex rings, analogous to rising buoyant “thermals” in the Earth’s atmosphere. In this work, we model stellar downflows as dense, negatively buoyant thermals and develop an analytical theory describing their evolution. We verify the theory with 2D cylindrical and 3D cartesian simulations of laminar, axisymmetric thermals in highly stratified atmospheres. Our results show that dense thermals fall in two categories: a stalling regime in which the droplets slow down and expand, and a falling regime in which the droplets accelerate and shrink as they propagate downwards. We estimate that solar downflows are in the falling regime and maintain their entropy perturbation against diffusion until they reach the base of the convection zone. This suggests that entropy rain may be an effective nonlocal mechanism for transporting the solar luminosity.