Bridging the Prandtl Number Gap: 3D Simulations of Thermohaline Convection in Astrophysical Regimes

Abstract Thermohaline convection (also known as fingering convection or thermohaline mixing) occurs in stellar radiation zones where a sufficient inversion of the mean molecular weight is present. This process mixes chemicals radially and occurs in a variety of stars, including near the luminosity bump on the red giant branch and potentially in polluted white dwarfs. Previous efforts to characterize this process using 3D simulations have been restricted to regimes far from actual stars: the Prandtl number Pr—the ratio of the kinematic viscosity to thermal diffusivity—assumes values as low as 10 −6 in stars, but 3D simulations have been restricted to Pr ≳ 10 −2 . For this reason, disagreements between observations and simulations are routinely dismissed as stemming from this Pr gap. This Letter bridges this gap and demonstrates that 3D simulations of thermohaline convection can be performed in stellar parameter regimes. Using a suite of simulations spanning previously studied regimes with Pr ≳ 10 −2 down to Pr = 10 −6 , we demonstrate that the chemical mixing model of J. Brown et al. remains consistent with 3D simulations across both regimes. Therefore, tensions between this model and observations cannot be dismissed as resulting from a Pr gap and must be resolved by considering additional physics.